Here we see Squidward staying up and reading instead of going to sleep. Squidward himself might smugly point out that he’s reading a book, not looking at a light-emitting screen, and use that as an excuse to feel superior. If so, he would be tragically mistaken. There are clearly lights on in the room that he’s using to read while staying up late, and that light will have an effect on his clock much in the same way light from a screen would. After all, most homes are bright enough in the evening to significantly mess up sleep-related processes, like melatonin production. Though Squidward would never accept it, his efforts to prevent circadian disruption pale in comparison to those of his neighbor Patrick, who blocks light by being a starfish who lives under a rock.
Originality: 3 out of 5. Not being able to put your phone down is a classic meme topic.
Quality: 4 out of 5. Slight cognitive dissonance caused by the “scrolling through social media” text coupled with the image of him reading a book, but it gave us a chance to talk about how room light exposure matters for circadian rhythms, which is what we’re all here for.
Let us start by noting that Homer’s perception of his sleep here may be skewed: many people with insomnia overestimate how long it takes them to fall asleep, and underestimate how much sleep they actually get. It may be that a more accurate version of this meme would be “me all night vs me four hours before my alarm goes off”– which is still, to be perfectly clear, a miserable experience. It’s miserable even if you’re objectively getting more than 6.5 hours of sleep per night but perceiving that you’re not sleeping much at all (also known as “paradoxical insomnia”). We love targeting sleep improvements through light exposure over here, but if you’re relating hard to this meme, you’ll probably want to get yourself some cognitive behavioral therapy.
Originality: 3/5. This, too, is a pretty typical sleep meme topic.
A vigilance lapse means that something popped up on a screen in front of you for half a second and you didn’t even register it. This is bad if you are, for instance, driving a car.
People on four hours of sleep a night also fail to get better at subtraction and addition tasks, despite days of practice (see: circles staying flat in the below):
And yes, caffeine can counteract “getting worse and worse at things” to an extent, but so can naps. As the authors of a recent review on fatigue and caffeine write,“It is important for caffeine consumers to understand that caffeine at any dose is not a chemical substitute for adequate healthy sleep.”
Originality: 4.5/5. Nice shout out to shift workers.
Quality: 2.5/5. Inscrutable indenting decisions. Objectively bad sleep practice.
Here’s a fun fact: You probably get way less light exposure during a normal work day than you would if you were out camping.
“Sure,” you say. “That’s no surprise. At home, I have walls around me to block the sun. If I’m camping, I presumably have fewer walls.”
“You don’t understand,” I say, leaning in. “You get way, way less light exposure.”
I’m basing this off a famous circadian experiment from Ken Wright’s group at the University of Colorado Boulder, in which they compared the light people get in modern electrical lighting environments with the natural light they get while camping.
It’s not a 1x or 2x difference when you go from modern light exposure to camping light exposure. It’s a 13x difference:
Thirteen times more light exposure during the day! And this in the winter! It’s nuts.
Imagine turning your current daily light exposure down by a factor of 13, or to 8% of its current brightness. Two things would probably be true. First, it would be hard to see, so you’d bump into things. Second, and most important from a circadian perspective, it would be hard for your brain to tell the difference between day and night.
After all, the signal telling your brain that it’s day (light) would be just a tiny fraction of what it was before. It’d be like turning a faucet down to just a thin drizzle. You can tell it’s on if you look for it, but it’s an easy thing to miss.
In a sense, we’ve already done this with the shift from natural lighting to indoor, artificial lighting. We’ve given up the firehose of light that is sunlight exposure in favor of a much muted signal from our indoor lights and devices.
If you look at the lighting figure above from Stothard et al., it’s ridiculously easy to see where day starts and stops in the camping conditions (black line). But the picture is muddled for modern electric light (gray line): Day seems to start fairly clearly, but where does it end? There’s this blunted peak in light exposure during the day, and a long, ambiguous tail of light exposure stretching out into the night hours. There’s not really a clear day/night divide.
