circadian rhythm, body clock, sleep

Sleep Series – Part 3: Serotonin, Melatonin, and your Circadian Rhythm

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Introduction – Your Circadian Rhythm:

My previous post explained what sleep scientists have measured during sleep using EEG, and scientists’
current state of interpretations. Included in that post was my definition of objective sleep quality: going through the  proper sequence of sleep cycles during the night with sufficient amounts of both stage 3 sleep (slow-wave sleep) and REM sleep. This post will focus on what drives you to be awake during the day, and makes you sleep at night: your circadian rhythm.  The term circadian rhythm comes from the latin words “circa” which means “around”, and “dia”, which means day. So, a circadian rhythm is any biological process that cycles around the Earth day, which is 24 hours.

Sleeping definitely fits the bill. Whether you are consciously aware of it or not, your activity patterns, sleeping included, are connected to the astronomical motions of Earth. Modern technology, however, can interfere with this process, as discussed below. Imagine a world without technology: nights would be very dark, and days would be very bright. This dark/light cycle is the primary driver of your circadian rhythm (which includes sleep),  the maintenance of which is a critical component of your overall health and longevity.

To begin, let’s take a look at the circadian rhythm with this diagram I snagged from Wikipedia:

circadian rhythm, body clock
Thank goodness for that 22:30 suppression! ;)

As you can see from the above diagram, in a mostly-natural setting, humans (and most animals) have a daily rhythm (i.e. the circadian rhythm). I can definitely relate to this diagram, which is backed up by research. As an early riser, I find my greatest mental work occurs in the morning, and by the afternoon I can get somewhat foggy-headed, and begin to crave more physical activity. I have, however, trained early in the mornings but I find it takes time (a couple weeks or so) to adapt to intense early-morning training, whereas afternoon training always seems natural.

This rhythm is driven by various environmental stimuli, but by far the largest impact, as mentioned above, comes from the natural daily light-dark cycle. During the day, when the sun is out and it is bright, light entering the eye and projecting to the retina (basically a movie screen at the back of the eye for the brain) causes light-sensitive receptors containing melanopsin to communicate directly with the suprachiasmatic nucleus (SCN) in the brain. The SCN is the major regulator of the circadian rhythm, and it is light exposure – or the lack thereof – that lets your brain know whether it is day or night. The process is illustrated below:

circadian rhythm, light, scn
Wikipedia, once again.

The SCN then communicates with the pineal gland, which either releases melatonin (in darkness), or shuts off melatonin production (in brightness). Melatonin is the hormone that makes you sleepy, and has a huge effect on sleep quality. Without it, quality sleep just won’t happen correctly. In the pineal gland, melatonin is produced from serotonin. These two hormones are inexorably linked, with serotonin (amongst other hormones) producing feelings of being awake and alert, and melatonin getting you ready to sleep deeply.

Serotonin and Melatonin:

The process of creating melatonin in the pineal gland during darkness actually starts with an essential amino acid (a building block of protein) called L-tryptophan. L-tryptophan can do a lot of things, but one is that it can be converted in a two-step process to the neurotransmitter serotonin. L-tryptophan is thus a precursor to this whole process. As it is a building block of protein, it can be found in a variety of foods. The highest concentration of L-tryptophan is found in egg whites, but it is present in all animal foods, most nuts & seeds, and even fruits like bananas (granted, a very small amount in bananas). Sufficient tryptophan in the diet is critical to ensuring that the body has the building blocks to create serotonin which then turns into melatonin and allows for the high-quality shut-eye we all need.

Once there is sufficient L-tryptophan in the diet, how is serotonin created then? Well, most of it is made in your gut, surprisingly. In the brain, which is what we’re concerned with when it comes to sleep, serotonin production is directly related to how much bright sunlight you are exposed to. The brighter the sunshine, and the longer an individual is exposed to it, the more serotonin the brain will produce. Serotonin is thought to produce feelings of well-being and happiness, which might explain the prevalence of depression during the darker winter months. So, during the (hopefully) bright sunny day, your brain produces and stores up serotonin from L-tryptophan. Then, when the sun gets lower in the sky, the SCN senses this dimmer light and the pineal gland starts the synthesis of melatonin from serotonin. Shortly after melatonin is secreted into the bloodstream and you start to feel sleepy. The process is illustrated in the infographic below:

melatonin, seratonin, melatonin synthesis, L-tryptophan, sleep, healthy sleep, infographic, circadian rhythm
I hired mmartist at to make this for me! Highly recommended.

