Circadian Rhythms: Zeitgebers, Entrainment, and Non-Photic Stimuli

For all the unchecked randomness in this world, there are at least some things you can count on. The sun always rises and it always gets dark, and that’s something life – all life – has learned to rely on. Our internal clocks, known as circadian rhythms, tend to match up with this established external cycle. In essentially all known forms of life, from the earliest cells and bacteria to plants and mammals, the circadian rhythm is characterized by a period of around 24 hours.

You might recall a previous MDA series on how blue light can affect our circadian rhythms, and what we can do to maintain normal, natural levels and timing of blue light exposure. Long story short – it turns out that our exposure to blue light is akin to exposure to daylight, and getting too much – or too little – at the wrong times can disrupt our natural circadian rhythm and affect the quality of our sleep by changing when melatonin is secreted in our bodies. In other words, blue light is a major human zeitgeber (the ten-dollar word of the day); an exogenous cue that synchronizes our internal clock. But it’s not just light that affects our circadian rhythms.

The master mammalian circadian pacemaker is located in the hypothalamus, in a section known as the suprachiasmatic nucleus (SCN). Lesser circadian pacemakers with their own 24-hour cycles, sometimes called slave oscillators, have also been located in the eyes, pineal gland, liver, intestines, and other organs, but the SCN is said to synchronize them, employing over 20,000 neurons in the process. The SCN receives input via three main pathways: the retino-hypothalamic tract, which directly delivers photic (light-derived) information; the geniculo-hypothalamic tract, which indirectly delivers photic information; and the raphe-hypothalamic tract, which uses serotonin to deliver non-photic information to the SCN. The SCN tells the pineal gland to secrete melatonin. Both photic information (like blue light) and non-photic information (like temperature, social cues, food availability, to name a few) act as zeitgebers with the ability to entrain (circadian synchronization in accordance with an outside cue is called entrainment) internal clocks. We’ve already covered the photic side of things (which also happens to be the most powerful), so let’s take a look at how some non-photic zeitgebers affect your internal clock and what you can do to entrain your own circadian rhythm.


Just as the sun rises and falls, the availability of food come in cycles, too. Indeed, research suggests that food availability cycles entrain organisms’ circadian rhythms. The classic example is the “early bird” who “gets the worm.” How does it “know” to wake up at the hour most advantageous? The bird doesn’t actively plan to wake up at a certain time and head out for grub(s). The availability of the food (in this hypothetical case, early morning) conditions the bird’s circadian rhythm to prompt an early morning wakeup. We see this in rodent models, which display distinct “food anticipatory activity” in the hours just before their regular mealtime, even if the SCN is damaged or removed, offering evidence of a separate, independent “food-entrainable oscillator” that responds to food intake schedules. We see similar results in mammal and primate studies.

As for humans, food-modulated C-peptide increases correlated with reductions in melatonin, the “sleep hormone”. In another study, patients with night eating syndrome (those folks who eat in their sleep) also showed delayed melatonin secretion. (Side note: They also showed delayed circadian onset of hunger-blunting leptin as well as advanced circadian onset of hunger-stimulating ghrelin secretion. Leptin release was delayed by one hour and ghrelin was bumped up five hours! Eating at night may have weight-gaining implications, but this is another post entirely). Eating late at night can phase shift your circadian rhythms by blunting melatonin secretion. So if you’re having a tough time sleeping consider not eating too late.


In animals, activity levels affect circadian rhythm. Using body temperature regulation as an indicator of circadian phase shifts, we see evidence of the zeitgeber effect of physical activity in nocturnal animals put on a daytime activity schedule. Whereas nocturnal animals typically have high body temperatures and activity levels at night, forcing them onto a diurnal activity schedule causes body temperature to rise during the day and fall during the night. Their circadian rhythms are adjusting to the new schedule.

Evidence on exercise’s circadian effects in humans is less conclusive, but still present. One study identified nightly exercise (three 45-minute bouts of cycling) as an effective phase delayer of melatonin/sleep onset, and another showed similar results. One group of researchers was able to effectively convert night workers onto a daytime sleep schedule by controlling light input and using hourly bouts on the bike to phase delay the onset of circadian melatonin and body temperature rhythms. Strangely, one study showed that evening and nocturnal exercise actually resulted in melatonin increases, or advances in circadian phases – I wish the authors had given details on the exercise protocol so we could understand the apparent anomaly. Still, most studies do show that delaying the onset of circadian melatonin rhythms is achievable through exercise. Whether that’s a desirable outcome is up to you to decide. Night workers may want to play with nocturnal exercise.