This matters for our health. There’s a notion in circadian rhythms science of your circadian “amplitude.” Roughly, you can think of amplitude as a measure of how confident your body’s clock is about the time it thinks it is. Give your circadian clock a clear day/night signal, and this will boost the amplitude. Keep it on a constantly changing, dim-light-round-the-clock kind of schedule and the amplitude goes down.
In other blog posts, I’ve talked about your phase response curve, which tells you which direction (earlier, later) light will push you when you get exposed to it. But you can also think of the amplitude response curve, which tells you whether your amplitude will go up or down if you get light at a certain time. Generally speaking, the amplitude response curves in our models tell you to go outside right smack in the middle of the day if you want to boost your amplitude as efficiently as possible.
So this Thanksgiving, get some outdoor light. Sure, yeah, get some exercise while you’re out there if you want. But simply being outside and in the light is a good thing: It’s building stronger, more robust rhythms in your brain. And if you happen to fall asleep hard after eating a big meal– well, part of it might be that your circadian clock’s a little more confident that day is over and it’s time to snooze.
For almost a century and a half, it was thought that the mammalian retina had just two types of photoreceptors: rods and cones. That assumption was not proven to be false until studies in the late 1990s proved the existence of a third kind of mammalian photoreceptor that differed greatly from rods and cones. These new types of photoreceptors were retinal ganglion cells (RGCs) that were intrinsically photosensitive (ipRGCs)— or in other words, naturally sensitive to light.
Though the official evidence to determine that ipRGCs actually existed did not come until much later, this third class of photoreceptor had already been hypothesized in 1927, nearly seven decades earlier, by a graduate student named Clyde Keeler. During one of his studies, he examined the behavior of mice that lacked nearly all rod and cone function as a result of severe retinal degradation, which left them functionally blind. Keeler noticed that despite the lack of rods and cones, the mice still had a very strong and significant pupillary constriction in response to light, and he determined that this response must have been the result of some third photoreceptor in the retina. The lack of concrete evidence for a whole new photoreceptor at the time resulted in this pupillary response being explained away by other scientists. However, in 1999, Russell Foster and his team would revisit Keeler’s work armed with a new host of tools.
Foster et al. worked with mice, much like Keeler did, but in their case, the mice being observed were genetically engineered to not have any rods or cones. Yet regardless of their missing rods and cones, the rats still displayed strong pupillary light reflexes and were even able to shift their circadian rhythms with shifting light exposure schedules. With these studies complete, the presence of a third photoreceptor was almost confirmed, but some still weren’t convinced because nobody had found another light-sensitive molecule (opsin) in the mammalian retina yet.
The discovery of melanopsin in the photosensitive skin cells of frogs occurred in 1998, and in the following four years studies determined that the very same opsin was being expressed in a small percent of RGCs in both mouse and human retinas. This discovery allowed scientists to easily mark ipRGCs and confirm their existence, which finally put to rest the debate of whether or not there was a third class of photoreceptor.
So they exist, but what do they do?
IpRGCs differ greatly from rods and cones when it comes to how they work. Their main function in the body is to signal the intensity of ambient light levels (irradiance) to the brain. These signals are largely used for non-image-forming visual reflexes that are subconscious, such as pupillary constriction, neuroendocrine regulation, and synchronizing daily circadian physiological rhythms to environmental light. This means that the way ipRGCs respond to light by themselves is also quite different from rods and cones.
As mentioned before, these photoreceptors use melanopsin as their photopigment. and that makes them more responsive to light at around 480nm (blue light). In the graph below, you can see that this wavelength is significantly different from the best wavelengths for stimulating rods and cones (panel b).
Although ipRGCs function as photoreceptors themselves, it was found that they additionally receive synaptic input from the circuits of rods and cones. This means that ipRGCs have both an intrinsic light response coming from melanopsin and an extrinsic one that is mediated by synaptic input from rods and cones. The light response caused by melanopsin is markedly different from that of rods and cones: ipRGCs have both an intrinsic and sluggish light response as well as an extrinsic, rod/cone driven, rapid photoresponse. There is an ongoing debate about the relative significance of this extrinsic synaptic input and the role rods and cones play in determining our circadian rhythms.