Melatonin is a powerful hormone.  The varying concentration of melatonin in the blood throughout the day carries out many important biological functions to the circadian rhythm, and also acts as the body’s most powerful broad-spectrum antioxidant. Quite literally, the release of melatonin at night is imperative to mop up the junky chemical remains of biological functions that occur during the daytime (this is the antioxidant function). This is so important, in fact, that a lack of melatonin due to circadian rhythm disturbance leads to an increased risk of all cancers. Moreover, timing of sufficient melatonin concentrations has a large impact on sleep quality. This means that even if you do manage to fall asleep with low melatonin concentrations in your blood, you will experience less of the critical stage 3 and REM sleep stages that are the most physically and mentally restorative processes.

It is clear that maximizing the natural production of serotonin in the brain during the day, followed by maximizing the production and release of melatonin at night is extremely important to produce the most restful, restorative, and disease-fighting sleep. Indeed, what we truly need are bright days and dark nights to optimize this system. It is the contrast in brightness between day and night that drives our body clock. In a completely natural setting, such as the environment that humans evolved in, a high day/night contrast is the default. With modern artificial lighting and all the electronic gizmos we shine directly into our eyes, the game has changed, and melatonin suppression is becoming a serious problem.

The Impact of Artificial Light on Your Circadian Rhythm:

Before launching into the effect of different artificial light sources, a little background physics is needed. Along with the brightness of the light, the colour of the light has an effect on how much it impacts our circadian rhythms. I fully explain (in a friendly way) the physics of light in this article, and the electromagnetic spectrum here, so for this post I will just say that red light is on one end of the colour spectrum, and blue light (also violet) is at the other. Blue light has by far the most powerful impact on our circadian rhythms, and red light has a relatively small effect. The colours in between are, well, in between. A view of the light colour spectrum is below:

em, electromagnetic, electro-magnetic, spectrum, em spectrum
I love science so much! Oh man. On the left are the higher-energy blue/violet light rays, and on the right are the lower energy red light rays.

Very generally speaking, regardless of colour, artificial lighting is much dimmer than the brightness of a sunny day, so one result is that much less serotonin is produced if you’re in a mostly-indoor environment as compared to the outdoors. According to this handy chart, an office environment is about 500 lux (lux is just a unit of brightness), whereas a sunny summer day is 50,000+ lux. This means that going outside when it’s sunny is literally over 100 times brighter than staying inside. Even a cloudy day is over 10 times brighter than a cubicle. To produce sufficient serotonin, an outdoor-level brightness that is rich enough in blue light is required, and the longer the better (to a point, obviously). This can be done artificially, such as with light boxes, which are lights specifically designed to put out the full range of colours evenly at 10,000+ lux to replicate daylight. However, it’s much easier and more effective to just go outside mid-day if you have the option. That being said, artificial light boxes are successful in treating the winter blues if used properly. I personally shoot for at least 30 minutes outdoors during daylight hours every day, regardless of the weather. If I can’t for some reason, I use my light box.

light box, light therapy, blue light, sad lamp
If you’re stuck inside all day, these light boxes can help to produce serotonin. Credit goes to Adam Barney for the photo.

Now, depending on the time of day, artificial lighting can actually be too bright, especially if the source is rich in blue light. At night, or even late in the evening, artificial light (especially blue light) suppresses the release of melatonin from your pineal gland, which delays your circadian rhythm and keeps you awake later and decreases the quality of the sleep you get. What’s worse is that backlit screens, such as on your smart phone, computer screen, and television screen, are fairly bright and very rich in melatonin-suppressing blue light, especially the new LED backlit screens. Not only that, but the whole point of these devices is to get you to stare directly into them. This means that staring at your smart phone/computer screen/television late into the evening will suppress melatonin and rob you of valuable sleep (quantity and quality).

led screens, screens, blue-light, blue light, blue-light screens, blue light screens
This guy is not going to bed anytime time soon…Credit goes to Duane Storey for the photo.

