The Science of Sleep: Understanding Your Circadian Rhythms

Let's make it plain. Circadian rhythms are daily biological rhythms of physiology and behavior, generated inside the body. We all have them. Think of it as daytime physiology and nighttime physiology. In my teaching, circadian rhythms are the secret to sleep health. The 2017 Nobel Prize in Physiology or Medicine went to three scientists who, working in fruit flies, uncovered the molecular mechanisms behind circadian rhythms.^[6]

Where does yours live? The master clock in your brain is called the suprachiasmatic nucleus. It sits just above the point where the nerves from your eyes cross, and a direct nerve pathway from your eyes tells it about light.^[21]

Left alone, that clock runs a little long. In people living in carefully controlled lighting, it averaged about 24.2 hours.^[1] So it needs a daily nudge, mostly from light.

Picture an orchestra. Before the conductor walks on, it isn't music, it's noise. The master clock is the conductor, and the clocks in your organs and tissues, such as fat tissue, are the instruments.^[5] When they drift out of step, we call it internal desynchrony. That's why at NEOS we're sleep and circadian rhythms specialists, not just sleep coaches.

Two forces decide it. The first is sleep pressure: the longer you stay awake, the more your body wants to sleep. In a study in cats, a chemical called adenosine in the brain rose during sustained wakefulness and fell slowly during recovery sleep.^[7] I call the process that keeps track of the sleep your body needs your sleep accountant.

In a sense, wakefulness is toxic: our brain is like an engine, and it produces exhaust.

The second force is the body clock. Researchers call this the two-process model, and it remains a useful framework.^[2]

As I teach it, we are built for roughly a 16-hour day and an 8-hour sleep window. As sleep pressure builds, the clock pushes back harder for wakefulness, which keeps you steady. At night, both push for sleep. After about four hours of sleep, most of the pressure is gone, and the clock carries you through the second half of the night. In my teaching, that is the sleep maintenance zone.

Ever lie awake in bed? In my experience, when people can't fall asleep when they want to, it comes down to one of two things. They haven't built up enough sleep pressure, or they're trying to sleep at the wrong time on their clock.

Does the afternoon slump ever catch you, mid-meeting and reaching for another coffee? It is not a lack of willpower. A review concluded that the post-lunch dip is real and can occur even without lunch, though only for some people and some tasks. A high-carbohydrate lunch worsens it.^[4]

In my teaching, for a regular sleeper the lull comes about six hours after waking. Rough numbers.

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Then comes the second wind, the one that shows up just when you are trying to wind down. In small lab experiments, people tracked after a night without sleep showed a big overnight sleepiness peak and a smaller afternoon one. Between them sat a "forbidden zone" for sleep, centred at around 8 to 10 p.m. in those studies.^[8] Researchers call it the wake maintenance zone. Usually in the hours just before your normal bedtime, it is a window when people rarely fall asleep, even when they try.^[20]

People tell me, "I can fall asleep anywhere, anytime." In my experience, that is usually sleep deprivation, not a talent. If you often doze off at the wrong moments, talk to your physician.

Let's talk about light. Bright light is regarded as the primary time cue,^[9] and its timing matters. Here is the rule I teach: light early in your night pushes your clock later, and light after your body-temperature low point, think early morning, pulls it earlier. A lab study of 21 healthy people found that pattern. Very bright light before the low point caused delays, and light after it caused advances, with about five hours between the biggest of each.^[3]

Think of it as travel. From Vancouver, a delay is like flying to Maui, and an advance is like flying to Toronto.

Everyday evening light matters too, because it delays the clock. In one small lab study, reading on a light-emitting e-reader before bed, compared with a printed book, meant longer to fall asleep and a later body clock.^[11] In a retrospective analysis of 116 healthy 18-to-30-year-olds in a research unit, ordinary room light before bed suppressed melatonin. It started later in 99% of people and lasted about 90 minutes less than in dim light.^[10]

What about blue-blocking glasses? I recommend them in my coaching: good amber lenses for the two hours before bed. The evidence is mixed: a 2023 Cochrane review could not say whether blue-light filtering lenses improve sleep quality.^[12] So let's dim the house too, and treat the glasses as one tool, not a cure. Never wear them while driving.

