Categories
Circadian science Shift Work

Running the numbers on health care shift workers

At Arcascope, we’re focused on helping shift workers get more sleep and feel better with circadian-specific interventions. In particular, we’ve spoken to a lot of shift workers in the healthcare industry about their experiences working nights. 

A lot of the lessons we’ve learned are echoed in this recent survey we ran with 218 nurses who work night shifts (both rotating and fixed nights). Here’s a tour of what we found:

Increased risk of error 

A full 65.1% of nurses we surveyed either agreed with or strongly agreed with the statement “Being tired on night shift makes me more likely to make a mistake.” More than a quarter said they strongly agreed with the sentiment, compared to less than 5% who said they strongly disagreed.

Heightened turnover risk 

Nearly half of the nurses—49.5%— said they’ve thought about leaving their current job for reasons specifically related to working night shift. 

When we asked them what made them want to quit, the number one cited reason was sleep loss. 

Negative effects on health and wellbeing 

More than half of the nurses in the survey, 53.7%, said they agreed with the statement “Working the night shift has negatively impacted my health and wellbeing,” with only 5% strongly disagreeing. 

This lines up with a past survey we conducted, where we asked nurses who work the night shift what symptoms they found most disruptive. The table below summarizes what we heard then: in general, feeling exhausted and not being able to fall asleep and stay asleep are major pain points for these workers.

Absenteeism…or presenteeism? 

38.5% of nurses working the night shift surveyed said they agreed with the statement “I’ve missed work because of health problems due to shift work (e.g. sleep loss, fatigue, etc.).” That said, 50% said they disagreed or strongly disagreed with the statement. 

On the one hand, society needs critical workers like night shift nurses in order to function. On the other, given the risks of fatigue and error that come with lost sleep, it may be that there are people showing up to work who are putting themselves at risk by getting behind the wheel to drive in. 

Interestingly, our population for this survey skewed a bit older, with 45-60 being the largest age group represented. 

This is interesting in part because the pain points we found here agree with the literature: You don’t tend to see shift workers being better at handling the night as they get older. Studies have found that night shift workers who are older tend to follow less adaptive sleep strategies, possibly because they have a harder time sleeping in. 

If you’re a nurse struggling with shift work, or a health system looking for a solution to get your people sleeping more, we want our app Shift to lend you a hand. Reach out to get on our pilot waitlist. 

Categories
Circadian science Shift Work

Feeling Flat

Over and over again, I’ve heard shift workers say something like the following: 

“I have no rhythms.” 

“It’s just a constant experience of bleh.”

“I get off shift, and more than tired, I just feel… flat.”

Ah,” I think when I hear this. “The amplitudes of their circadian rhythms have been squashed.” To steal an analogy from an older blog post, it’s like they’re rocking back and forth just a little bit on a swing at the park, instead of getting some nice height and momentum. It’s like they’re a yo-yo that’s only making it a fraction of the way back up to your hand. Or experiencing one of those dud bounces on a trampoline, where you only go up a few inches while the friend next to you goes flying. 

There are lots of ways of talking about this idea of lost amplitude.

You can think of the amplitude of an average daily activity profile, where you visualize a person’s normal day by taking the average of what they do over multiple days. For somebody with an irregular schedule, the amplitude (or maximum height) of this profile is going to be lower than for somebody with a super regular schedule: An irregular person’s periods of high activity will get canceled out by periods of low activity, whereas somebody who’s super regular will have all their high activity happening at roughly the same time. 

You can also think of amplitude in terms of the brain’s suprachiasmatic nucleus, or SCN, where the core timekeeping of your body’s daily rhythms occurs. If all the neurons in the SCN are like “Yes! It’s daytime!” they’ll send a clearer, stronger signal to the rest of your body, whereas if half of them are like “It’s day!” and the other half are like “No, it’s night!” the signal will be… not so clear. Kind of muddled, really. A dud bounce. 

I was thinking about circadian amplitude as I made my way through this recent paper from Zhang et al. in eBioMedicine. In this paper, the authors track the temperature and activity patterns of both day and night shift healthcare workers, and look at the ways in which they differ from one another.

For instance, the probability of a day worker resting at different times of the day in their dataset looked a little something like this: 

While the same plot for night shift workers looked like this: 

The black dashed average line for night shift workers maxes out around 0.6, while the same line for day workers hits ~0.9. In other words, we see a lower amplitude of the probability of resting (and a much more chaotic picture overall of night shift activity overall).

This is that “average daily activity profile” I mentioned above. But the authors also looked at patterns in chest temperature over the course of the day, finding 24-hour rhythms in 70% of the day shift workers and only 48% of the night shift workers. 

What’s that mean? Well, it could reflect the fact that temperature and activity are correlated—you move a bunch, your temperature goes up—so the story we see in activity could simply be making itself known through temperature as well. But it could also be capturing a bit of the SCN discord I brought up earlier. Your body’s clock contributes its own 24-hr pattern to your daily temperature profile, so a flatter signal coming from your brain could make for a flatter body temperature pattern too. 

I’ve written elsewhere about how more amplitude—or a clearer difference between night and day—seems to be linked to lots of good things, like lower cardiovascular disease. One nice thing about amplitude? It’s not fixed over your lifetime: it can change, dynamically, with your actions. Our app Shift isn’t just aimed at shifting the time of your body’s clock: we also want to help you to boost amplitude as well. Want to give it a shot, for yourself or your employees? Reach out at inquiries@arcascope.com

Thanks to the authors of Digital circadian and sleep health in individual hospital shift workers: A cross sectional telemonitoring study for an enjoyable read!

Categories
Circadian science Technology

Visualizing MESA: Part 2

We’ve already looked at the Multi-Ethnic Study of Atherosclerosis (MESA) dataset—an absolute treasure trove of sleep data, available from the NSRR at sleepdata.org—once, through the lens of sleep duration. But what about other dimensions of sleep health? After all, sleep regularity may be just as important as sleep duration in a number of contexts.

We once again teamed up with Ryan Rezai, a data scientist and student at the University of Waterloo, to visualize some MESA data. Once again, all the plots below were made by Ryan to highlight some intriguing trends in the MESA dataset. As always, we think there’s a lot of value in looking for pictures that can help you grapple with the complexity of multifaceted, complex phenomena like sleep.

Let’s start with the basics:

First we need to define sleep irregularity. We can do this in a number of ways. In the plots below, we’ve defined it in the ways MESA does—as either the standard deviation in total sleep duration (sd24hrsleep5) or the standard deviation in bed time (sdinbedtime5). 

