Welcome to our Circadian FAQs page! Here, we’ve compiled answers to the most common questions about circadian rhythms, helping you unlock the secrets of your body’s internal clock. Whether you’re new to the topic or seeking in-depth insights, you’ll find valuable information to better understand and optimize your daily rhythms.

What’s a circadian rhythm?

A rhythm in your body that (i) repeats about once every 24 hours (ii) without needing something external to drive it, (iii) while being able to adjust and adapt to new “time zones”, and (iv) being robust to changes in temperature (Dunlap, 2004). The name comes from “circa diem,” meaning “about a day” in Latin.

Lots of things in your body exhibit circadian rhythms, but not every rhythm is “circadian.” For instance, if you get up and go to the gym every morning, you’ll see a spike in your heart rate that happens about once a day, but that rhythm is externally driven—by you choosing to get up and go to the gym. It’s not a circadian rhythm.

An example of something that is a circadian rhythm is your body’s production of the “night” hormone melatonin (Dunlap, 2004). Even if you cut yourself off from the world to go live in an underground bunker for ten weeks, your body will continue to produce melatonin approximately once every 24 hours. If you fly across time zones, the timing of your melatonin rhythm will shift until it adjusts, or “entrains,” to the new time zone. And it’ll keep chugging along with a period of about 24 hours even if you raise your body’s temperature—a fact that’s pretty cool when you think about how much temperature changes mess up mechanical clocks.

What has a circadian rhythm?

Immune response, metabolism, peak strength, alertness, sleep drive, temperature, melatonin, cortisol, you name it (Panda, 2020). Lots and lots of things in your body exhibit these rhythms.

Are circadian rhythms the same as sleep?

They’re related, but not the same thing. Your circadian rhythms help set the stage for times when you can and cannot easily fall asleep, while your sleep affects your circadian rhythms in several ways, including by restricting light exposure during the hours you’re asleep (because your eyes are closed) (Dijk, 1995).

Why does light matter?

Light is the number one time cue—or “zeitgeber”—to your central circadian pacemaker, though other factors can affect your body’s timekeeping system as well, including exercise and other forms of activity. Food is a zeitgeber for peripheral clocks, like those in your liver (Panda, 2020).

Central clocks? Peripheral clocks?

There are actually clocks all throughout your body. The central pacemaker is in your brain, and it’s called the suprachiasmatic nucleus (SCN). The SCN is the conductor of the orchestra of your body’s clock, and it listens to light primarily. There are peripheral oscillators in your skin, stomach, liver, and other organs, which follow the signal from the SCN, but also listen to other signals, like when you eat (Dunlap, 2004).

How do you measure a circadian rhythm?

You can try directly measuring things that exhibit circadian rhythms, like your body’s temperature, except then you’re likely to run into the issue of “masking”—which you can think of as the idea that temperature (and other signals) are affected by a lot more than just your body’s internal clock, and if you’re not careful, you can mistake those other signals for circadian rhythms when they aren’t.

Here’s an example: Melatonin gets squashed in the presence of light. If you’re trying to find the time when melatonin starts to rise in your body, and you don’t control for the presence of light, you might just end up finding that melatonin starts to rise at the exact time the light gets turned off. That’s not you measuring something circadian: that’s you measuring “time you decided to turn off the light.”

That’s why most researchers use dim light melatonin onset (DLMO) to measure circadian rhythms. DLMO takes lots and lots of melatonin measurements from people in constant, dim, controlled conditions, over a period of time stretching hours. One timepoint isn’t enough: You need to get lots of samples in order to identify the time when melatonin appreciably rises in the dark in order to benchmark a person’s circadian clock in this way.

DLMO has been around for a while, and there are cool new methods being developed to measure circadian rhythms from blood samples, skin samples, and gene expression patterns.

At the company, our niche is non-invasive methods to predict DLMO: we’ve collected lots and lots of DLMO samples in order to build and train ML models that can predict melatonin onset using signals passively taken from your phone (and wearables if you’ve got them). We’ve published on this a couple times, including in (Huang, 2021) and (Cheng, 2021).

What does “entrainment” mean?

Entrainment, as a picture, looks like this:

Entrainment and circadian rhythms

It’s the process of your circadian rhythms stretching and adapting themselves gradually to match a new repeating zeitgeber signal; e.g. a new schedule. This is what happens when you cross time zones or switch to a new wake up time. In the above picture, the pink and blue curves are entraining to fall in line with the repeating gray curve. The blue curve is entraining faster than the pink curve because it’s getting lined up quicker.

Here’s how I think of entrainment: Imagine you’re walking on a sidewalk, and you’re doing that thing where you step first with your left foot and then with your right foot exactly once in each sidewalk square. (I can’t be the only one who does this. Please back me up here.)

Probably the gait you walk with when you’re trying to hit exactly 2 steps per sidewalk square isn’t exactly the gait you’d be walking if you were on a seamless stretch of concrete, without any squares. You’re regulating your gait a little bit, so that you can get that sweet, sweet two steps per square in.

Now imagine you reach a driveway. There are no sidewalk squares for a stretch, and when you hit them again, your feet are out of sync with the pattern of squares—maybe you step first with your right foot instead of your left, or maybe you have to take three steps in a single square because of where your feet landed. But gradually, over the course of the next few squares, you get back into a left-right (new square) left-right (new square) pattern.