Yes, the other clocks in your body. Let's start with meals. As I teach it, one clock is set by light and another by when you eat. In a 13-day lab study of ten young men, six days of meals five hours later delayed the blood-sugar rhythm by nearly six hours. Melatonin and cortisol, markers of the master clock, did not move.^[5]

My rule of thumb: breakfast like a king, lunch like a prince, and keep dinner the smallest meal. What I focus on is when the big meals land.

Exercise can shift clocks too, depending on when you do it.^[9]

Whenever you get to set your own schedule, if you stay up later and later and sleep in longer and longer, you are a night owl. If you cannot sleep in even on days off, you are an early bird. If you don't know whether you're a night owl or an early bird, you're probably neither. You're somewhere in between, and the advice you need is different again. Scientists call this your chronotype.^[13] In my experience, one-size-fits-all approaches lack impact, because we're all individuals: advice that suits an owl can backfire on an early bird.

I came to this field honestly: I'm an extreme night owl, and as a young person I couldn't sleep. I avoid blue light early in the night and get light in the morning. It can work: in a small study of eight adults, after a summer camping trip with only natural light, body clocks ran earlier, and later chronotypes moved the most.^[15]

Trouble starts when work forces an owl up early. Late chronotypes build sleep debt on workdays and catch up on days off. Researchers call the gap between workday and day-off timing social jetlag.^[13] The good news: your timing can be moved.

Get bright light soon after waking, ideally outdoors, if you're a night owl. If you're an early bird waking too early, flip it: dim mornings, light in the late afternoon and early evening. In between? You're one of the lucky ones: light at either end of the day won't throw you off as much. We suggest a change, then watch your data to see whether it's moving you in the right direction. If it isn't, we course-correct. In dark winters, I teach adding a light box. With an eye condition, a mood disorder or low winter mood, talk to your physician first.

In a 10-day lab study of 10 adults, eating and sleeping about 12 hours out of step with the body clock lowered leptin by 17%. It raised blood glucose 6% despite 22% more insulin, and reversed the daily cortisol rhythm.^[16] That was a short lab study, not everyday life.

If you work shifts, this one is for you. A large BMJ review linked shift work with accidents, type 2 diabetes and coronary heart disease, though the original studies showed mixed results.^[17] Within any shift system there are ways to adapt: 24-hour shifts, rotations, evenings and permanent nights. See the night-shift sleep schedule guide.

Timing also shapes risk on the road. Road accident risk shows a clear daily rhythm, with a major peak around 3 a.m.^[18] In UK data, sleep-related crashes peaked around 2 a.m., 6 a.m. and 4 p.m.^[19] Those peaks sit near the overnight low and the afternoon lull. The good news: these windows are predictable, so we can plan around them. Managing people who work around the clock? See sleep health for corporations.

Think of it as a day that tells your clock what time it is. Build a sleep window and protect it with what you do during the day, and at night when you can't sleep. Do those two things, and your sleep accountant will balance the books. Over time, that anchors your clock. This is my method, not a study result. It's your goal, not ours: where are you on your journey, and how far do you want to go?

Be patient with yourself: give it four to six weeks of protecting your sleep window. That is my coaching experience, not a study result. Shift workers: napping works differently, see the night-shift guide above.

And remember: it's never too late to improve your sleep, and your sleep is never so bad that it can't be improved.

Sometimes habits are not the whole story, and that is okay. If you have kept a steady routine for several weeks and still cannot sleep when you need to, talk to your physician. Some people have a circadian rhythm sleep-wake disorder, such as delayed sleep-wake phase disorder, advanced sleep-wake phase disorder, shift work disorder or jet lag disorder.^[22][23] A physician can assess and diagnose these. Loud snoring or gasping at night also needs a physician. I don't recommend melatonin or sleep medications as a fix. If you're using them, talk to your physician before changing anything. If you are not sure where to begin, the free sleep assessment is a good first step. It is not a diagnosis.