So how does sleep irregularity, as defined above, relate to Epworth Sleepiness Scale self-reports (ESS) in the dataset? Like this: 

Looking at this, I feel pretty confident that there’s a trend here! As sleep irregularity increases, so does ESS, up until you get up to pretty profound variability in sleep regularity (180 minutes = 3 hrs standard deviation—you might be a shift worker at that point).

It’s even more remarkable when you look back at sleep duration and ESS (see last blog):

Not only does sleep regularity seem to have a clearer trend with ESS than sleep duration, it also seems to have a slightly higher amplitude effect, as shown on the y-axis. Right off the bat, this is a clue that we might be wanting to pay more attention to sleep regularity when we talk about the experience of sleepiness.

If we look at both bedtime irregularity and sleep duration simultaneously, we can notice something else interesting: 

Namely, that even for people sleeping quite a long time (e.g. 500 minutes), greater bedtime irregularity is linked to greater feelings of subjective sleepiness. 

What might be going on here?

We know a person’s subjective experience of sleepiness doesn’t always line up with how restricted their sleep has actually been. For instance, people on four hours of sleep a night tend to get worse and worse at reaction time tests, while their subjective sleepiness grows for a while but eventually levels off. Maybe irregularity makes people more reliably aware of just how sleepy and impaired they are because their irregularity means they’re more likely to be awake during periods of time when melatonin is at a high concentration in their body. Or maybe irregularity is having a dampening effect on their body’s circadian rhythms, making them more likely to feel exhausted and flat. There’s plenty of work to be done here in the future, but I’ll take off my Hat of Speculation now.

Beyond sleepiness

These 3-D sleep plots can be used to visualize more than just ESS. Take, for instance, this plot of total apneas over the course of the night as a function of sleep duration and sleep regularity: 

Ok, wow! That’s a clear picture, albeit perhaps not a surprising one in some ways. After all, you’d expect a longer duration of sleep to mean more opportunities for apneas. That said, it is interesting how, once you get above about 300 minutes of sleep (5 hours) or so, holding sleep duration fixed and increasing irregularity seems to correlate with increased apneas. 

Something similar appears to hold for sleep irregularity, sleep duration, and apneas per hour, with more sleep irregularity linked to a higher rate of apneas—at least when you ignore people sleeping around 200 minutes a night (which, to be clear, is probably not that many people):

Sleep irregularity and heart rate

Lastly, we might be interested in how sleep irregularity correlates with heart rate. After all, recent research has shown the risk of a cardiovascular event is more than twice as high in irregular sleepers as it is in regular sleepers. When we look at the irregularity in sleep duration, it sure looks like there might be something going on with average heart rate and irregularity in how long you sleep:

This trend also seems to hold when you look at the correlation between bedtime irregularity and average heart rate:

For both of these plots, the standard error is smallest between 0 and 180 minutes of standard deviation in sleep irregularity—and like we noted earlier, three hours standard deviation in sleep irregularity is a lot! (What’s going on when the standard deviation of bedtime irregularity is around 300 minutes? I sure as heck don’t know. But since that’s a standard deviation of five hours in sleep irregularity, odds are good that that’s not the typical sleeper.)

On the whole, it seems pretty clear: People who care about their overall sleep health shouldn’t sleep on sleep regularity.

With thanks to these resources:

Zhang GQ, Cui L, Mueller R, Tao S, Kim M, Rueschman M, Mariani S, Mobley D, Redline S. The National Sleep Research Resource: towards a sleep data commons. J Am Med Inform Assoc. 2018 Oct 1;25(10):1351-1358. doi: 10.1093/jamia/ocy064. PMID: 29860441; PMCID: PMC6188513.

Chen X, Wang R, Zee P, Lutsey PL, Javaheri S, Alcántara C, Jackson CL, Williams MA, Redline S. Racial/Ethnic Differences in Sleep Disturbances: The Multi-Ethnic Study of Atherosclerosis (MESA). Sleep. 2015 Jun 1;38(6):877-88. doi: 10.5665/sleep.4732. PMID: 25409106; PMCID: PMC4434554.

The Multi-Ethnic Study of Atherosclerosis (MESA) Sleep Ancillary study was funded by NIH-NHLBI Association of Sleep Disorders with Cardiovascular Health Across Ethnic Groups (RO1 HL098433). MESA is supported by NHLBI funded contracts HHSN268201500003I, N01-HC-95159, N01-HC-95160, N01-HC-95161, N01-HC-95162, N01-HC-95163, N01-HC-95164, N01-HC-95165, N01-HC-95166, N01-HC-95167, N01-HC-95168 and N01-HC-95169 from the National Heart, Lung, and Blood Institute, and by cooperative agreements UL1-TR-000040, UL1-TR-001079, and UL1-TR-001420 funded by NCATS. The National Sleep Research Resource was supported by the National Heart, Lung, and Blood Institute (R24 HL114473, 75N92019R002).

Categories
Circadian science Shift Work

A space for shift workers, Pt. 2

Here again is our blog feature where we look on the internet for what folks are saying about their shift work, and try to speak to their experiences with the power of circadian science.

First up: 

Reddit User:
My shift is usually 7pm-3:15am. when i get off work i shower, eat, and watch some Netflix until i fall asleep around 5am, but then i’ll only sleep until around lunch when my body naturally wakes me up and i can’t go back to sleep. i end up spending the rest of the day anxious and anticipating going into work and feeling like i can’t do anything productive because i’ll be too tired to perform at my job. i’m a card dealer so i need to be alert and focused during my shift to count cards and do the mental math for payouts.

Sometimes i can squeeze in a nap in the afternoon but it can make me more groggy when i wake up. i’ve tried forcing myself to stay up a little longer after i get home from work but i’ll still wake up around noon and then i’m stuck with less sleep than if i had passed out right when i got home.

Any advice? i get so much anxiety about sleeping through my alarms or being so tired at work i pay someone wrong and it’s overwhelming sometimes. i want to try to keep this job for at least a year so i can pay off my debt and student loan but it’s becoming mentally draining


Our Take:

Reddit user, you’re not alone. Many shift workers experience the exact same thing you do—waking up after only a short time sleeping, post-shift. What you’re describing sounds like your body’s circadian clock swinging to wake you up after you’ve drained your homeostatic build-up, or sleep hunger, from working the night shift. In other words, your body clock is still pretty well adjusted to a day schedule, so it’s trying to wake you up to match the day. By the time noon rolls around, it thinks your circadian sleep window has passed.