A recent case study:
In a recent research article by Mouland et al., their team assessed whether the effective light intensity registered by melanopsin (blue light ~480nm) was a more important determinant of circadian impacts than that of cones under realistic contrast scenarios. The ability to determine melanopsin’s contribution to circadian light responses comes from the evolution of a color science technique which is referred to with multiple names, such as receptor silent substitution or metamerism in colorimetry. Metamerism occurs when two colors appear to match under a specific lighting condition but have different underlying spectra.
This technique allows for the stimulation of specific photoreceptor classes, like ipRGCs. Mouland and colleagues quantified the circadian impacts of different photoreceptors by recording electrophysiological activity from the suprachiasmatic nucleus (SCN) of anaesthetised mice while they were presented with movies. The movies were either high or low contrast and had varying irradiances specialized for the distinct photoreceptor classes.
During the experiment, the energy response recorded from the SCN closely tracked with melanopsin-driven signaling across all conditions. In general, steps in melanopic irradiance were determined to be the most significant factor accounting for light-induced changes in SCN activity. The only cone-directed lighting patterns with significant impacts on SCN activity were low contrast movie conditions. Basically, this study suggests that cones do have an impact on the circadian signal going to the SCN in some conditions, but the influence of melanopsin on the circadian signal is far more consistent.
This blog post was written by Arcascope’s intern, Ali Abdalla. Thanks, Ali!
This post used Webvision as a major resource. Thanks to Dustin Graham and Kwoon Wong for the excellent review.
I recently got some blackout curtains for my bedroom. This was pretty long overdue: about thirty feet from my bedroom window is a cheerfully bright, energy-efficient street lamp, which—while great when I’m taking the dog out for a nighttime stroll—is the photic equivalent of somebody standing in my azaleas and playing “Seventy-Six Trombones” while I’m trying to sleep.
I’ve definitely started sleeping longer since I’ve gotten them. But I’ve also noticed that they’ve made it so I need to be even more careful about my other sources of light at night. The reason? They don’t just block my light at night. They also block light in the morning.
I’m thinking about this because it’s almost the end of daylight savings time, and, once again, there’s talk of making it permanent. As a quick guide: Daylight savings time (DST) is the one where the clocks move forward (so it’s lighter at night), while standard time is the one where the clock moves back (darker at night). The “Sunshine Protection Act”, introduced by Florida senator Marco Rubio, encourages states to observe a permanent version of DST, with the argument being that lots of good things could come out of just chilling it with the time change.
Permanent daylight savings time means not having to change the clocks, and not having to experience that gnarly “lose an hour” in the Spring. It means no confusion about how many hours offset we are from the time in the U.K. and no struggling to remember if you should say EDT or EST when you’re trying to coordinate a Zoom meeting across time zones. As a programmer, I’m generally in favor of anything that makes the totally miserable experience of interacting with dates and times in code even marginally easier.
But it also means—and I’m talking about permanent daylight savings time here—lots and lots of dark in the mornings.
This is bad. It’s bad because light at night is fundamentally different from light in the mornings, because our bodies are fundamentally different at night than they are in the mornings.
Light in the morning does a couple things, but one of the most important ones is that it “advances” our circadian rhythms. It tells our internal clock that night is over and it’s time to get a move on. It makes it easier to fall asleep at night.
And if you get a lot of light in the morning, it eventually advances you to the point where… it stops advancing you. You enter the part of your daily rhythm where light delays your clock. A sort-of “slow down, what’s the rush” period of your internal rhythm that starts in the mid-afternoon for most people and continues into the early morning.
And that slowdown period is the problem. Because while light in the morning is hitting you in the advance region, which you eventually get advanced out of, light at night is hitting you in the delay region, which is like a temporal sand trap. When you get light exposure in the delay region, your clock gets slowed down, which means you spend more time in the delay region. Which means you don’t feel tired as quickly, which means you get more light, which means you spend even more time in the delay region. It’s a feedback loop that spins out of control. It might be the reason that night owls exist.
So if we adopt permanent DST, we’re adopting a schedule where we get more light during the hours most people call night, and much less light in the hours we consider morning. We’re setting ourselves up to fall into the delay region sand trap: More light in the night, making us stay up later and get delayed, and far less light in the AM hours to counteract it.
