What makes us feel sleepy?

Perhaps you're reading this an hour after a coffee that cleared your head and has since stopped doing it. So what was it hiding?

Every hour you spend awake adds to a pressure for sleep¹1 Dijk, 2009, and sleep is what drains it. Most of that draining happens in the deep sleep of the first hours of the night¹⁶6 Borbély, 2022. All day, 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. pushes back, keeping you alert while the pressure climbs. It pushes hardest late in the day, just as the pressure is highest³3 Dijk & Czeisler, 1994. Around your usual bedtime it eases off, and the pressure the day has built is free to send you to sleep.

Tonight's sleepiness, in other words, is made during today. Once you can picture that store filling by day and emptying by night, the coffee that hides it for a few hours and then lets it back starts to make sense, and so does the slow Sunday night after sleeping in. If you've come here worn out and still not sleepy, that question has its own answer below, in Why can I be exhausted and still not sleepy?

Sleep pressuresleep pressure The body’s need for sleep, which builds the longer we stay awake and eases while we sleep. is what researchers call the part of sleepiness that depends on how long you've been awake. Researchers can't see it directly, so they follow the trace it leaves in the large, slow brain waves of deep sleepdeep sleep The slowest, most settled stage of sleep, when it’s hardest to wake someone. Most of it comes in the first part of the night.. The longer people have been awake, the more of this slow-wave activity their sleep has, and across a night it falls away¹1 Dijk, 2009. It peaks in the first sleep cycle, and each cycle after rises less high¹⁶6 Borbély, 2022. Most of the draining is done early.

But how could anyone tell it's the hours awake doing this, and not simply the time of day? The answer came from volunteers who agreed to lose track of time. For weeks they lived with no clue to the hour, on days stretched so that their sleep fell at every hour of the body clock in turn¹. Whatever hour their sleep fell at, how deep it ran followed how long they'd been awake, and hardly at all the time of day¹. In every sleep, whenever it came, the slow waves faded as it went on¹. That's the store, filling with each waking hour and emptying in sleep.

Two forces are at work. One is the pressure, rising steadily from the moment you wake. The other is your body clock, which pushes for wakefulness by day and lets go at night. Researchers draw the two together in what they call the two-process model²⁶2 Borbély et al., 20166 Borbély, 2022. It's a model, a way of setting two forces against each other, and no one has found them as two separate parts of the brain. Even so, it has predicted the timing and depth of sleep well across many experiments for more than thirty years²⁶. In it, the clock pushes hardest for wakefulness late in the day, when pressure is highest, and hardest for sleep late in the night, once pressure has run down³⁷3 Dijk & Czeisler, 19947 Münch et al., 2005.

One of the neatest ways to see those two forces at work was also one of the most tiring to take part in. Twenty-two volunteers, after a night without sleep, lay down for seven minutes every twenty minutes and tried to sleep, around the clock, for a day or more⁸8 Lavie, 1986. With a whole night's sleep missing, you might expect them to drop off every time they lay down. Sleep came unevenly instead. They fell asleep most often at two points in the body clock's cycle: the large one at night, and a smaller one in the middle of the afternoon⁸. Another laboratory found the same two peaks in fifteen people living in time isolation, the second about nine to ten hours after the first⁹9 Strogatz et al., 1987.

The evening was stranger still. In the few hours before their usual bedtime, the volunteers found sleep unusually hard to reach, even with all that pressure behind them, because the clock's alerting signal was at its strongest⁸¹⁰10 Dijk et al., 1999. Researchers came to call that stretch the 'forbidden zone'. For those volunteers it ran from about 8 to 10pm⁸, and yours comes wherever your own clock puts it, a few hours before your usual bedtime.

So if 3pm is your sleepiest hour and 10pm your clearest, both forces are doing what they do. The second wind is real. It comes from your clock, which keeps you alert through those last hours and then lets the day's pressure through. The clock has its own page, What is the body clock, and how does it decide when I'm sleepy?. If bedtime often finds you alert, there's Why am I wide awake at bedtime?

When you drink a coffee, the caffeine blocks the receptors adenosine works through, so its signal can't get through⁵5 Reichert et al., 2022. It doesn't take adenosine away, and it can't undo the hours you've been awake. Going by how caffeine works, then, the pressure you've built goes on building out of sight, and it's still there when the caffeine fades. The same cup can also work differently from one day to the next. How much it lifts you depends on how much pressure you've built and how long it's been since your last coffee⁵, which suggests a short night changes what that cup can do for you. A coffee in the afternoon works on the sleepiness you have then. One taken late reaches into tonight as well: too close to bed, caffeine typically delays sleep, shortens it and makes it shallower, with less deep sleep⁵¹¹11 Clark & Landolt, 2017. And your cup may not do to you what it does to the person across the table, because people differ a great deal in how caffeine affects them, partly because of their genes⁵¹¹.