The good news is that you can shift your body’s clock, so that your circadian sleep window happens when you want it to (like in the hours after your shift, instead of during your shift). Might make life a little easier, huh? You may be thinking to yourself that the process of “shifting your clock” sounds complicated, and you really don’t have time to add more to your plate. Well, here are two pieces of good news for you.

First, while understanding your unique circadian clock is complex, with a lot of moving parts, we’ve been working on a way to make it simple: simply hook our app up to the data collected from your phone (and wearables, if you own one). Living a circadian-aware life is something that anyone can accomplish, whether you work day shifts, night shifts, or somewhere in between. 

Second, it’s not about “the time it takes” to fix your clock— it’s about “the time” itself. Your normal activities throughout the day like eating, exercising, and looking at screens are all sending signals to your clock. When you start timing these activities correctly, the signals can help shift your circadian sleep window to where you want it to be. So what we’re really trying to say is, it’s not so much about the what of your day, it’s about the when of it.

Oh, and that grogginess you feel after a nap in the afternoon? That’s called sleep inertia, and it can be worse at some times vs. others. We can warn you when sleep inertia’s likely to be worse by tracking your body clock’s time.

There are a lot of things we’ve learned about what can help shift workers adjust to their schedules. If you’re looking for where to start, that’s where our app, Shift, comes in.


Reddit User:
I’ve been doing overnights for almost 3 years now (22:15-0645). It’s been decent for the most part, but the one thing I’ve consistently had an issue with is staying asleep. I’ll get off work, come home, eat breakfast, and be asleep by 08:45. I have no trouble at all falling asleep, but I wake up around 14:00 all the time. I was wondering if you guys have had similar problems? What did you do to stay asleep? Ideally I’d like to sleep until 15:45, as that would give me 7 hours of rest. Thanks for any help.


Our take:

Once again, this sounds like a problem caused by a body clock that’s scheduling sleep too early in the night (and missing out on the window of time you actually have available to sleep). 

Let’s dive into the science a little more. There are two main forces that work together to keep you asleep. One is the homeostatic sleep drive, or “sleep hunger,” which builds while you’re awake and the other is the circadian sleep drive, which rises and falls about every 24 hours. When you’re well-adjusted to sleeping on a day schedule, there’s a hand-off from your homeostatic sleep drive to your circadian sleep drive. 

Think of this as a baton relay race, where the baton is your sleep, and the track is the length of time you’re hoping to be asleep for. That makes your homeostatic sleep drive the first racer, and your circadian sleep drive the second racer. When your homeostatic sleep drive nears the end of its turn, there’s a hand-off to your circadian sleep drive in order to keep you asleep until the finish line. However, if your internal circadian clock is out of whack—or, analogy time, like the second racer isn’t where they need to be for the hand-off—the passing of the baton doesn’t go so smoothly. And even though you were hoping to stay asleep for the whole race, the fumbled hand-off wakes your body up hours earlier than you wanted.

There are many negative effects that happen when your circadian clock is off track with your schedule, and waking up in the middle of the night is just one of them. Luckily, this does not have to be permanent. The fact that you have the power to throw your clock a little off track, also means that you have the power to get it back on track. That’s what we’re here to help with.

Categories
Circadian science Sleeping troubles

Sleep Regularity + Students = An A+ Idea

College, as any student can attest to, is a hectic life.

Look no further than my past spring semester for a prime example of this. Taking multiple difficult classes while managing club activities and a social life was a whirlwind that was both exhausting and invigorating at the same time. The thorn in my side, however, was Advanced Calculus, a class for proving the underlying mechanics that we used in Calculus 1 and 2.

In my defense, Advanced Calculus is a difficult class even for math majors. They say if you’re able to pass it, you can get a math degree! I’m relieved to say I managed to scrape by and am on my way to finishing up my undergraduate degree in math (and computer science) next year.

I passed—isn’t that all that matters? Yet, with all the things I want to do in my college years, I found myself wondering: is there room for more optimization? I couldn’t help but think of the story of the British Cycling Team and their transformation from being the worst cycling team in the sport to utterly dominating it in a few years. What did they do, exactly? They accumulated little optimizations that eventually compounded into huge advantages. Some of these tiny changes include wearing more aerodynamic racing suits, testing different massage gels for faster recovery, and (drumroll, please) using better  pillows and mattresses for sleep.

I’m convinced now more than ever that sleep is a severely underrated optimization in anyone’s life (exponentially more so for shift workers). Yet for students, the reigning sentiment is that we must fuel ourselves with Monster in order to work late into the night, whether that’s studying for finals, cranking away on projects, or catching up on assignments. Sacrificing sleep to stuff our heads with lecture material is tradition, but does this really work?

Well, maybe not. In “Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing”, Phillips et al. were able to find a compelling correlation between sleep regularity and average GPA by using a metric called the Sleep Regularity Index. This index calculates the probability of an individual being in the exact same state (aka sleep or awake) 24 hours later at any point in time. They found that an increased SRI correlated with a higher GPA. In fact, during a 30-day period, there was a considerable gap in GPA between the regular and irregular sleepers: 3.72 (Regular) versus 3.42 (Irregular). Importantly, they found no such relation for GPA and sleep duration. In other words, you might be throwing your grades off track simply by staying up later than usual, even if your overall sleep duration averages out to a decent amount.

Now, association doesn’t necessarily mean causality. There’s a lot that goes into a grade. Sleeping regularly by itself won’t be the reason why you ace a final, but it can certainly tilt the odds in your favor. One fact the study highlighted was that an irregular sleep schedule had the same effect as traveling westward by two to three time zones. I imagine that being jet-lagged isn’t the most optimal state to study in. And that feeling of jet lag could negatively compound upon itself over multiple days, adding up to a pretty big effect.

A nice thing about the sleep regularity index is that you don’t need Advanced Calculus to calculate it—there are multiple versions of code to calculate it online. And if sleep regularity is key, as it sure seems like it is, then optimizing everyday life around a consistent bedtime is probably for the best. That’s something for college kids everywhere to keep in mind when life gets hectic again this fall!

This blog post was written by Jessica So, one of Arcascope’s interns. Thanks, Jessica, for your hard work!

Categories
Circadian science Lighting

What matters besides light?

A Guide to Non-Photic Zeitgebers

We can all agree that the difference between night and day is, well, night and day when it comes to light. The progressive change in light present in our environment is subconsciously tracked by our bodies and that information is used to help time our sleep-wake cycles. Of course, not everyone has the physical ability to perceive the changes in light which occur over the course of the day. Yet some blind individuals are still able to entrain accurately to their environment without these crucial photic cues. How is that so?