This is what tanked permanent DST the first time we tried it. I’m not sure why this doesn’t always get brought up as the very first point against permanent DST, but we’ve totally done it before. In 1973, anywhere from 57-73% of people supported staying on DST during the winter. So they did it, in January of 1974. By the time February and March rolled around, only 19-30% of people still thought it was a good idea, while 43% said it was actively bad.
What changed? People experienced what happens to your body when you have to kick off your day in the dark of night. They drove to work and caught the bus to school, while the sun waited to rise until 8:00 am. They didn’t like it, and rolled the decision back before the next winter came around.
You might say, “well, time is a fake idea. Who says you have to start your day before 8:00 am?” This is a fair point. We could, societally, shift the normal times we do things to match whatever schedule we wanted. In China, where the entire country is on the same time zone, places like Kashgar (in the far west) have shifted their normal operating hours to reflect the fact that the sun might not come up until 10 am.
But it’s a lot tougher to change social standards of when school and work “should” start in every town in the country than it is to pass a bill changing the time that appears on your phone. Which is why we shouldn’t do it: Permanent DST will put us on schedule where our traditional social standards for when things should happen are at odds with our biology, sabotaging our sleep and circadian health.
If we want to stop the whiplash of changing the clocks twice a year, why not do permanent standard time? I’m in favor of this. It reduces confusion the same way permanent DST does, but without the corresponding damage to our internal rhythms. Sure, it might mean that 9:00 pm is dark, even in the summer. But darkness at the right times is a healthy thing. And from a safety perspective, there are lots of street lamps and other sources of light at night these days that are very good at their jobs.
Which brings it back to me and my blinds: I’ve needed to be more careful about my other sources of light at night lately because my blackout curtains mean I don’t get woken up by the sun. That’s not a big problem: I can wake up in the dark and yank them open myself, like one of the townsfolk in the first song in Beauty and the Beast.
But if I get too much light at night from non-streetlamp sources, like watching Succession on my computer or looking at Succession memes on my phone, my ability to wake up in the dark in the morning is going to be less reliable, jeopardizing my exposure to that vital morning light. And I’m lucky that there’s even morning light to get: with permanent DST, I could be hopping on my first calls of the day while the sky is still black outside.
My point is that social pressures already make it hard for us to get the darkness we need at night (let’s face it, screens are fun) and the light we need in the morning. We shouldn’t make it harder for ourselves with a change to a system that’s already failed once. Permanent daylight savings time is a no-go. Permanent standard time? Call me.
I love my Apple Watch. The ability to track my exercise, heart rate, activity levels, and sleep has enabled a real awareness of how my physical and mental health changes over time. The ability to track personal health data over long time periods outside of laboratories is one of the most exciting developments of the last decade. I believe this data will usher in a new era of personalized, precision health which just wasn’t possible in the past. At Arcascope, we are at the forefront of developing algorithms to turn the data collected by wearable devices into insights that improve people’s lives.
With that being said, the current state of things just isn’t all that satisfying when you think about what’s being left on the table. So much of the data being collected is uninterpretable. Knowing my current heart rate is cool, but what can I do with that information? The part of this that bothers me the most is that so much of this data is focused on the past.
Here is a screenshot from Apple Health showing my sleep over the last month. You can see that I had some wake periods at 3 am at the beginning of the month. But how does this information really help me?
Sleep tracking in particular reminds me of a weather app that can only tell you yesterday’s weather. Clearly, a weather prediction service that could only tell you the weather from 24 hours ago wouldn’t do well against the Doppler radar. It is useful to be able to say exactly how hot it was yesterday, and interesting to know how that compares to years past, but I really want to know if I should bring an umbrella with me when I leave the house.
I can tell that I didn’t sleep well last night from the fact that I am feeling tired. Having a device to quantify exactly how poorly I slept can be useful for tracking long-term trends, but it isn’t all that useful on a day-to-day basis.
Another snapshot from my Apple Health data. Doesn’t this remind you of a weather app pointing out how this weather’s month compares to historical trends? What about today? Or how about tomorrow?
