What is the body clock, and how does it decide when I'm sleepy?

After dinner the tiredness can get so strong that the sofa starts to feel like bed. Then, somewhere around ten, it goes. By the time you lie down, your mind is clear and busy. If you're reading this late, you may be right inside that turn, in the stretch when the clock pushes hardest to keep you up⁸⁹8 Dijk & Czeisler, 19949 Dijk et al., 1999.

Two things are working as a pair. One is a sleep pressuresleep pressure The body’s need for sleep, which builds the longer we stay awake and eases while we sleep. that builds from the moment you wake, so by evening it's strong. The other is your body clockbody clock The timekeeper in your brain that runs on a cycle of about a day, setting when you feel sleepy and when you feel alert. Daylight, especially in the morning, keeps it in step with the clock on the wall., which sends its own signal through the day and night. Together the two set when you sleep and how deeply⁵5 Borbély et al., 2016. Through the day the clock works against that growing sleepiness, and around your usual bedtime it stops.

There is, and it keeps time in more places than you might expect. The one in charge is deep in the brain, in a small cluster of cells in the hypothalamus called the suprachiasmatic nucleus. It acts as the master clock for the body's daily rhythms, and it takes in light directly from the eye¹1 Mohawk et al., 2012. Researchers worked most of that out in rodents, and what we know about people is the textbook view based on their work.

Then there's how widespread the timekeeping is. When researchers take cells and tissues out of the body and grow them in a dish, far from any brain, they keep going through their daily rhythm¹. Nothing tells them what hour it is, and they keep one anyway. Cells and tissues all through the body keep time like this¹. So what's left for the brain's clock to do? It keeps them together, the way a conductor keeps an orchestra in time. In animals whose master clock has been removed, the clocks in different tissues drift out of time with one another¹. When you eat can nudge the clocks outside the brain too¹.

People noticed this inner timekeeping long before anyone could find it. In the 18th century the astronomer Jean Jacques d'Ortous de Mairan shut a mimosa plant away in constant darkness and watched its leaves go on opening and closing on their daily rhythm¹⁰10 Nobel Assembly at Karolinska Institutet, 2017, with no sun to tell them when. Centuries later, the 2017 Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for working out how the clock is built inside a cell, in fruit flies, and the same principles run in human cells¹⁰.

Left to itself, your body clock keeps a day of its own, and it runs a little longer than the world's. In most people its own cycle lasts about twenty-four hours and nine minutes²¹¹2 Duffy et al., 201111 Czeisler et al., 1999, though some people's run shorter than a day². Those nine minutes come round every day, so your inner day keeps ending after the outer one. With nothing to pull it back, your evenings would slide later night after night.

What pulls it back is light, and the eye has its own cells for the job. Alongside the rods and cones we see with, there's a third kind of light-sensing cell¹1 Mohawk et al., 2012. It responds slowly and needs fairly bright light, which suits it to reading the day rather than a passing flash¹, and the rods and cones feed into it as well. Mice with no working rods or cones still reset their clocks to light¹²12 Do & Yau, 2010, which shows this pathway running on its own, apart from sight. And in some people blinded by the loss of their rods and cones, light still set their clocks and switched off their melatonin¹², though they couldn't see it at all.

By ten in the evening you've been awake longer than at any other point in the day, so the pressure to sleep is at its height. Why, then, is that so often the hour a second wind arrives? Researchers went looking for the answer by pulling the clock and the pressure apart.

In one experiment, eight men lived in time isolation for over a month on long artificial days, so that their sleep fell at every hour of their body clock's cycle⁸8 Dijk & Czeisler, 1994. That let the researchers measure the body clock and the sleep pressure separately, and the two turned out to matter about equally to how unbroken sleep stayed through the night⁸. Put back into an ordinary day, their timing looks almost back to front. The clock's push to stay awake peaks in the evening, just as pressure is highest. Its push to sleep peaks in the early morning, when pressure has nearly gone⁸⁹9 Dijk et al., 1999. So the two balance each other. That's how you can stay alert through a long day, and sleep on through the hours when you've already had most of the sleep you need.

Your evening has a chemical marker too. The pineal gland makes melatonin, the body's signal of biological night: it's released during the clock's night, and light both resets its rhythm and switches its production off¹³13 Gooley et al., 2011. In eight adults living their ordinary lives, it began to rise roughly two hours before they fell asleep⁴4 Wright et al., 2013. You might expect sleep to come easiest in those last evening hours, with the pressure high and the night signal close. When researchers measured how readily people fall asleep across the clock's cycle, they found the reverse: in the hour or few hours before people's usual bedtime, falling asleep is harder than at any other time¹⁴¹⁵14 Strogatz et al., 198715 Lavie, 1986. They call this stretch the wake maintenance zone, or the forbidden zone for sleep.