Due to the melatonin suppressing effects, artificial light, though beneficial in many ways, can be carcinogenic if the timing is wrong (light at night). Although it may seem strange to consider light a carcinogen, the evidence was strong enough that in 2007 a working group funded by the World Health Organization concluded that “shift-work that involves circadian disruption is probably carcinogenic to humans”. This means that, ironically, the doctors and nurses who engage in night shift-work are at a higher risk of cancer due to a lack of melatonin production. This obviously extends to other night-shift professions, but the irony is strongest when it comes to medical professionals being at an elevated cancer risk. I will provide tips on how to mitigate this issue in the section below.

So, how much light at night is bad? Well, that all depends on three factors:

  1. The intensity (brightness) of the light source: Brighter lights suppress melatonin more than dimmer lights. According to one study, exposure to room lighting of just under 200 lux (typical) for a few hours before going to sleep was enough to significantly suppress melatonin production. Moreover, another study on rats showed that sleeping in a room at only 1 lux (still ~1000x higher than starlight) was enough to disrupt circadian rhythms. (Granted, the study was on rats, but rats are mammals like humans, so the results can be cautiously extrapolated to humans as well.).
  2. The duration of light exposure: The longer you are exposed to the light source, the more melatonin production will be suppressed, but the first few minutes of light exposure has a much greater impact than subsequent minutes, ie: your first 10 minutes of light exposure has more of an effect than the second ten minutes of exposure. Also, ten separate 50 second bursts of light (not unlike late-night texting) is enough to increase alertness and cognition.
  3. The colour spectrum of the light source: Light sources richer in blue suppress melatonin much more than light sources poor in blue. So, backlit LED screens (smart phones, televisions, computer screens) are much more effective at keeping you up at night by suppressing melatonin as compared to candles or campfires.

How to Optimize Your Circadian Rhythm and Sleep Better:

With all we’ve learned about the circadian rhythm and the effect of light, let’s optimize. The below tips assume that you have a typical day, ie: you wake up in the morning and go to sleep at night. I will then give suggestions for dealing with the modern, chronically-lit world in addition to some tips for night shift workers.

How to Optimize in an Ideal Situation:
  1. Get enough L-tryptophan in your diet: Any good source of protein will provide this. Meat, fish, eggs, and even nuts and seeds will do the trick.
  2. During the day-time, go outside without sunglasses and let the sun shine into your eyes: This will ensure that you produce enough serotonin to tell your body that it’s day time. I shoot for at least 30 minutes a day outside. Sunglasses block it out, so try for at least part of your time outside to be bare-eyed. Bonus points if you get at least some of this bright sunshine early in the morning.
  3. Before you intend to sleep, start avoiding bright light: Ideally, the only source of light at this point would be firelight (candles or something).
  4. Sleep in a pitch black room: Anything beyond moonlight will suppress melatonin to a degree, so make your bedroom as dark as possible. If you live in the countryside with no artificial lights outside your window, this is no problem. If you live in a modern city, see below.
  5. If you wake up in the night, minimize light exposure: Exposure to light in the middle of the night will signal to the SCN that it is day too early, so it will be tough to get back to sleep, and it will compromise sleep quality. If you have to go to the bathroom, use a minimal amount of light so that it’s not dangerous, ideally a dim reddish light (small incandescent light bulb). Better yet, minimize fluid intake before bed to prevent this from happening, or have a well known clear path to your bathroom so light is not necessary (be careful though!).
How to Optimize When You Live in the Modern World:

On top of the above:

  1. Install f.lux on your computers, smart phones, and tablets: There is a glorious free program called f.lux (for Mac, Windows, Linux, and iPhone/iPad) that upon telling it where in the world you live, when the sun is down it will shift the output colour spectrum of the screen to a much redder-spectrum as opposed to the normal blue. It’s set-it-and-forget-it. When the sun goes down, the screen shifts to red. When the sun comes back up, the screen shifts back to normal. You get used to it in no time, and actually start to love it. For Android users, choose the program Twilight.
  2. If you can’t go outside during the day-time, use a light box: Good light boxes are effective at producing serotonin. It may weird you out to turn on an intense light every day “for your health”, but I have been using one on-and-off for years and I really appreciate it during the dark winter months, or when my schedule doesn’t allow for enough time outside. Here’s one on that is really popular. Here is a similar one for Canadians on
  3. If you must be exposed to blue light at night, try blue light blocking glasses: Ok, this is where people start to think I’m a little silly, but these things do work. I use these when I’m reading at night, or if I’m in a situation that involves a lot of blue light exposure at night (seeing a movie, usually). Here’s the most popular model on, and here’s the same on