Okay, enough of the weather prediction analogy. I’ve already pushed that analogy further than I should. First, unlike the weather, we actually have control over our behavior, and what we are doing now will change the forecast for our physiology tomorrow. Also, these variables are much more predictable than weather.
The technology we have developed at Arcascope can answer questions like:
What separates the days where I am at my best, from the ones where I am struggling?
How can I alter my behavior now so that I will sleep better tonight?
When is it best for me to stop drinking coffee for the day?
When will it be best for me to study, exercise, eat and relax?
When is the best time to take my medication to minimize side effects?
When should I avoid high stakes activities because my chances of making a mistake are highest?
We believe this is the future of personalized health tracking. We also think it’s a heck of a lot more exciting than looking back at yesterday.
Back when I was in college—waking up at 4:50 am for crew practice, staying up until 1:00 am working on problem sets, and sleeping in until noon on my days off—I could eat pretty much any food in any amount at any time of the day. And I mean anything, anytime. Think: bootleg s’mores made out of saltine crackers and ice cream toppings. Cooked in the microwave. At 6:45 in the morning.
Something changed in graduate school, when I stopped having wildly irregular sleep schedules and got my circadian act together. I just… didn’t feel hungry after a certain point in the evening. And if I tried to eat something very late in the day—say, a midnight snack— it actively made me feel… kinda bleh.
My philosophy around food has always been to eat when you’re hungry. But once I was on a regular schedule, I stopped feeling the constant, slow-burning hunger I’d had back when I was up at all hours. I still felt hungry, but only at certain times of the day. The rest of the time I wouldn’t really feel hungry at all. You might say that my eating and hunger patterns became more strongly rhythmic.
Enter circadian rhythms. It makes sense to think that our bodies might be more prepared to handle food at some times (like when we’re awake), rather than others (like when we’re supposed to be asleep). And the same way light at night confuses and disrupts the central clock in our brain, so too could food around the clock confuse and disrupt the peripheral oscillators in our organs. Buying all this, how might you avoid this disruption? Eat what you want, but only over a portion of the day.
This is time-restricted eating, or TRE: the idea of keeping all your eating to the same window of time every day. Usually this window is 8-10 hours long. So if you get up and start eating at 8:00 am, you might restrict your food intake to ten hours a day and stop meals after 6:00 pm. Or you might hold off on eating until 11:00 am, in which case you’d wrap up food for the day around 7 – 9:00 pm. You might only do this for five days of the week, or you might do it every day. Regardless, this isn’t about actively trying to cut calories. It’s not about how much you eat, it’s when.
TRE came out of the work of Dr. Satchin Panda, a professor at The Salk Institute and author of The Circadian Code.
Not everyone can manage TRE with their job (night shift workers, for one, often have to fuel whenever they can), and people with underlying health conditions should talk to doctors before giving it a try. But as someone who fell into a time-restricted eating schedule somewhat by accident, I have no plans of quitting anytime soon. I’ve lost a fair amount of weight since my undergraduate days, and it’s not because ever stopped eating s’moretines. I’ve just gotten to the point where my body has a much clearer signal than it used to for when it wants food and when it doesn’t.
So take a look at Dr. Panda’s website, or book, and consider if TRE makes sense to try out. After all, the timing might be right for you.
Sleep regularity is a gauge of how consistent a person’s sleep patterns are, based on the day-to-day variability in their sleep–wake times. In general, having poorer sleep regularity, or irregular sleep patterns, has been shown to lead to many adverse outcomes in metabolism, mental health, and cognitive performance. Low sleep regularity has even been linked to increased inflammation. In order to avoid these and other complications, you want to increase your sleep regularity by aiming to get into bed at the same time every night.
How can we score sleep regularity?
There are at least five different metrics that can be used to quantify sleep regularity, each capturing different aspects of it and useful in its own way. The five measures of sleep regularity that we’ll look at in this blog post are listed below:
Individual Standard Deviation (StDev)
Interdaily Stability (IS)
Social Jet Lag (SJL)
Composite Phase Deviation (CPD)
Sleep Regularity Index (SRI)
Traditionally, the most common overall metrics that have been used to assess sleep regularity are quantified by measuring sleep deviations in sleep patterns from an individual’s average. Examples of overall metrics are StDev and IS, both of which compare sleep from each day to an average sleep–wake pattern, and SJL, a metric that compares two average sleep patterns (workdays and free days).