Light is the strongest “zeitgeber”— or, environmental cue that provides input to the circadian clock. Our bodies use the signals from zeitgebers to try to synchronize our internal clocks with our environments. For example, the decrease of light over time that you experience while watching a sunset can communicate to your body that nighttime is approaching. If you keep the lights on instead, your body’s clock can be delayed and production of the hormone for night, melatonin, can be suppressed. Because of this, it’s important to think about what signal your light exposure is sending to your internal clock (cough, screens at night, cough). But light is not the only player in town. There are other zeitgebers besides light that influence your circadian time.

Before getting into the weeds, it will help to have a mental map for how your body’s clocks are organized. Our circadian clocks are composed of a central clock and peripheral clocks. The central clock acts as the command center located in the suprachiasmatic nucleus (SCN), sending signals to the various tissue-specific peripheral clocks spread throughout the body. The phase of each peripheral clock is influenced by both the central clock and factors specific to that system, allowing different biological functions to be coordinated through the central clock while being autonomous enough to respond to stimuli specific to that system. For example, as we’ll discuss below, the metabolic clock is driven by both the central clock but is also affected directly by meal timing. Some signals may primarily affect peripheral rhythms, while others can affect both peripheral and central rhythms.

Zeitgebers, such as light, communicate with SCN
Zeitgebers, such as light (a), communicate with the SCN which houses the central circadian clock. Other signals, like meal timing (b) can relay their own signals to the body’s peripheral clocks in organs and tissues (c and d).

Before we talk about these non-light inputs to your body’s time, let’s talk about phase-response curves. The standard way to track how a zeitgeber advances or delays your central circadian pacemaker is through phase-response curves (PRCs). These graphs represent the timing of a zeitgeber stimulus (x-axis) and its quantitative effect on the timing of a circadian biomarker (y-axis), like shifts in melatonin timing or core body temperature minimum. A phase-response curve can tell us when a zeitgeber will advance you, when it will delay you, and when it will have essentially no effect. 

Phase-response curve
Phase-response curve showing circadian time of exercise vs. phase shift, determined by presence of 6-sulphatoxymelatonin (aMT6s) measurements. Participants performed 1 hour of moderate treadmill exercise at the same clock time for 3 consecutive days.

Exercise has been shown to induce central clock phase changes dependent upon the timing of activity. One study in particular, which produced the PRC above, recruited 101 physically active adults to investigate the effects of exercise on circadian phase. Participants were put on a 90 minute ultradian light schedule (60 minutes light/wake followed by 30 minutes dark/sleep) for approximately 6 days. Baseline aMT6s (urinary melatonin) measurements were made for individuals and averaged across the whole sample of participants. The difference between the individual and sample phase was determined and then subtracted from the external clock time of exercise to calculate a “circadian time” adjusted for interindividual differences. 

Each participant performed 1 hour of moderate treadmill exercise at the same clock time for 3 consecutives days following baseline, isolating the effects of exercise on phase. Moderate exercise was quantified as 65-75% of the heart rate reserve calculated for each participant based on their individual maximal heart rate. This study concluded that exercise at 7am and between 1pm and 4pm causes phase advances to aMT6s timing, whereas exercise between 7pm and 10pm provoked phase delays. 

These results can be read from the PRC above by identifying the selected time along the x-axis and reading the corresponding point’s phase shift, quantified by the y-axis. For example, exercising at 7pm (19 h) results in approximately -0.75 h phase shift. The negative value indicates a delay, whereas a positive would indicate an advance. Thus, exercise at 7pm causes a little less than an hour phase delay based on the above PRC. 

A similar pattern of phase advance and delay regions can be observed in the human response to light, especially in studies with parallel protocols or similar duration of stimulus. Intuitively, the analogous behavior of morning advances and evening delays produced by light and physical activity makes sense—light and activity are often correlated, so if they told really different circadian stories, it would be pretty strange. The authors of the exercise PRC research note that the phase shifting strength of exercise is comparable to that of light exposure, making exercise another great tool for your circadian management tool belt.

Phase-response curve showing Dim Light Melatonin Onset
Phase-response curve showing Dim-Light Melatonin Onset (DLMO) determined circadian time of melatonin supplement vs. phase shift. Participants were held on an 3.5 hour ultradian light-dark cycle (1.5 hours dark/sleep period followed by 2 hours light without sleeping) with administration of 3.0 mg of melatonin following the dark/sleep period. 

Oral melatonin supplements have also been shown to shift central clock outputs like dim light melatonin onset. In general, melatonin dosing tends to do the opposite of what light exposure would do at the same time—delaying you when you’d be advanced by light, or advancing you when you’d be delayed by light. While most people don’t think of melatonin as something that can be mistimed, the phase-response curve tells us that melatonin at the wrong time might have no phase shift whatsoever, or could delay you when you’d really prefer to be advanced (e.g. if you want to fall asleep faster at night). 

Meal Timing Study, factors controlled by light.
A visual depiction of results from a meal timing study showing which factors were controlled by light (and the SCN, purple, top), and which were controlled by meal timing (green, bottom). Larger arrows mean more significant control by that input.

Eating patterns are interesting. When you keep light exposure patterns fixed and change the timing of meals, the vast majority of relevant outputs—melatonin, cortisol,  hunger, triglycerides, and genes like PER3 and BMAL1—stick with the patterns set by light (in other words, they follow the SCN). A few of the players track with meal timing, however: glucose, and, to a lesser extent, PER2 and insulin patterns, showed significant phase shifts in response to the food time changing.

This means that misaligning your meal timing with your light timing could result in a kind of desynchrony, in which your light and your food are sending two different time-keeping signals, and in turn throwing your metabolic processes out of whack. This desynchrony—or, specifically, avoiding this desynchrony—could be the reason why time-restricted eating (TRE) has been found to improve cardiometabolic health. 

So light’s not the whole story. On the one hand, that means there are more circadian-relevant factors to have to worry about; on the other, that means we have more knobs to turn as we try to help people achieve optimal circadian health. Can’t get bright light? Why not try exercise instead? Need more than just a light dose? Here’s the right time for melatonin, for you. At Arcascope, we want to use all the tools available to us to help you optimize your external and internal cues, in search of synchronous bliss.

This blog post was written by Carrie Fulton, one of Arcascope’s interns. Thanks, Carrie, for your hard work!