So why does the sleepiness come back around four in the afternoon? Nobody has yet measured a same-day 'crash' as such⁵. What we do know is how caffeine works, and that points to the simplest explanation: the pressure was waiting behind the coffee all along. If you drink coffee every day, something else may add to it. Going without caffeine leaves regular drinkers drowsy, tired and less alert, usually from twelve to twenty-four hours after their last dose¹²12 Juliano & Griffiths, 2004. Researchers who study adenosine suggest that some of the sleepiness regular drinkers feel during the day may be a mild form of that withdrawal, an idea they haven't yet tested⁵¹³13 Weibel et al., 2021. How long caffeine lingers is the subject of How long does caffeine stay in your system?

A nap in the evening spends pressure you'd otherwise have taken to bed with you. In a lab study of healthy young men, sleep after an early-evening nap took longer to arrive and had less deep-sleep activity⁴4 Werth et al., 1996. There wasn't enough time between the nap and bedtime for the pressure to build back up¹⁴1 Dijk, 2009. That study's men took a proper nap in a lab. A twenty-minute doze in front of the television hasn't itself been measured, so its exact cost is still an open question. The picture suggests it spends at least some of what you'd built for the night.

Earlier in the day, and after too little sleep, a short nap can do real good. After nights cut to about five hours, young good sleepers who took a ten-minute nap at 3pm were less sleepy, more alert and performed better straight away, with some of the benefit lasting more than two hours¹⁴14 Brooks & Lack, 2006. If naps are part of your days, Are naps good for you? goes into when to take them and for how long.

Getting up at ten instead of seven leaves you three fewer hours awake by your usual bedtime, and so less pressure behind you when you lie down. No study of sleeping in has measured that, but it follows directly from how the pressure builds.

Sleeping in seems to move the clock later too, and researchers have measured that part in teenagers. When 15- to 17-year-olds went to bed an hour and a half later and woke three hours later at the weekend, the evening rise of their melatonin had moved about forty-five minutes later by Sunday¹⁵15 Crowley & Carskadon, 2010. Adults' clocks haven't been tested this way. If they shift too, the clock would keep sleep away until later on Sunday evening.

If your morning in bed follows a week of early starts, a night up with a child or a run of late shifts, you'll know why you took it. It may help on Sunday night to know why sleep is slow to come, and that dread of Monday isn't the only thing at work.

The body makes up for a bad night mainly by sleeping deeper, above all early in the night¹⁶1 Dijk, 20096 Borbély, 2022, and only partly by sleeping longer. That's why you can sleep soundly the next night and still wake not much later than usual. In one trial, volunteers spent five nights in a sleep lab with four hours in bed each night. Then came a single recovery night, and some of them were given ten hours in bed¹⁶16 Banks et al., 2010. Those given ten hours slept about nine, longer and deeper than usual, and yet their alertness still hadn't fully returned¹⁶.

Over a longer run of short nights, how you feel and how well you manage can drift apart. In a two-week trial, people kept to four or six hours in bed did steadily worse on attention tests, while their own sleepiness ratings rose at first and then barely moved¹⁷17 Van Dongen et al., 2003. After the first few days, people stopped feeling much sleepier, even as their attention kept slipping. The trial measured only those two things, attention and how sleepy people said they felt. In the two-process picture, the pressure goes on building underneath after the feeling levels off. What brings alertness back is the fuller nights that follow, perhaps more than one¹⁶. For the day itself, there's How do I get through the day after a bad night?

Part of the answer is in the word 'tired', which covers two different things. Sleepiness means being likely to drop off. Fatigue means feeling drained. The two are related but different, though everyday speech calls both of them 'tired'¹⁸18 Shen et al., 2006. A day can leave you with plenty of fatigue and too little sleepiness. That's how you can lie there exhausted and wakeful at once. If that's a familiar night, Why am I wide awake at bedtime? looks at what may be going on. If it has gone on for a while, Do I have insomnia? is the place to turn.

Every hour you spend awake today goes into tonight's sleep. Sleep spends it most freely in the first hours of the night¹1 Dijk, 2009, and by morning the store is low again.