StDev: lower is more regular / StDev⬇ = Sleep regularity⬆
This score is just the standard deviation of your sleep metric of choice, like sleep onset, sleep offset, or sleep midpoint. The standard deviation captures the variation of a quantity from its mean.
IS: higher is more regular / IS⬆ = Sleep regularity⬆
This metric uses sleep-wake data (can also use rest-activity data) over a period of days to measure the stability of a person’s sleep-wake rhythms. It does this by comparing the pattern of daily sleep activity to the average pattern across many days.
SJL: lower is more regular / SJL⬇ = Sleep regularity⬆
Social jet lag is a metric that quantifies the mismatch in the average mid-sleep timing between workdays and free days. Negative SJL values represent earlier mid-sleep timing on weekends than weekdays while positive values indicate the opposite.
Two newer measures of sleep regularity are CPD and SRI. These two fall under the category of consecutive metrics, which means they measure variability in sleep–wake patterns between consecutive days. The circadian system makes adjustments daily, and consecutive metrics were developed in order to utilize day-to-day information and more accurately predict circadian disruptions associated with poor sleep regularity.
CPD: Lower is more regular / CPD⬇ = Sleep regularity⬆
Composite phase deviation is a metric that was created with shift workers in mind. CPD quantifies circadian disruption where sleep is both irregular (rotating shifts) and mistimed (sleeping in daytime). This metric uses an individual’s chronotype to determine optimal timing of sleep. The chronotype then helps to quantify how “mistimed” they are. The regularity aspect is calculated using the difference between mid-sleep timing from one day to that of the prior day. In order for CPD to be derived it requires data that has one main sleep session per day or some other daily sleep variable, like sleep duration.
SRI: higher is more regular / SRI⬆ = Sleep regularity⬆
The sleep regularity index is a measure based on binary sleep-wake time series. It measures the similarity of a person’s sleep-wake pattern from one day to the next. The scale for this metric ranges from 0 (random) – 100 (perfectly regular) and it represents the percentage probability that an individual will be in the same sleep/wake state at any two time points. It’s important to note that this metric does not account for total sleep time so a person that (hypothetically) sleeps 0% of the time will still be able to get an SRI value of 100.
So I’m regular, that means I’m healthy right?
Well, not quite. Depending on the kind of variability you have in your sleep patterns and the method used to record your sleep, different metrics may tell you very different stories regarding your sleep regularity. Context is very important when making a decision about which sleep regularity metric to use.
Just think about what would happen if you increased the variability in your work week sleep timing, but maintained a consistent average. Your SJL score would stay the same, while your other metrics would likely shift to indicate greater variability. The ordering of days also matters. In Fischer et al. they shuffle days around to show how consecutive metrics can give you different stories on regularity than overall metrics do.
In order to properly assess sleep regularity for yourself or your patients, it is necessary to understand the little things that go into calculating each of these sleep metrics. A variety of unknowns, such as the type of data being gathered or the length of the data set, can cause these metrics to disagree with each other. The good news is that you’ve got lots of options to choose from.
I woke up feeling groggy and lazy almost every day in the past school semester leading up to this summer. I was already doing research on sleep at that point, and I had a general idea about what I was doing wrong, but when I tried to dive deeper into the hundreds of articles on sleep science, I just found myself getting tired and confused. The last few months working as an intern at Arcascope have taught me how to get in sync with my circadian rhythms and taught me of the many subtleties that are involved with supporting your circadian system.
When I started learning about sleep, I found myself asking friends about their habits and experiences with sleep out of curiosity. I quickly realized that almost everyone I talked to had experienced frustrating sleep problems at some point during their lives and had no idea how to deal with them because they were just never taught enough about sleep. I decided that I wanted to give them some of the knowledge that they will need to improve their sleep quality by reflecting on certain essential components of sleep that I have learned about throughout this past summer.