References

  • St Hilaire MA, Klerman EB, Khalsa SB, Wright KP Jr, Czeisler CA, Kronauer RE. Addition of a non-photic component to a light-based mathematical model of the human circadian pacemaker. J Theor Biol. 2007 Aug 21;247(4):583-99. doi: 10.1016/j.jtbi.2007.04.001. Epub 2007 Apr 4. PMID: 17531270; PMCID: PMC3123888.
  • Celine Vetter, Frank A.J.L. Scheer, Circadian Biology: Uncoupling Human Body Clocks by Food Timing, Current Biology, Volume 27, Issue 13, 2017, Pages R656-R658, ISSN 0960-9822, https://doi.org/10.1016/j.cub.2017.05.057. (https://www.sciencedirect.com/science/article/pii/S0960982217306231)
  • Youngstedt SD, Elliott JA, Kripke DF. Human circadian phase-response curves for exercise. J Physiol. 2019 Apr;597(8):2253-2268. doi: 10.1113/JP276943. Epub 2019 Mar 18. PMID: 30784068; PMCID: PMC6462487.
  • Burgess HJ, Revell VL, Molina TA, Eastman CI. Human phase response curves to three days of daily melatonin: 0.5 mg versus 3.0 mg. J Clin Endocrinol Metab. 2010 Jul;95(7):3325-31. doi: 10.1210/jc.2009-2590. Epub 2010 Apr 21. PMID: 20410229; PMCID: PMC2928909.
  • Burgess HJ, Revell VL, Eastman CI. A three pulse phase response curve to three milligrams of melatonin in humans. J Physiol. 2008 Jan 15;586(2):639-47. doi: 10.1113/jphysiol.2007.143180. Epub 2007 Nov 15. Erratum in: J Physiol. 2008 Mar 15;586(6):1777. PMID: 18006583; PMCID: PMC2375577.
  • Mohawk JA, Green CB, Takahashi JS. Central and peripheral circadian clocks in mammals. Annual review of neuroscience. 2012;35:445. doi:10.1146/annurev-neuro-060909-153128.
  • Wehrens SM, Christou S, Isherwood C, Middleton B, Gibbs MA, Archer SN, Skene DJ, Johnston JD. Meal timing regulates the human circadian system. Current Biology. 2017 Jun 19;27(12):1768-75.
  • St Hilaire, M.A., Gooley, J.J., Khalsa, S.B.S., Kronauer, R.E., Czeisler, C.A. and Lockley, S.W. (2012), Human phase response curve to a 1 h pulse of bright white light. The Journal of Physiology, 590: 3035-3045. https://doi.org/10.1113/jphysiol.2012.227892
  • Kripke, D.F., Elliott, J.A., Youngstedt, S.D. et al. Circadian phase response curves to light in older and young women and men. J Circad Rhythms 5, 4 (2007). https://doi.org/10.1186/1740-3391-5-4

Categories
Circadian science Lighting Sleeping troubles

Light at night is bad, people.

Imagine you’re on a swing on a playground with a friend standing behind you.

This friend is not a jerk, so they’re going to push you when a normal person would push you on a swing—right when you’re at the end of your backwards motion and ready to start moving forward again. They, like a normal person, are going to stay out of your way the rest of the time.

If they push you a little bit late or a little bit early, no big deal. If they push you way early, or way late—like, for instance, when you’re still very much in the middle of swinging backwards— that’s a different story. Imagine having your blissful, carefree swing interrupted by smacking into someone standing right in your path. Imagine them actively pushing you back in the direction you came from, right when you least want to be pushed that way.

That person is light exposure. The case where they give you a boost, speed you up, get you to a bigger swing: that’s light exposure during the day. The case where they slow you down, get in your way, reduce the size of your swing: that’s light at night.

And okay, okay: it’s never that simple. “Day” and “night” mean different things based on your body clock’s current time. Light exposure somewhat slowing down your body’s clock is part of a normal day. The same light can affect different people in different ways.

That said, I love this analogy because it captures something I think about a lot in the context of circadian rhythms. The secret of a good swing is having a clear difference between your forward and backwards motion. Shoves are good in the forward direction, and not-so-good in the other direction. Similarly, more and more it seems to me that the secret to healthy circadian rhythms is having a clear difference between the active and inactive parts of your day. When your body wants light, get lots and lots of light. When it’s time for dark, get the darkest dark you can.

I’m thinking about this today because I just read “Light at night in older age is associated with obesity, diabetes, and hypertension” by Kim et al. in the journal SLEEP. Using a dataset of 552 community-dwelling adults aged 63-84, they looked at how exposure to light at night is associated with cardiovascular disease risk factors. They found significant associations between light at night and obesity, diabetes, and hypertension, but no such associations between average light over the course of the 24-hour day and those same risk factors.

In other words, the fact that it’s at night matters. When you get light at night, you muddle the difference between night and day. You lose that good swing.

Caption: Light (left) and activity (right) in the light at night (LAN, yellow), and no light at night (No-LAN, purple) groups. The LAN group has more light at night as well as lower activity during the day, and less of a difference between day and night.

This paper isn’t the only paper to look at light at night—many others exist—and I don’t want to get in trouble for confusing correlation and causation (the above are correlations only). That said, there are plenty of possible mechanisms by which light could increase your risk of cardiovascular disease.

One is by throwing off your internal clock, so your metabolic machinery is not firing on all cylinders, or is rising and falling in a way that’s mistimed relative to when you’re eating.

Another is by suppressing production of melatonin, which your body produces naturally once a day, but won’t produce if it thinks it’s still light outside. Melatonin has a number of properties that are important for circulatory and metabolic health, so it makes sense that having less of it around may not be the greatest thing for you.

But at the highest level, I find real value in thinking about that swing analogy. There are some things in our body that are meant to be dynamic and strongly rhythmic: breathing, walking, heart rate. Our 24-hour rhythms are no different. Get that good swing by getting lots of light during the day. Keep that good swing by turning them all off at night.

Thanks to the authors of Kim et al. for their great work! We really enjoyed the read.

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Circadian science Sleep Meme Review Sleeping troubles

We Rate Sleep Memes, Pt. 2

Back by popular demand, here is part two of the “We Rate Sleep Memes” blog series. If you’re just now tuning in (check out part one here), the title pretty much says it all. We take sleep-related memes we find online, we use them as an excuse to talk about sleep and circadian science, and we rate ’em. Let’s kick it off with:

Meme #4

Here’s the annoying thing: Resting is good. I’m not about to be out here telling you not to rest. In fact, a study that looked at people who regularly sleep less than five hours a night during the work week found that weekend catch-up sleep might help compensate for the bad effects of not sleeping during the week.