Social Cycles

Can interactions between organisms cue circadian entrainment? Most likely. Researchers generally agree that circadian clocks developed primarily in response to the daily cycles of the environment – and, for most organisms, the environment includes not just light, dark, and temperature, but also the rhythms of and interactions with predators, prey, parasites, and community. For example, when two previously isolated deer mice, each with a different circadian rhythm, were placed in a common enclosure, they developed a mutual synchronization of their internal clocks. Honeybees forage in synchronicity with the rest of their colony, but isolated members tend to drift away from established foraging schedules, suggesting an important role for social entrainment. Members of numerous species show signs of metabolic synchronicity when in close contact with each other, including honeybees, beavers, bats, and even humans. In fact, something called the Social Rhythm Stability Hypothesis proposes that disruptive social events (deaths, break-ups, even minor disturbances of a person’s normal routine or of those around them) can entrain a person’s circadian rhythm and, in sensitive individuals, lead to bipolar disorder. It’s also interesting to note that one common treatment for bipolar disorder, lithium, is also one of the only known substances to directly affect a person’s circadian rhythm.

We are incredibly social animals – just think of how important the village, the tribe, the community, the family and language are to our identity as humans – and we are shaped by our interactions with others. It’s no surprise that social cues can have physiological effects on our circadian rhythms, too.

Temperature Cycles

Temperature cycles often correspond with light and dark cycles, but there is evidence that temperature acts independently on certain species’ circadian rhythms. Temperature cycles entrain the rhythms of drosophila, a type of small fly; of the leafcutter bee; and of the circadian-mediated locomotion patterns in certain lizards. Note, though, that these guys all share a common trait: they are not homeotherms. They rely on exogenous sources to regulate their body temperature. Very few mammals, other than maybe the pocket mouse and a kind of heterothermic (exhibits partial self-regulation of body temperature) bat, show significant circadian response to external temperature cycles, probably because they are largely homeotherms with the ability to self-regulate body temperature, as well as the temperature of the SCN pacemaker. When you put an SCN in a Petri dish and expose it to temperature fluctuations, the neurons respond. Thus, the human SCN is shielded from ambient temperature fluctuations, but body temperature fluctuations (even those driven by the clock itself) may affect the clock. Human core temperature is related to our circadian rhythms, but the ambient, environmental temperature cycle to which we are subject does not seem to affect the rhythm.

Regardless – extreme heat does make it hard to get to sleep, and I’ve always preferred crisp, cool bedroom air, so temperature does matter. Just not so much to our master pacemakers.

Sound Cues

It turns out that sound cues play a potentially large role in human (and other species’) circadian cycles, but I’ll discuss that more thoroughly tomorrow.

So What Does This All Mean?

Photic input remains the primary determinant of human circadian rhythm. You keep artificial (especially blue) light usage to a minimum as you approach your bedtime, make sure to get some natural (or even artificial blue) light in the mornings, and you’ll have taken care of most of your sleep-related circadian rhythm concerns. Just keep in mind that non-photic input matters, too – perhaps not as much as light, and the effects in humans are still being tested – and playing around with the peripheral rhythms might give you an edge. Tinker, as I often suggest (or even thinker, a la the Healthcare Epistemocrat), especially if you suspect your rhythm is off and you’re getting bad sleep. Otherwise, don’t get too haphazard with the hacking. Our circadian rhythms are pretty hardy, but it’s always smart to exercise caution when messing around with physiological systems our best and brightest are still figuring out.

The following hacks are worth testing:

  • Intense exercise right before bed may suppress melatonin and delay the phase cycle. If your find yourself restless and too alert after your late night workouts, try earlier workouts.
  • Conversely, if duty beckons and you need to be alert and awake one night, an extensive exercise session enjoyed right before normal bedtime should suppress melatonin and prevent sleepiness.
  • Eating also appears to suppress melatonin secretion, so if you’re having trouble sleeping consider eating earlier in the evening.

You’ll just have to find what works for you. The good news is that we are pretty darn adaptable, and fretting over workout or meal timing (as long as we don’t run marathons before bed or eat entire meals at 3 AM) is probably tougher on us than simply allowing our circadian rhythms to respond, react, and adapt to all the various zeitgebers we’re faced with every day.