    blue light blocking glasses, blue light, circadian rhythm, melatonin
    UVEX blue light blocking glasses. You know you want these…because they work!
  4. Use black-out blinds or drapes in your bedroom: Keep that streetlight OUT of your bedroom. It is suppressing melatonin, and lowering the quality of your sleep. Here are some great blackout curtains on, and here are the same on My rule of thumb: if you can see your hand in front of your face, your room is not dark enough!
  5. If you don’t want/can’t have blackout curtains, use a good sleep mask: These are great for when you’re traveling or sleeping somewhere different than usual. No need to freak out over some light-leaks if you have one of these beauties! Here’s a great one on, and here’s the Canadian counterpart at
black out blind, melatonin, sleep
Here’s how a black-out blind should look. No leaks though! Keep it closed when you are sleeping.
Tips for Night-Shift Workers:

The first thing to recognize if you’re a night-shift worker is that this is not an ideal situation. A lot of important work is done at night in the modern world, so it is necessary, but credence should be paid to mitigating the dangers associated with melatonin suppression. Here are my tips:

  1. When working at night, use a light box: Ironically, this is the one situation where I would suggest bright-light (including blue light) exposure at night. In this non-ideal situation, you want your brain to think that it’s day-time, so soak up some intense light.
  2. Two hours before you’re done, start wearing blue-light blocking glasses: This way, your brain will think the “sun” is going down.
  3. When you do sleep, make sure your bedroom is pitch-black, or wear a sleep mask: You’re trying to convince your brain that it’s now night-time, so your sleeping environment must match that of a normal night.

There is also the option of supplemental melatonin for night-shift workers, or workers on a rotating schedules (nights AND days…). I hesitate to recommend these supplements as they are somewhat controversial, and I would rather people just learn to produce it as naturally as possible. Please, by all means, do some research on melatonin supplements and see if they are right for you.


There is no doubt that human beings are animals like any other (just with highly evolved cortices in the brain), so we are designed to thrive on the rhythm of night and day. Using modern technology, we can also high-jack this system and cause real harm to our bodies and minds. With some proper mitigation strategies, we can exist in the modern world, enjoy our high quality of life and modern technology, and still get the quality sleep we need every night.

I would also suggest to most healthy people to not sweat it if you don’t sleep perfectly for a night or two. As I’ve talked about before on the the Context of Vices, humans are designed to be able to be stressed in various ways, but then we need to recover. High-quality sleep is necessary for recovery, so knowing how to manufacture a good night’s sleep is a great tool to have in your arsenal.

Personally, I aim to follow the above guidelines six nights a week (at LEAST five), so I don’t stress it when there’s one night in a week where I’m up late in light, especially if it’s time spent with friends. As long as I get my recovery nights, it all comes out in the wash. There is more to producing a quality night’s rest, but you’ll have to read the next post in this series to find out. Teaser: it has to do with stress management.

Thank you for reading! I hope you learned something, and that the information presented can be of use to you.

– Graham

19 thoughts on “Sleep Series – Part 3: Serotonin, Melatonin, and your Circadian Rhythm”

  1. This is an absolutely excellent article. Thank you so much. It’s helped me understand a lot. Incidentally, when I last had my prescription varifocals renewed, I opted for a coating that blocks blue light as this is meant to prevent macular degeneration. I said to the optician, surely we evolved to cope with blue light as in the ancestral environment we’d have been outside in daylight from dawn to dusk? She replied yes, but now with computers, phones etc. we have so many more sources of blue light that exceed the bounds of natural daylight. Anyway, my point is, when I wear the glasses for walking during the day, the coating is blocking or reducing blue light and I wonder if this is affecting serotonin production – especially in winter? I’ve had a separate set of distance glasses made up which don’t have the coating as I want to receive maximum blue light to manufacture serotonin in the winter months!

    1. Anne,

      Thanks for the encouragement! I’m glad you found the article helpful. I apologize for taking so long to reply to you. I’ve been moving!