I want to begin this informational recap by stating what I think is the most important factor involved in getting good sleep: Get enough light and get it only when you are supposed to. Getting light, especially bright light, at the wrong times causes an advance or delay to your circadian clock which essentially means that you’re giving yourself jet lag without ever having to leave your room. Light is the strongest signal to the human circadian system and it can do some amazing things when used correctly. Bright light has the ability to affect the amplitude of your circadian clock, and if you time it right, can allow you to cross time zones faster than you otherwise would. However, this kind of entrainment schedule can be especially hard to follow given the pervasiveness of screen use at night. The presence of almost any light, particularly above 50 lux, has been shown to have a melatonin suppressing effect that can make the process of falling or staying asleep difficult.
Using light as effectively as possible is not just about timing and brightness but also about daily regularity. Previous light history has been shown to affect your circadian clocks current sensitivity to light, which makes paying attention to your light exposure important to maintain your healthy sleep habits. Poor sleep regularity has been linked to many health complications ranging from subdued cognitive performance all the way to inflammation. Recently, some new metrics have been created to quantify sleep regularity which may give clinicians the chance to make more well-informed decisions regarding recommendations for their patients’ sleep health.
The timing and brightness of light, as well as the regularity of your sleep schedule, are three components that have an immense influence on overall sleep health. My personal experience in school so far has left me lacking this key knowledge on how to promote my sleep efficiency. Learning about what to be cognizant of in regards to my sleep habits has greatly increased my alertness in the early morning, and I still have lots of room to improve upon my habits. The crux of my argument here is that just being aware of the things that can mess with your sleep will inherently allow you to avoid bad situations and improve the quality of your sleep.
This post was written by Arcascope’s intern, Ali Abdalla. Thanks Ali!
In this blog post, I want to point to some common symptoms of circadian disruption. The role of circadian rhythms in sleep is subtle, and many times issues that are attributed to other sources really have circadian factors as the root cause.
The two process model:
To understand the interaction between sleep and circadian rhythms we need to discuss the two primary drivers of sleep: the homeostatic sleep drive and the circadian drive to sleep.
The homeostatic sleep drive (called “Process H”) describes the build up of sleep pressure the longer you are awake. This drive builds up anytime we are awake, and the higher it gets, the harder it is to stay awake. In contrast, when we are asleep, the homeostatic sleep drive decreases.
Let’s imagine what would happen if this were the only component driving sleep. Then people would operate a bit like my iPhone: Active as long as your battery lasts, then asleep/recharging as long as it takes to get back to full battery… or at least until you get yanked off the charger.
If I wanted to adjust the “sleep schedule” of my iPhone I could just adjust the time that I pull it off the charger. I could also use it more (burn up the battery, turn the screen to full brightness) if I wanted to make it “go to sleep” sooner. This probably doesn’t match your experience with sleep. Being more active during the day doesn’t ensure that you will go to sleep earlier (although it can help). And staying up an hour later doesn’t necessarily mean that you’ll wake up an hour later the next morning. From experience, you’ve probably already learned that sleep duration isn’t just a function of how tired you were when you fell asleep.
This is because the homeostatic sleep drive is one of the two processes which control our natural sleep cycles. An iPhone has no issues with jet lag, shift work or sleeping on Sundays! That’s because it doesn’t have…
Circadian Rhythms: The Second Process
The more subtle process which controls our sleep cycle is the circadian clock, also called “Process C”. Circadian rhythms in humans act to help us sleep in a single block at night by modulating the sleep drive according to the body’s internal clock time. Much more about this process below.
So what are signs that your sleep issues are being driven by your circadian clock?
I wake up at 3am and can’t get back to sleep
One of the most common sleep disturbances is waking in the middle of the night and not being able to get back to sleep. Very often circadian rhythms play a role in this annoying occurrence.