It shows in how sharp people are, as well. In a small lab study, 31 young adults kept awake for about two days did better on attention tests in those evening hours than they had earlier in the day, though they'd been awake longer¹⁶16 Shekleton et al., 2013. If you're worn out by ten and yet something keeps you up, this evening window is probably the second wind you're feeling. If that's where you are tonight, Why am I wide awake at bedtime? has more on what helps. By the time your usual bedtime comes, the window has passed, and sleep comes more easily than it would have an hour or two before.

Light reaches you at different hours on different days: the window over breakfast, the street on the way to work, the lamp beside you late in the evening. Which hour it arrives in decides whether it moves your clock earlier or later. Researchers mapped this by sitting 21 adults in a lab under hours of very bright light, each at a different point in their body clock's cycle. Light in the late evening and the first half of the night, before the body's lowest temperature, pushed the clock later. Light after that point, toward and into the morning, pulled it earlier³3 Khalsa et al., 2003. No long stretch of the day left light without effect, though at the turning point between later and earlier it caused no shift³. That lab light was far brighter, and lasted far longer, than most days bring, so the size of those shifts may not apply to yours. The direction does. Since the clock sets when sleepiness comes, light late in the evening tends to push your sleepiness later, and morning light tends to draw it earlier.

What happens when ordinary days change? For a small study, researchers took eight adults to the Colorado Rockies. The eight spent a week of their ordinary lives, and then a summer week camping, with only sunlight and campfires⁴4 Wright et al., 2013. Out under the open sky, the light reaching them rose more than fourfold⁴. By the end of that one week, every measure of their inner clock had moved about two hours earlier, and their melatonin began to rise near sunset⁴, at the hour the light outside was going.

Evening light works the other way, and it needn't be bright to matter: ordinary room light before bed is enough to make melatonin start later¹³13 Gooley et al., 2011.

If you're an owl, you probably know the nights of lying awake at eleven while someone beside you drops off in minutes, the mornings that arrive like the middle of the night, and the way you come alive in the evening just as everyone else is fading. Larks live the same thing turned around. Both are part of a wide spread: across American adults, the middle of weekend sleep spans almost ten hours, and the middle half of people have theirs between about 2:24am and 4:15am²¹21 Fischer et al., 2017. A midpoint of 4am is someone asleep at midnight and up at eight.

Some of where you fall in that spread comes with you from birth. Finnish researchers asked more than eight thousand pairs of adult twins whether they were morning or evening types, and found that genes accounted for about half the differences between people⁶6 Koskenvuo et al., 2007. Since then, gene hunters have searched the genomes of hundreds of thousands of adults and found hundreds of places linked with being a morning person⁷7 Jones et al., 2019. Each of those places matters only a little: the five percent of people carrying the most morning variants sleep on average about twenty-five minutes earlier than the five percent carrying the fewest⁷. Added together, the common variants the gene study could measure explain about a seventh of the differences between people⁷, well short of the twins' half. Nobody has fully explained that gap. Part of it is that the gene study counts only the variants it could measure, and the two methods differ in other ways too. What both find is that a real share of whether you're a lark or an owl runs in families. That part is something you came with.

The rest comes from the life you're living: age, sex and the light people live in explain a large share of the differences, alongside genes⁷²²22 Wittmann et al., 2006. Across a national sample, weekend sleep moves later through childhood and the teenage years, is latest around age 19, and then drifts steadily earlier, until people of 70 sleep at much the same times as 10-year-olds²¹.

Each of them opens a gap between the clock inside you and the day outside. After a flight across time zones, the day outside has moved all at once, and the clock inside is still set to the place you left. The clock times sleep, hormones, body temperature and metabolism, so you feel that mismatch for as long as it lasts¹⁰10 Nobel Assembly at Karolinska Institutet, 2017. Sleepiness, hunger and wakefulness turn up at the wrong hours. Night work asks you to be awake through your clock's night and asleep through its day, when its push runs the other way⁸⁹8 Dijk & Czeisler, 19949 Dijk et al., 1999. How do I get over jet lag faster? and How do I sleep well on night shifts? take each in turn.

A weekend can open the same gap on a smaller scale, and you may know it best on a Monday, when the alarm rings while your clock still thinks it's night. In 15- to 17-year-olds sleeping at home, one ordinary weekend of going to bed an hour and a half later and sleeping in three hours later pushed their clocks about forty-five minutes later by Sunday²³23 Crowley & Carskadon, 2010, and an hour of morning light on the weekend mornings didn't stop it²³. Why can't my teenager fall asleep before midnight? has more for parents. Adults weren't studied, though the same may happen to them and be part of what makes Monday mornings so hard.

The clock inside you runs a few minutes long, and each day the light that reaches you tells it where the morning is. The time you grow sleepy is partly what you were born with and partly what your days keep setting, and tomorrow, when the curtain opens or the back door swings out onto the first gray light, it will be setting it again.