      Interesting that your glasses block blue light. I’ve never really heard of that. I’d say that, depending on how effective your glasses are, that would certainly reduce serotonin production. Maybe you could take them off for a portion of your time outside? Or go with your long distance glasses for a period? I’m just spit-balling here :) I don’t wear glasses, so I haven’t had that challenge.

      Hey – at least they block blue light at night, if that’s a problem :)

      Thanks again for your comment!


      1. Hi Graham, thanks for your reply.

        Just FYI I enquired further of the optician (who’s actually an eye doctor so I thought she’d know) about whether the blue light filter in my lenses stops the brain manufacturing serotonin. She referred me to the manufacturer of the lenses, Essilor. I’ve sent a query to them but so far don’t have a reply.

        I don’t know if I can post a commercial link here (if not, please delete) but this is what they have to say about blue light (scroll down to the section called ‘The Risk of Blue Light’):

        They say that good blue light is the blue-turquoise part, which is necessary for regulation of the sleep/wake cycle, mood and cognitive performance (so presumably it promotes serotonin manufacture, although they don’t say so directly). The blue-violet component of daylight however was recently identified as being harmful to retinal cells, according to them, which can accelerate the onset of age related macular degeneration.

        In relation to UV light, they say that the risk of damage to the eyes from UV light has increased due to ozone depletion in the atmosphere. I don’t know if this also applies to the effects of blue-violet light. I’d have thought we evolved to cope with blue-violet light personally.

        I don’t know what the science is behind all this. They are a commercial company so might be pushing this as an expensive extra when ordering lenses.

        They say that some blue light protection lenses filter all blue light, including blue-turqoise light. They don’t specifically state that their lenses block only blue-violet light, so I’ve written to ask them.

        I’ve no idea whether the blue light from computer screens, tablets, etc. is blue-turquoise or blue-violet light!

        1. EDIT: I just read up a little more, and they mention that UV can be reflected off the lenses themselves, which I hadn’t considered. I suppose lenses aren’t natural, per se, so it may be a unique situation.

          Again, as I’m not an optician, I’m out of my league, but maybe some UV protection is not a bad thing (I’d still take out the lenses from time to time and get unfiltered). As for the blue-violet, I’m not sure, but I’m much less concerned about it.

          Original reply:


          Sounds like you’ve been really researching this! Good for you!

          I’m getting slightly out of my league here, but I can offer a little insight, if you’re interested :)

          First of all, check out my post on ozone depletion:

          In short, I think where most people are living, the effects of ozone depletion are overstated (although the phenomenon is very likely real).

          So, here’s the thing. I haven’t looked at the research that these manufacturers are quoting, but I get an inkling that there was some experiment where they isolated a particular patch of wavelengths (the blue-violet range) and noticed damage to a retina.

          Sunlight is always a mixture of all wavelengths, so this experiment (if that is the case) is not reproducing natural situations. For natural sunlight, yes, I think we have evolved to cope with this to a certain degree.

          Damage is not always bad. Sometimes it means that it forces our bodies to adapt and become stronger. If it’s a natural stressor, that is very likely the case, and it’s how I feel about sunlight.

          That being said, obviously you can overdo it on the sunlight, but I believe that you notice this when it happens. You can put on sunglasses or a shirt (for your skin) or seek shade when this occurs, but it’s not a good idea to avoid it entirely.

          With the proper diet and lifestyle, you can handle these stressors, and become stronger for it. If you have a reason to believe that you are especially sensitive to a particular stressor, then you can deal with that with modern technology, but if you’re good to go, I would, personally, err on the side of dealing with the natural stressors in a responsible manner (ie: don’t overdo it, but don’t avoid it).

          I hope that helps. And for the computer screens – I bet it’s a mixture of both. There’s very rarely a pure cutoff of wavelengths of light. It’s almost always a spectrum of all colours, with varying amounts of each. Computer screens are blue-heavy, but they have the reds and yellows as well. It’s all a mix with varying amounts of all colours.

          I’m not sure what to suggest for your contact lenses, but I think you’re on the right track. The filters might be unnecessary, but as I’m not a trained professional in this matter (and just a scientifically educated health enthusiast), I can’t responsibly recommend any particular action here.