What’s annoying is that sleeping in on the weekend can cause its own problems. Basically, you end up jet lagging yourself without actually going anywhere. This “social jet lag” can mess with your mood, grades, metabolism, and lots of other important things.

So the real answer is probably to do everything you can to get to the point where you don’t need to recover sleep on the weekend. Keep to the same schedule, every day. Yeah, yeah, I know—easier said than done, especially with work and life commitments. But if you can make your sleep life more regular, expect it to make a lot of other things in your life better too.

Originality: ⅘ Nice callout to the weekend

Overall quality: ⅘ Dog very cute


Meme #5

Heads up: If you’re doing this, and doing it a lot, you miiight be building up an association between “being in bed” and “not going to sleep.”

Don’t get me wrong, creating imaginary situations that will probably never happen before bed is a time-honored tradition. As far as I know, pretty much everyone does it. Boromir, son of Denethor, isn’t incorrect here.

But it’s a problem if your brain’s whirring so much on your pillow that you start to think of bed more as “the place where I am stressed out about imaginary scenarios” than “the place where I sleep.” That association can make it harder and harder to actually fall asleep when you want to.

If this sounds like you: Get out of bed. Have your imaginary scenario thoughts in a nice chair in the living room somewhere. Keep the lights dim or off altogether as you do it. Wait to get into bed until you’re just about falling over yourself with sleepiness.

Then, when you look like this:

…land right on into bed.

Originality: 2/5 Yes, yes. We all know about Night Thoughts.

Overall quality: ⅘ Boromir very tragic and noble.


Meme #6

Facts o’clock: Your body’s internal clock sends different signals for sleep at different times of the day. These different signals mean you’ll sleep for different lengths of time depending on when you fall asleep.

And since your body’s clock is always updating and adjusting itself, it probably won’t send the same signal at the exact same time every day. This can make it hard to pick up a pattern in why you’re sleeping four hours one nap, and 20 minutes the next.

Another thing that can make it hard to find a pattern? How long you sleep also depends on how much you’ve been awake and asleep recently, on top of the signal from your body’s internal clock. So there are a lot of moving pieces, which can make it seem like you’re playing “nap roulette”, when it’s really “nap you could do a better job of predicting the duration of if you were keeping close tabs on when you’ve been sleeping and the time your body’s clock currently thinks it is.”

*green goblin voice* Listen here, Spiderman: biology is complicated and can seem random, but it might not be as random as you think

Originality: 3.5/5. Definitely been done before as a topic, but “nap roulette” has strong brand energies.

Overall quality: ⅘. Nice meme. Now if you’ll please excuse me, I have a picture I need to sell to J. Jonah Jameson.

Categories
Circadian science Lighting Shift Work Sleeping troubles

What do shift workers do & what might they do?

So you want to help shift workers feel better—sleep better, be safer, have fewer of the long term chronic health problems that go hand in hand with shift work. How do you do it? Where do you start?

As some of the most circadian-wrecked people around, shift workers have been the topic of no small amount of research. Yet one incontrovertible, “best” strategy has failed to emerge for what shift workers should do. There are plenty of reasons for this, but the short answer is: it’s complicated! There are a lot of possible shift schedules a person can be on, and a lot of variation from person to person in how those shifts will affect them. In this blog post, I’ll try to chip away at the complexity a bit by covering what’s currently known about strategies for shift work, and what shift workers might do in the future.

Rather conveniently, a lot of the ways you try to help shift workers can be framed as a choice between two alternatives. So let’s start with one of the biggest “versus” there is out there.

Homeostat vs Circadian Interventions

There are two main forces that conspire to make a person feel sleepy. One is your sleep hunger, or sleep homeostat—basically, a build up of “need for sleep” that accrues when you’re awake, and drains when you’re asleep.

Figure 1 from Bailey et al., 2018. The two process model has the sleep homeostatic (red line) getting bigger while you’re awake, and going down when you’re asleep. The entrance and exit to and from sleep is regulated by the body clock (black curves). Waking up in the middle of the night? It might be because your circadian clock isn’t quite aligned to where you want it to be.

The other is your body’s circadian clock, which sends an extra strong signal once a day to tell you to go to sleep. These aren’t the only things that make a person sleepy, but they explain a lot of the phenomena we see in shift work contexts. This way of thinking about sleepiness (homeostat plus circadian) is called the “two process model of sleep.”

You could classify the strategies around helping shift workers into two camps, based on which of these two forces—homeostat or circadian— they’re primarily targeting. If you want to have a low sleep homeostat going into the night shift, for instance, you probably want to sleep as close to before your shift as you can. So you might try staying up until 1:00 pm on the day after your shift, building up a ton of sleep pressure, then falling asleep for most of the afternoon and evening, waking up right when it’s time for work. Naps and caffeine would also fall under the header of “mostly targeting the sleep homeostat.”

Targeting the circadian clock, however, means moving your rhythms to promote sleep at a time you actually can sleep. This means phase shifting your clock, which can be achieved by doing the kinds of activities that matter to the clock (getting light exposure, avoiding light, exercise, etc.) at the right times.

These methods aren’t mutually exclusive by default, but they can be in conflict at times. A lot of what decides that is the direction you choose to move your clock in.

Advancing vs delaying the clock
Two directions for shifting the clock, from Burgess et al. 2002. Large white rectangles correspond to night shifts. Small white rectangles correspond to 3 hour pulses of bright light aimed at delaying (left) or advancing (right) the clock. The pulses move in time because the person’s circadian clock is shifting in response to the signals from the previous days.

A totally day-adjusted person will probably have their peak fatigue hours occur sometime in the early morning; say, 3:00 am. If they go on a night shift, those peak fatigue hours are happening right in the middle of work hours. (Not exactly ideal). So you could shift their rhythms so that their worst hours no longer happen at 3:00 am.

Way #1 to Achieve This: Shift them later, or delay their clock. Move it so they’re feeling the biggest circadian drive to sleep at, say, 9:30 am, after they’re home from work.

Way #2: Shift them earlier, or advance their clock. Move it so they’re feeling the biggest circadian drive to sleep at, say, 5:00 pm, or before they go to work.