This is a pretty big topic and new research is still coming out. This post was meant to introduce you to the topic at large. Let me know if it’s something you’d like to see covered in more depth in the future.

Stay tuned for tomorrow’s post on sound cues and circadian rhythms. Grok on!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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48 thoughts on “Circadian Rhythms: Zeitgebers, Entrainment, and Non-Photic Stimuli”

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  1. Good stuff. If I ever do find myself in a position where my melatonin secretion is compromised, I’ll take some oral melatonin. It usually only happens once a week or less but it is very effective for getting a good sleep.

  2. Early to bed, early to rise makes a man healthy, wealthy and wise!

  3. Some excellent stuff here. I talk about the importance of sleep to my readers all of the time and how activities such as meditation help with that. But this is really insightful (next level) stuff which I have a duty to pass on to them.

    It’s been a while since I’ve been here and left my 2 cents worth, but now that I’m here again today, I can see why I stopped by the last time.

    Thanks again

    – Shaun of Stay-Fit Bug

  4. I keep it simple. Live a primal lifestyle and go to bed when I am tired and wake up when I wake up.

    This allows me to feel rejuvenated as soon as I wake up in the morning and be full of energy throughout the day.

    I usually go to bed a little bit past sunset and wake up a little bit past sunrise.

    1. I’m curious about how you accomplish this. Do you go to bed at 5 or 6pm in the winter? I really like the idea, but just can’t figure out how to do – or hack – this for my modern lifestyle. The best I’ve come up with is just being consistent – 10:30-6:30, year round, which has me at least getting up w/i half hour of sunrise (before or after) most of the year.

      1. I do not go to bed at 5 or 6 pm in the winter. It is summertime so that is all I am thinking about. Right now the sun goes down just before 10 so I am in bed ready to fall asleep around 11 most nights.

        I have only been primal for 3 months so I am not sure when I will go to bed in the winter. Will it be earlier then 11? I would assume so. Will it be 6? No, not a chance. I won’t be tired by then.

        Could it be 9? Possibly. I have always usually gone to bed between 10 and 11.

        I just say after sunset right now because it is the summertime. So 10:30 year round is definitely ideal.

      1. Hey Jeff,

        What do you have against me? Is there anything you want to ask me?

        Yes, you are correct. I do NOT have a job – whats to give? And, YES, I do live with my parents. You seem to care a lot about these 2 facts. Why?

        I am really curious…

  5. About a year ago my whole sleep process was destroyed. Never tired, never awake, can’t sleep more than 4 hours a night. Life destroyed.

    It occurred with a severe anxiety episode, along with obsessively picky (“healthy”) eating which led to malnutrition. I don’t think the radioactive iodine I swallowed for a thyroid scan helped either.

    To those of you still with functional brains and SCNs, don’t take this information for granted.

  6. Getting good sleep is so paramount to feeling good and maximizing your greatest potential both physically and mentally that it is probably the biggest, most simplest contributer to your health.

    I sometimes forget how important good sleep is and I’ll be up late studying for an exam . Even though I’ll find time to nap the next day, getting good uninterrupted sleep makes me feel much better than replacing lost sleep time with a nap. Not only do I function physically and mentally better, but I can wake up with my mind so much more at peace. I think getting enough sleep every night plays a huge role in peoples’ ability to deal with stress!!

  7. Great post! Just fyi, the anomalous exercise study that you mentioned does indeed give a detailed account of the exercise protocol in the methods section of the paper. I’m just curious…what other details do you think would be important to know?

  8. What about night people? Those that feel better if they go to bed at..say.. 2am and then sleep late. The night people in my family have never changed no matter how long they have had to get up early for jobs, school, etc.

    And what makes some people so groggy and grouchy in the morning? Is there anything that can change this? I am talking about those who get enough sleep but still have difficulty waking up.

    Maybe no one knows.

    1. The length of people’s circadian rhythms varies- those with shorter circadian rhythms tend to go to sleep earlier. This tendency can be genetic: the study of a family of people who all went to bed extremely early actually led to the isolation of a gene which helps regulate the length of the circadian cycle, which they had a mutated variant of.

      A short blurb about a study of circadian rhythm length:

    2. I know it’s not universally true, but most of the “night owls” I know have a tendency toward procrastination, impunctuality, generally … inertia. Just keepin’ on doin’ what you’re doin’…

      For me, it’s tied in with a lack of discipline. I can go to bed at 10:30 every night for years, but when dh is out of town, I go to be at 2am again. B/c I just don’t have the discipline to stop doing what I’m doing, and inevitably stay up until I’m so tired I can hardly move myself upstairs.