          Either way, I hope this helps :)

          If you want to learn how to eat to reduce damage and maximize benefits from sunlight, I have another article here that has been quite popular: (this is more about skin, but I suspect it would help eyes too).

          Good luck!


  2. Dear Graham

    I have been looking at the possibility that lack of sunlight in winter could be contributing to my sleeping difficulties. I first noticed that when I took a trip to the North of Norway and when in Denmark on an overcast winter day I sleep very poorly. I also noticed that when in the Caribbean, I can sleep wonderfully. I am looking for something to simulate light on dark days and am considering therapeutic glasses such as these They would be easier to pack than a light box and can be used whilst moving about. They use a blue light and are intended for use in the morning. Do you have an opinion on this? I am also interested in your notes on glasses to cut out the blue light from the TV and will try this.

    1. Hi Jackie,

      Thanks for the comment. You know – I was talking to some friends of mine who work night shifts in hospitals, and they were saying that when the lights are off for the patients, it makes them sleepy! I instantly thought of the potential for light-therapy glasses, and was considering looking into them, and if they didn’t exist, perhaps develop them.

      Turns out they exist! Thank you for showing me. I have never used them, but the theory is sound. If they truly are as effective as a light box, and are more convenient, then they get my vote of confidence. Please let me know how it goes if you try them out. I’m very curious.

      I consistently use my orange blue-light blocking goggles after 9:30 pm, and have been very successful with these. That said – the other side of the equation is getting enough blue light during the day. If you can’t do so naturally, then these products are very effective.

      Thanks again,


      1. Thank you for replying. Luminette are not the only therapeutic light glasses – there is also a Danish make My only concern is some research that suggests that blue light exposure could be linked to macular degeneration . The Belgian glasses seem to be in the specific light range that is said to be only beneficial and both makes state that they are UV free.

        1. Hmmm…well, I’m not a complete expert in the matter, but I’ll give you my take.

          We’re designed to take blue-light in the eyes. It could cause small amounts of damage (like Vitamin D creating UV light), but with proper diet and sleep habits, you’ll recover fine. It’s not like these things will be on 24/7. We’re talking 30 minutes, tops, right?

          I’d say the improvement in your sleep would be worth it. If you notice any pain while wearing them, by all means stop! I doubt that’ll happen though. But as for blue light – I go outside in the bright sun as much as possible with no sunglasses, which would be 1000x+ what these glasses would put out (granted, with a more ‘full’ spectrum, but including UV), and my eyes are great. I’m 31 (young, but not that young) and have no signs of degeneration.

          It’s all about stress-recovery. If you were blasting your eyes 24/7 with a bad diet and little sleep, then yes, undoubtedly your eyes would be damaged. I can tell from your writing that that would not be the case for you.

          Just my 2 cents :) Hope it works out!

    1. Hey Allison! Thanks for the comment.

      Interesting about the eye question. I think it’s along the lines of what I think for skin: provided you consume a low-inflammatory nutritious diet (see Sunlight Series Part 10: Sunburn Resistance Through Diet), I think your eyes are up to the task of taking in sunlight. That article doesn’t give any specific recommendations, or present any compelling evidence that the sun will destroy your eyes. I think it’s healthy.

      Rule of thumb: If it’s a natural stressor (the sun is very natural), and the way you’re doing it feels good, it’s great. If you get any significant pain from exposing, or over-exposing, your eyes to the sun, then I would start thinking about sunglasses. Also – it’s best not to stare directly at the sun. The pain from doing that makes it obvious that it’s too much. Just do what feels natural – have your eyes unprotected and sunlight entering them, but not direct sunlight (for any significant length of time anyways). If you need to take a break from the sun here and there too, that is perfectly ok :)

      I’ve never experienced any problems, and have only had positive results from sunlight entering my eyes.

      Thanks again for the comment!

      1. Your suggestion makes a lot of sense to me. Lately, I’ve been gauging in my body reactions to determine when to stop sun exposure. It seems like the best indicator because I’ve never gotten any major sunburn of sunsickness and I feel great afterward. Thank you for your response 😀

  3. Awesome post. That infographic is great, too! One thing I try to do every day is go for a walk at lunchtime and make sure my eyes see the sun. Also, no smartphones in my bedroom!

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