Way #1, or delaying the clock, is often called “compromise phase position.” The idea is that it’s a compromise for the night shift life—you’re not totally shifting to a nocturnal schedule, but you are getting the time of day when your clock maximally promotes sleep to be outside your work hours. You can do this by blasting yourself with light in phase delay portion of your body’s daily rhythms, which for a person who’s still pretty adjusted to the day schedule is going to be in the afternoon and evening. Note that this is where we start to conflict a bit with the homeostat-targeting interventions: If you’re keeping yourself in a super bright environment in the hours before your shift, you’re probably not sleeping the whole time you’re at it.

Figure 4 from Burgess et al. 2002. This schedule is a compromise between work days and off days where the schedule on off days is still very late. Shaded rectangles are sleep windows, the large white rectangles represent five night shifts (11pm – 7am), and the L symbols are recommended times for light exposure. The black triangle markers show the timing of the core body temperature minimum, or the point at which your body sends its strongest signal for sleep.

Way #2, or advancing the clock, does not come with the same homeostat conflict. To advance the clock, a person still relatively well-adjusted to a day schedule would want to avoid evening/afternoon light and get tons of it in the morning. A “sleep after 1:00 pm” intervention in which people were also dosed with bright light in the latter part of their shift saw a 3 hour shift in the timing of the circadian rhythm biomarker, dim light melatonin onset (DLMO). In other words, you can target the homeostat right before a shift and promote an earlier phase shift at the same time.

Figure 1B from Chinoy et al. Black bars are scheduled sleep periods, gray bars are periods of dim light exposure, shaded bars are work shift hours, and white rectangles are periods of enhanced light during the night shift. By sticking the brightest light in the phase advance period for these people, their clocks should shift to better adjust them to sleeping from 1:00 pm to ~8 or 9:00 pm.

There’s evidence that both strategies can improve upon a baseline of undirected, “do what you want” advice to shift workers. Advancing the clock plays nicely with “sleep before shift” strategies, but you could also take a pre-shift nap, while mostly delaying yourself in the lead up to it. You could also try splitting your sleep—sleeping right after your shift, and then again right beforehand, and using your non-sleep time to steer your clock in one direction or another (though depending on what your personal time zone is, this may be a bit difficult—those hours might be times when you’re more or less insensitive to light).

So how do we begin to choose a strategy to recommend? Well, there’s one missing dimension to all the research touched on so far that we haven’t discussed yet.

Non-shift workers vs. shift workers

All of the shift working studies cited above looked at non-shift workers who were brought into the lab and put on simulated shift work protocols. Typically, being a shift worker was an exclusion criteria for the study: No real shift workers allowed.

There’s a very good reason for this, which is that shift workers have wonky circadian rhythms. You bring shift workers into a lab and look at their dim light melatonin onset timings, and you can see coverage over almost all the 24-hour clock. This means that you wouldn’t expect a nice clean scientific result to come out of putting them all on the same schedule: What’s good for someone would almost certainly be terrible for another. Focusing only on non-night shift workers (who are, it should be said, a good model for “just starting out on the night shift workers”) means you’re able to better parse a signal from the noise.

But it also means that you miss out on a very important piece of information: Namely, that only a tiny fraction of shift workers phase advance themselves in the real world. Many of them don’t follow particularly great strategies, but the ones who are better adapted tend to be very delayed.

From Gamble et al. 2011. Gray shaded areas are nigh shift work schedules, red shaded areas are typical sleep times. From a large number of responses, the Night Stay, Nap Proxy, No Sleep, Switch Sleeper, and Incomplete Shifter strategies were identified as categories. No Sleep and Nap Proxy shift workers tended to be worse adapted, while Incomplete Shifters and Switch Sleepers were better adapted.

This result comes from work in night shift nurses that looked at the different strategies that real nurses employ. In that research, the “most adapted” nurses were the ones who basically did this compromise phase position strategy, where they were very late types on their off days. Nurses who stayed up all night before a shift or napped during the day on their off days tended to be worse adapted— worse mood, increased cardiovascular risk, you name it. Counterintuitively, the least adapted nurses also tended to be older and more experienced on the job.

When you step back and think about shift work in a vacuum, the truly best strategy from a health perspective would probably be for shift workers to shift their lives entirely to align with night work, sleeping during the day even on the days they have off. In that sense, it would be like living in the United States but pretending you worked the same hours as a person living in Tokyo. With good enough blackout curtains and strong enough willpower to ignore the FOMO of diurnal life, you truly could fully adapt to a night-living lifestyle.

A tiny fraction of real shift workers do this. But most don’t, and the vast majority want to sleep at night during the days they’re not working. The better adapted nurses in the Vanderbilt study achieved this by being pretty extreme night owls on their days off. The poorly adapted nurses, the older ones who tended to stay up all night or nap on off days— they might be the ones to benefit most from a phase advancing schedule, which appears to have worse discoverability (nobody really does it in the real world) than the delaying schedules. In other words, if one direction isn’t working—as it appears not to for the ill-adapted shift workers—try going the other way.

Time now for my caveat that this is all, once again, pretty complicated. You can be an extreme night owl on your off days right up until the moment you have to work a 7am to 7pm shift. Your actions during your off days and off hours are constantly shifting your circadian profile, so that the thing that works for you one week might not work for you the next week. None of these studies could look at DLMO changing day-by-day in the real world, because none of them had the ability to track DLMO cheaply and in real-time. What do you want to prioritize—safety on the commute? Safety during shift? Ability to sleep well and feel good? Putting one of these above the other can give you a different answer. It’s a lot.

Enough already! What should I do?

Listen, if there was a one-size-fits-all easy solution to all of this, we wouldn’t have made an app for it. I would just have emailed everyone this blog post and done that thing where you brush your hands together in the international sign of “all done here.”

Here’s one rule-of-thumb, though: If you’re adjusted to a day schedule, and you’ve got a one-off night shift tonight before going back to the day schedule, you’re not going to be able to meaningfully shift your body’s circadian clock in the next 8 hours. You’re going to want to bank as much sleep as you can in the hours leading up to it and be aware of when your peak fatigue hours are going to occur. Our app can help you with that.

For everyone else, this is where our app comes in. Shift builds on this history of research to design plans unique to your body clock. You can choose which ones to try, and give feedback on the ones you like and don’t like. Want to help us move the needle on getting shift workers to a healthier place? Reach out for early access.

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Categories
Circadian science Sleeping troubles Technology

One Simple* Rule for Understanding Your Sleep

My friend sent this to me the other day:

“If I go to bed at 10:30 pm, I often wake up at 4:30 or 5:30 am and can’t go back to sleep. But if I go to bed at 1:00 am, I fall asleep easily and can sleep until 9:00 am. Throughout the work week I’m running on 5-6 hours of sleep, and only on the weekend do I get to recover. Could it be that my natural cycle is just different from what social norms say?”