      I know common sense tells me that there should not, biologically, be any such thing as a “night owl.”

      As for being groggy, I spent years hardly functioning in the morning. The first thing that helped was getting *enough* sleep. For me, that meant 8-9hrs – 7.5 wasn’t enough. The second was getting rid of sugar. WHAT a difference that made! First, it meant I didn’t have to sleep as much – 8hrs worked, IDK why. Second, maybe it was the lack of crashed blood sugar every morning? b/c I just feel 100% better every morning when I’ve eaten well the day before.

      JME, FWIW

      1. Well, as is sometimes the case, common sense in this case does not true as far as science has been able to tell.

        The time people naturally desire to go to sleep and wake up simply varies in the human population, due to biological variation, and regardless of discipline or procrastination.

        Granted, before the advent of effective lighting, the “early birds” were probably going to bed when the sun went down at 6pm while the “night owls” were staying up until 10. . .

      2. Not sure how I feel about the thought that there’s no evolutionary rationale for a night owl… I imagine that might be an assumption. If our distant ancestors lived in tribe with ferocious beasts around them and competing tribes, isn’t it safe to say that some cave-dude had to work the graveyard shift even then for protection sake?

        Just a thought.

    3. I wondered about those of us who feel compelled to stay up to 2 am also. I’ve always had trouble waking up early, even as a child… maybe the fact that my dad worked 3rd shift and the house didn’t shut down until late had a lot to do with it. I always liked to entertain the thought that maybe I wasn’t meant to live on the east coast and had some beautifully romantic excuse to move to closer to the Pacific Ocean. *sighs and laughter*

  9. I wear contacts or glasses all the time. At my massage therapist sister’s suggestion, I now make sure to get at least a few minutes of daylight on my “naked” eyes. I am sleeping more soundly than ever.

    Practically speaking, weather permitting, I take my morning cup of coffee outside and sit with my glasses on top of my head.

    1. Great suggestion; I just did this this morning. (No coffee, lol, but my eggs.) Unfortunately, the sun doesn’t rise above the trees until almost 8 and it was 6:45. I wonder if the benefit is there if the sun has risen but I can’t see it yet?

    2. *siigh* I wish my vision were good enough to do this safely…

      On the upside, my contacts are gas-permeables which means they’re much smaller than my iris — just a bit bigger than my pupils usually are. So hopefully I get some light leaking in from the edges!

  10. Im not sure what Mark means by Blue Light…is there an earlier post/blog addressing that topic???

      1. He means blue light as part of the spectrum you are receiving into your eyes. Like daylight as opposed to firelight. If you go back up to the article he seems to have a link to the previous article just under the picture. Recommend you read it, it was VERY interesting.

  11. Since I work at night, I know these effects very much. When I come back to work from a holiday, my internal clock is totally destroyed for a week. Then my brain slowly connects the dots, and it works again. I have been trying to sleep at night in the weekends, but it just gives you a somewhat “jetlag” at workdays. I’ll still keep doing it here at summer, since I need my Vitamin Ds :). The best thing about working at night is that I can choose to sleep as much as I please (no alarm clock), but the worst thing is that your body somewhat shuts down, and you feel weaker.

  12. I’ve had massive problems with this stuff since I was about 8 years old. Just wouldn’t fall asleep before 3AM. Learned to manage it until some doctor told me he could easily fix it. Then I was about 20yo. That really wrecked the whole thing and now I usually sleep every other night for about 16 hours, which works ‘ok’.

    For a while I had one of those dawn simulator alarm clocks, worked very good, at least for waking up. Might be something to try.

  13. I’m VERY fascinated to read about the food connections. I had insomnia for the first time in my life during pg#4, then again during pg#5, & unfortunately it continued postpartum. No idea wha tto do about it. I just kept thinking about it, and thought that it seemed like it was b/c I had had enough sleep, but at the wrong times.

    I’d go to bed at 10:30, be awake till 11:30, wake up at 4:30, be awake till 6:30, sleep till 10, be exhausted and have to nap mid-day… it just seemed like my sleep cycle needed a “reset.” I don’t know how I came up with the idea, but I decided to try to use breakfast to reset my inner clock. I decided to get up at 6:30 and eat a full breakfast, no matter how tired I was, for one week. I gave myself permission to then go right back to bed, if necessary.