That period of time when you’re stretching or shortening your gait to get back into your original groove is you “entraining” to the new pattern of sidewalk square tiles.

Making the analogy to jet lag: the time you spend walking on the square-free driveway is like the dim, lightless environment of a long flight, and your return to sidewalk squares that are misaligned with your gait is like your arrival in a new time zone, which is misaligned with your body’s rhythms from before the flight.

I like this analogy because it sets us up to intuitively answer other questions about circadian rhythms, including:

Could we adjust to a 16 hour day?

Probably not (Wright, 2001). Our clocks are pretty hardwired for a roughly 24-hour day. Using the analogy from above, imagine if you were trying to walk two steps per sidewalk square, and I slowly started making the sidewalk squares longer and longer. Eventually, your legs wouldn’t be long enough to hit 2 steps per square, no matter how hard you tried. Similarly, if I made the sidewalk squares shorter and shorter, you’d eventually start tripping over yourself, to the point where you couldn’t get into any sort of walking groove.

That’s what happens to your body’s circadian clock when the day gets more than a little shorter or longer than 24 hours. By 22 or 26 hour-long days, odds are pretty low that we can adjust to them.

So what happens if you can’t or don’t entrain to a schedule? If you’re walking on the sidewalk and you’re me, you pretty much give up, ignoring the pattern of sidewalk squares and the joy that comes from getting two feet in each one.

In circadian parlance, you “free run.” That means that you continue to move forward at a period that’s slightly different from the 24 hour day. At first, you’ll probably only drift a little bit—maybe waking up 12 minutes later every day. But one month of that is a six hour shift in your body’s internal time relative to where you started; the equivalent of going from the central U.S. to London. And if you keep free running, you’ll eventually cycle back around to where you started, having gone through all “effective time zones” of the world, without even having had to leave your house.

With all the inputs we feed our body’s clock around the clock, how are we even able to stay in sync at all?

In some sense, the physics of the circadian clock set it up for stability. Let’s assume you’re a person entrained to a day schedule, with no circadian weirdness going on, getting a regular pattern of light and dark that pretty much matches the solar day.

Now, assume that your clock gets slightly out of sync for whatever reason, and is running a bit behind schedule when Monday morning rolls around. When the light hits your eyes through your window that A.M., it’s going to be arriving during the “phase advance” region of your body’s circadian clock. The phase advance region is when your body’s clock speeds up in response to light, and the presence of light sooner than your body was expecting light will have a corrective effect on your circadian rhythms (Dunlap, 2004). Light at that time will send the signal of “it’s time to get a move on” to the rhythms in your body.

Similarly, imagine your clock is running a little ahead of schedule, and you’re feeling sleepier than usual even though the sun is still up. Keeping the lights until your normal bedtime means you’ll be getting light during the “phase delay” region of your clock, when light has the effect of slowing your rhythms down.

In both of these examples, small disruptions to your body’s rhythms are naturally corrected by sticking to a consistent light schedule.

There’s a key word in the last sentence, which is consistent. The dark undercurrent of light exposure at night is that it can actually make it kind of hard to keep that consistency. After all, we live in a world where light is under our control. So when you get light at night, and it delays your clock, your “time when you’ll feel sleepy” gets shifted later. And if you’re not sleepy, you’re more inclined to keep the lights on, because you want to be awake and do things. So then light delays you even more.

This is a positive feedback loop with a negative end result: You end up much more delayed relative to where you would be in a camping environment, without the power to control your light exposure. This kind of phenomenon could be the reason night owls exist to the extent they do in modern society: if you’re more sensitive to light (and some people are really sensitive to it), you are more susceptible to falling into the quicksand of light at night delaying you.

How does Arcascope track circadian rhythms?

Less technical answer: We use lots of long term data from devices people already own (i.e. phones) to figure out when melatonin onset is going to happen each day, then we use melatonin onset as a benchmark for other rhythms in your body.

More technical answer: We simulate a differential equation model of the SCN to predict the cumulative effects of phase shifts from all “zeitgebers” taken from your devices (Forger, 2017), then we polish the output of the PDE model with a neural net to predict circadian time.

Should we do permanent Daylight Saving Time?

No. See www.srbr.org/advocacy/daylight-saving-times-presskit/


Bowman et al. “A method for characterizing daily physiology from widely used wearables” Cell Reports Methods (2021) 100058

Cheng, P et al. “Predicting circadian misalignment with wearable technology: validation of wrist-worn actigraphy and photometry in night shfit workers” Sleep (2021) zsaa180

Dijk, DJ et al. “Contributions of the circadian pacemaker and the sleep hoemostate to sleep propensity, sleep structure, electroencephalograpic slow waves, and sleep spindle activity in humans” Journal of Neuroscience (1995) 3526

Dunlap J et al. “Chronobiology: Biological Timekeeping” Sinauer (2004)

Forger D. “Biological Clocks, Rhythms and Oscillations” MIT Press (2017)

Huang, Y et al. “Predicting circadian phase across populations: a comparison of mathematical models and wearable devices” Sleep (2021) zsab126

Panda, S “The Circadian Code” Rodale Book (2020)

Wright K. et al. “Intrinsic near-24-h pacemaker period determines limits of circadian entrainment to a weak syncrhonizer in humans” PNAS (2001) 98 14027