Why are some nights better than others?

A night like that can feel like a gift from nowhere, and it's tempting to give the credit to whatever came last: the cup of tea, the new pillow, the phone left in the kitchen. Yesterday's hours did most of the shaping, while you were busy with other things. From the moment you woke, a pressure to sleep was building with every hour you stayed up²2 Dijk, 2009. The time you got out of bed was doing its own work too. It set the hour your body clock would let that pressure have its way³3 Burgess & Eastman, 2006. When the two meet well at bedtime⁴4 Borbély et al., 2016, sleep tends to come easily and run deep. The rest is mostly the ordinary give in a living body, which seldom sleeps the same way twice.

How much do a good sleeper's nights really vary? Researchers pooled the records of 2,404 healthy adults in the US and Australia, leaving out anyone with signs of insomnia. Each wore a small motion sensor on the wrist at home for a week or two. Over those nights, each person's sleep typically strayed about an hour and a quarter either side of their own average, seventy-seven minutes on the sensor¹1 Messman et al., 2022, and plenty of nights went further than that. So if your nights swing, a good one here and a restless one there with no pattern you can see, an hour longer or shorter than your usual is an ordinary swing for good sleepers like these.

The youngest swung most: the 18-to-24s had the most variable sleep of any age group¹, perhaps because their days run less regularly. Most of the people in the study were young, White and college-educated. If you're older, your own nights may run a little steadier. And what drives the swings? The researchers point to a mix of things around us, in our minds and in our bodies¹, rather than any one cause. If you did everything you meant to and still slept badly, that fits what they saw too: poor nights come to good sleepers as well¹, and there may be no one thing you could point to.

From the moment you woke yesterday, something began to gather that sleep researchers call sleep pressuresleep pressure The body’s need for sleep, which builds the longer we stay awake and eases while we sleep.. They can see it in the slow, large waves of the sleeping brain. The longer someone has been awake, the more deep slow-wave activity their sleep has when it comes²2 Dijk, 2009, and it drains away again as they sleep through the night². Every hour awake adds a little. A long, full day, up early and busy until evening, brings plenty of it to bed, enough for a good stretch 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..

A nap can draw on that store before the night has begun. In a lab study, healthy young men took a nap in the early evening. That night they were slower to fall asleep, with less deep sleep in its first hours⁵5 Werth et al., 1996. That may be part of the story behind a restless night after a doze on the sofa that ran on into the evening. A short nap early in the afternoon is a different case, and Are naps good for you? looks at it more closely.

The time you got up yesterday set the other force in your night: the 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 keeps its own time alongside the building pressure. You may know it as the second wind that comes in the evening, just when you'd expect to be running out of steam. Why would the clock push you awake then, of all times? It pushes hardest in the hours just before your usual bedtime⁶⁷6 Dijk & Czeisler, 19947 Lavie, 1986, keeping you awake while the pressure is already high, so you don't drop off in the early evening. Around your usual bedtime the push falls away. Then the two forces together give you one long, unbroken night²2 Dijk, 2009.

The time you get up can move that clock, probably through the daylight that meets you when you do. In one study, fourteen healthy adults spent two weeks waking at their usual early hour and two weeks waking three hours later. After the later fortnight, the evening rise of melatonin, the body's signal that night has come, arrived 2.4 hours later³3 Burgess & Eastman, 2006. It was as though their clocks had followed their mornings. A fortnight of late mornings is more than one weekend. Still, it may help explain why a couple of lie-ins can leave Sunday night slow to come.

So when you look back over yesterday, the hours you were awake and the hour you got up are a good place to start. They aren't the whole of a night, though. These two forces make up a strong model of when we sleep and how deeply, in use for more than thirty years⁴4 Borbély et al., 2016. Worries at bedtime, health that day and the night before⁸8 Åkerstedt et al., 2012 have their say as well.

After a short night, the next one can come on like a tide, heavy and deep. What you're feeling is the pressure you didn't spend the night before, still there. Sleep labs have watched it happen. Researchers kept healthy volunteers awake for thirty-six hours, and when at last they slept, their sleep ran deeper early in the night²2 Dijk, 2009. Left to sleep as long as they liked, they slept for more than twelve hours². That's a far longer stretch awake than an ordinary short night.

Something like it seems to reach into everyday life too. In Sweden, fifty people rated their sleep each morning for six weeks, and a short night beforehand tended to be followed by a better-rated one⁸8 Åkerstedt et al., 2012. Part of what made last night feel good may have been the short one before it.

This is true of people who usually sleep well, and they're who those studies followed. When 106 people with long-term insomnia kept diaries for a month, about a third of them had poor nights that came unpredictably⁹9 Vallières et al., 2005. For them, a bad night didn't reliably bring a good one. If that sounds like your nights, Do I have insomnia? is worth a read.

Perhaps yesterday evening held a glass of wine with dinner, a worry you took to bed, a walk or a coffee later than usual. Some of these show up in the night, and some hardly at all.

Alcohol helps people drop off, then breaks up the second half of the night, at every dose studied in healthy volunteers¹⁰10 Ebrahim et al., 2013. It shows in people's own homes too, where nights were slightly more broken after a drink within four hours of bed¹¹11 Spadola et al., 2019. Does alcohol help you sleep? goes further into why.

The worry counts for more than it might seem. Those fifty Swedes, rating their sleep each morning for six weeks, showed something else: stress and worries at bedtime predicted a poorer night more strongly than anything else⁸8 Åkerstedt et al., 2012. So if a night went badly after you went to bed worrying about something, the worry is a likelier reason than the pillow or the late screen. People differ a great deal here, though: some sleep through stressful spells largely untouched, while others sleep much worse¹²12 Kalmbach et al., 2018.

The walk seems to help a little. A single bout of exercise tends to bring slightly more deep sleep and moderately less time awake in the night¹³13 Kredlow et al., 2015. The gains are small, but they fit the sleep that so often follows a long day outdoors. If you walked yesterday, it may have added a little to last night, on top of what the day had already built.

Coffee is the hardest to judge from the inside. In a small study at home, healthy sleepers took a large dose of caffeine six hours before bed. Their measured sleep fell by more than an hour, yet their diaries showed no clear difference¹⁴14 Drake et al., 2013. How could anyone lose an hour and not notice? Perhaps because how a night feels the next morning can miss what happened in it. An everyday study points the other way, though. Among people having their usual evening coffee, caffeine within four hours of bed showed no link with how they slept¹¹. How long does caffeine stay in your system? puts the two side by side.

Often you can, and the parts of a good day worth repeating are the plain ones: getting up at around the same time, with some morning light, and a full day awake with no long doze in the evening. A steady wake time lets the light set your clock to the same hour, and the full day brings plenty of pressure to bed with you. Late coffee is a smaller matter, though it may be worth noticing. None of it guarantees a night, because some swing from night to night comes to everyone¹1 Messman et al., 2022.

And when a night turns out ordinary, or worse, it's the same give that every sleeper's nights have. The pressure it leaves behind tends to make the next night a better one⁸8 Åkerstedt et al., 2012.