    It totally worked. I felt like death the first day from lack of sleep, but that meant I slept like a log at night, and by the third day, I woke up great. I think there were a few afternoon naps the first day or two, and by the end of the week, I was solidly sleeping at night. Another year or so later, I fell into the same odd sleep trap again, tried re-establishing my sleep with food again, and it worked!

    So when I read about the zeitgebers (does this mean “time-givers?”) in the liver, I almost shouted! 🙂

  14. To touch on the subject of food and sleep, I found some research awhile ago, one study suggests that ghrelin helps promote slow wave sleep.

    Further, there are also studies showing how ghrelin reacts to meals of different macronutrient compositions. Protein basically surpresses ghrelin more slowly than carbs, but it supresses it for much longer.

    Carbs supress ghrelin the fastest, but there is a curious strong rebound in ghrelin levels 60-90mins after ingestion, perhaps helping to additionally explain why people can feel sleepy after a carb meal.

    Ghrelin’s response to fat ingestion is somewhere in between protein and carbs.

    There was also one study showing that the digestion of fat is important in ghrelin supression.

    When we put everything together, I believe as Mark says that eating before bed is a bad idea, especially anything that has alot of protein in it. If you want to eat shortly before bed I expect fruit is your safest bet.

    The cut-off point for protein and fat ingestion should be 5 hours before bed.

    1. I seem to be better off with an ounce of cheddar, a handful of nuts, or a boiled egg before bedtime. In my house growing up there weren’t any clear-cut rules on when the eating should stop, so I’d really like to hear more about this(especially as a nursing mom).

  15. I wonder if the social aspect of sleep is what makes yawning “contagious.”

    1. I thought yawning was a sign that you were oxygen deprived.

      Somehow I always imagined the contagious yawn was really the result of someone else nearby inhaling some of the yawner’s exhaled carbon-dioxide. It somehow sounds right in my head?

      “Curiouser and Curiouser!”

  16. I would really love a post about eating before bed . This was great nonetheless!

  17. The best book on the subject of circadian rhythms and diet and everything else related to sleep is …

    Lights Out: Sleep, Sugar, and Survival

    THE best book on all of this.

    It has the additional credibility of not being publicized, awarded, talked about, or promoted at all. Which generally means that it wasn’t part of anyone’s agenda, that it was independent, that it contains knowledge that the powers that be would rather we not be made aware of.

    See the MANY reader comments on “Lights Out” at Amazon. You’ll get an idea how

    1. I’m happy to see that someone mentioned this book! I have it, and while it’s entertaining in spots, many points weren’t elaborated on enough, I found.

      And I don’t know if T.S. Wiley follows her own protocol or not but I’ve seen some pictures of her and she’s not exactly slim herself.. dunno what’s up wi dat….

  18. The best book on the subject of circadian rhythms and diet and everything else related to sleep is …

    Lights Out: Sleep, Sugar, and Survival

    THE best book on all of this.

    It has the additional credibility of not being publicized, awarded, talked about, or promoted at all. Which generally means that it wasn’t part of anyone’s agenda, that it was independent, that it contains knowledge that the powers that be would rather we not be made aware of.

    See the MANY reader comments on “Lights Out” at Amazon. You’ll get an idea how the info in this book affects people.

  19. Two squirrels often show up in my campsite to try to scavenge shortly after I wake up, sometimes before. The bigger one is bolder even though I’ve thrown some rocks at it to try to scare it off after it ignored my hissing warnings when it was going for food I was saving for later. It craps all over the pile of boards I use as a table, even on one of the DVD cases I use to bust up herbs on. It crapped right on my sheets. I think that was this morning, when it came a couple feet away from my face, for a thrill maybe? I think it’s become addicted to sugar from eating the left over sauce from cans of beans (my free unlimited back-up staple).
    Two pigeons have recently been lurking in the rafters of the big open shed I’m squatting in. One crapped on my coat arm this morning and on my table.
    It’s like I’m being attacked by defecating animals. I hope mother nature is trying to improve my immune system, not make me bat-shit crazy (though it could be too late due to my in the dirt lifestyle and the fact I lived with a friend’s rat that was actually diagnosed with toxoplasma.)

  20. great post.

    i was the whole day picking up clinical trials and reviews just to reach the same conclusions that i have seen here.

    very good. will start reading your articles with more attention.