We can think of the homeostatic sleep drive and circadian sleep drive as executing a delicate hand-off in the middle of the night. Let’s walk through what happens when everything is in sync. Since homeostatic sleep drive increases whenever you are awake, the hours near bedtime are when the homeostatic sleep drive is at its peak , while the circadian sleep drive is opposing sleep– or at least not promoting it. This push and pull helps keep you awake through the evening hours even if you have had an active day. This also keeps your bedtimes consistent (and in a natural environment aligned with sunset). However, once you fall asleep, the homeostatic sleep drive begins to decrease steadily, and soon it reaches levels similar to those you had during the daylight hours. So why do you stay asleep?
Well, as the night progresses, the circadian process begins to take over the job of promoting sleep. This maintains an overall drive for sleep throughout the night. Finally, around dawn, the circadian drive to sleep drops enough for you to wake up. The handoff in the middle of the night between the homeostatic sleep drive and the circadian sleep drive is what allows for one, contiguous block of sleep.
If your circadian rhythms are out of whack, this handoff can be fumbled, leading to the annoying episodes of waking in the middle of the night and not being able to get back to sleep.
Trouble getting to sleep on Sunday
Another very common sleep disturbance is “social jetlag”. This is caused by the likely familiar practice of staying up later on Friday and Saturday night and sleeping in the next mornings. This move to later light exposures tells our circadian clock to shift later, so it creates the same effect as jet lag without you ever leaving the couch.
If you stay up three hours later on Friday and Saturday and sleep in a commensurate amount, you have effectively traveled from New York to Los Angeles for the weekend– a three hour shift west. The pain comes when you need to perform the reverse trip to get back on your workweek schedule. When you try to go to sleep at 10pm on Sunday night then, as far as your body is concerned, it’s 7pm. Worse yet: this will often move the Sunday bedtime into the dreaded wake maintenance zone, discussed next.
I try to go to bed a few hours earlier and I just can’t fall asleep
This is caused by the so-called “wake-maintenance zone”: in the hours leading up to bedtime, there’s a period of time where it’s hard to fall asleep. From an evolutionary perspective, this wake maintenance zone, which would occur as the sun was setting, could have existed to ensure we’d be awake and active while we still had some light to make our way to a shelter (or into a tree) before nightfall. In modern life, bedtime is rarely sunset, and this wake maintenance zone can fall in the 9pm-11pm range.
In conclusion: Your inability to move your bedtime up by a few hours may not have anything to do with mindfulness and have everything to do with how much sunlight you got the previous morning.
Whenever I am on a break from (school/work/obligations), I end up going to bed at 3am
This typically happens when the societal constraints that are keeping the circadian clock tethered to the sun are removed. Often, someone on a spring break (and without any reason to set an alarm), will find that their schedule starts to drift later and later each night.
This phenomenon can originate from a feedback loop between behavior and the circadian rhythm. Staying up later one night will delay the circadian clock through light exposure, which will tend to move bedtime the next day later. This is compounded if you sleep in later, as you are missing the morning light which can counteract the extra evening light the night before.
This cycle keeps repeating, slowly driving the bedtime later (for me, this was something like 30 minutes each night). This progression can be curtailed by hitting the circadian wall where the circadian drive to sleep is maximal. This drive, combined with the homeostatic sleep drive which has been building up all day and night, can induce you to finally fall asleep. That doesn’t always happen, though: delay yourself enough, and you might find yourself cycling all the way back to a day schedule.
I sleep better when I go camping
Finally, one example where you may have experienced the benefits of having healthy circadian rhythms. Many people find that they sleep better when they are camping. This is especially surprising, since this typically means leaving comfy mattresses and other sleep aids behind. Personally, I can fall asleep easily much earlier in the night when I am camping, and– even if I wake several times during the night to roll over– I wake near sunrise feeling much more refreshed than normal.
A big part of this effect can be traced to the therapeutic effects that camping has on circadian rhythms.These results come from one of my favorite circadian rhythms papers which will be the subject of a future blog post: Stay tuned.
Lately, I’ve been watching clips from the Olympics, getting misty-eyed when the athletes hug at the end, and then stalking the winners on social media: a normal Saturday night in 2021. I have a shirt that looks like that, I think as I scroll through photos of them lifting approximately three times my body weight. We’re not so different, you and I, I think as they hit a five-inch target from a football field away.