Reader, I pushed my glasses up my nose so fast and so hard that they are now deeply embedded into my face. A small price to pay for this gift of a question.

But before I can get into why I was so excited about my friend’s pain and suffering, I need to tell you about the two-process model of sleep.

The Two-Process Model, 10,000-ft-View Edition.

When you’re awake, you build up a “hunger” for sleep. This is your sleep homeostat, and it grows when you’re awake and drains when you’re asleep.

Two days of wake and sleep. Purple line goes up, you’re awake. Purple line goes down, you’re asleep. Shaded purple regions show sleep windows.

At the same time, you’ve got a circadian drive for alertness, which for day-adjusted folks tends to send its strongest signal for sleep in the middle of the night. The effect of both of these together is your sleep drive.

When the sleep drive hits an upper threshold, your body switches into “wants sleep” mode and tries to get you to go to bed. When it drops below a threshold, your body switches into “wants wake” mode and tries to get you to wake up. You can visualize this phenomenon as the zig-zag of the homeostat bopping between time-varying upper and lower thresholds set by the circadian clock. Hit the upper limit, fall asleep. Hit the lower limit, wake up.

Visualizing the two processes—homeostatic (purple) and circadian (orange/red).

Still with me? Ok, here’s the most important picture you’ll see in this whole blog post:

Ta-da.

Well, there you have it! My friend’s sleep problems in a nutshell. No further explanation needed.

I showed this graph to another friend of my mine, and he sent back this:

Okay, so maybe a little more explanation needed.

Simple model. Complex phenomena.

Let’s talk about the two-process model in the real world. For one, people don’t instantly pass out when they hit the upper threshold for sleep. People can push sleep back in a lot of ways— staying out of bed, keeping the lights on, or, as was the case in one old sleep study, repeatedly dunking their heads in ice water. We can call this a “wake effort”—making an effort to stay awake in the face of the tyrannical rule of the two-process model. In fact, it might not really feel like that much effort if you’re having fun on the internet.

The blue-dashed line in the plot below shows what would happen if my friend stayed up a couple hours later than her body necessarily wanted to, moving her bedtime from roughly 10:00 pm (no wake effort) to a bit after midnight (with wake effort). The blue shaded region shows her sleep when she delays her bedtime in this way.

Purple line: falling asleep right when the two-process model says to. Blue line: Staying awake a little longer. Two different choices, two different sleep durations (blue vs. purple shaded area.)

Here’s the thing: That blue shaded sleep area, the one that starts a little after hour 24 (midnight) and goes until a little bit after 8:00 am? It’s wider than the shaded purple sleep area, which starts around 10:00 pm (hour 22) and goes until maybe 4:30 am. In other words, she’s sleeping more by staying up later—almost two hours longer.

You can eyeball it if you look where the lines are hitting the red waveform on the bottom. The purple and blue curves are chasing it as it’s going down, and the “stay up later” blue curve is hitting it at a significantly later point. That means more sleep overall (“good,” in theory), despite a later bedtime (“bad,” in theory).

What does this mean? Well, for starters, it hints at the enormous complexity you can start to get at when you mix two waveforms and thresholding conditions. The wiggly line of circadian sleep drive and the zig-zag line of homeostatic sleep drive, while deceptively simple, can interact to generate some wild phenomena.

Let’s explore a bit. I’m starting my friend’s sleep homeostat at a fairly high value, because she told me she tends to need an alarm to wake up during the work week (and feels pretty tired all of the time). If I started her at a lower sleep homeostat (waking up better rested), the difference between the two sleep durations becomes a lot smaller (purple: 7.7 hours of sleep, blue: 7.9 hours).

Starting my friend at a low homeostatic value. Now there’s really not a difference in sleep duration between her staying up a few extra hours or not—both the purple and blue shaded regions have the same duration.

Give her a higher starting homeostat, on the other hand, and the no-resistance-to-sleep-drive curve (purple) has her falling asleep around 7:30pm, waking up at midnight, and staying awake all night until she passes out again in the early morning, while the “wake effort” curve in blue mostly stays the same:

Now there’s a huge difference in sleep between the two options! They’re basically complements of each other.

These are wildly different scenarios, and they’re arising from me gently nudging a number up or down a little bit. Imagine what happens if we make those upper and lower thresholds—representing the circadian drive for alertness—act the way a real circadian rhythm does: shifting, stretching, and bending in response to the signals you give it during the day. Imagine weakening the signal from the body’s clock, or adding the effects of noise:

Circadian madness

Mathematicians who study this kind of stuff have done some pretty great work looking at just how much complexity can arise from the simple rules of a sleep homeostat and a circadian rhythm. In my own life, I think of the two process model most often when I wake up in the middle of the night. Ok, I tell myself. My circadian sleep drive is probably running a bit late. If I hang out in the dark a while, it’ll swing in and kick me back into sleep. And what’s great is that it reliably does.

“Are my natural cycles just different from social norms?”

But let’s get back to my friend’s question. If I had to make a guess as to what was going on with her based on the two-process model, I’d say her circadian clock is delayed relative to where she wants it to be (and that she’s often waking up in the morning with a high sleep homeostat due to the chronic sleep restriction from her work.)

That might seem to be a vote for her natural rhythms just not jiving with her work hours, and there’s probably some merit to this. She’s described herself as light sensitive, which may be a trait that predisposes you to being more of a night owl. That could be shifting her rhythms later, and making it so her circadian clock isn’t where it needs to be when her sleep homeostat drops to a low value.

Yet one of the great things about circadian rhythms is that they can change. If they didn’t, we’d never get over jet lag. We’d stay on our starting time zone schedule for all time, regardless of where we went, when we slept, or when we went outside. The fact that our clocks can be disrupted also means they can be fixed.

Another way of thinking about it: My friend’s weekend rhythms would make her a pretty darn early bird if we abruptly transplanted her three time zones west to California. What’s the difference between living in California versus her home city on the East Coast? Well, a lot of things, but the most important one for her circadian clock is the difference in her light exposure.

So I’ve got her trying out a new circadian regimen to help herself sleep during the work week. My hope is that the two-process model, along with the fancier flavors of it that have spun up in research and at our company, can help people in the real world understand their sleep better. Simple rules can interact to make complicated behaviors happen, but at the end of the day they’re still simple rules. And simple rules might just have straightforward solutions, too.