How much sleep do I really need?

Eight is probably the number you measure yourself against. At night it turns into a countdown, and by morning into a tally: a short week set beside what you were told you ought to get. Either way it's standing in for a plainer question, which is whether you're all right. How your days go answers that better than any count of hours.

If you're reading this awake in the night, with the alarm a few hours off, it may help to know that your body is made to recover from one short night, and that the next sleep tends to run deeper.

The reason is sleep pressuresleep pressure The body’s need for sleep, which builds the longer we stay awake and eases while we sleep., a drive toward sleep that builds through the day. It grows the longer you're awake and is spent by sleeping²⁴2 Dijk, 20094 Borbély et al., 2016. A short night leaves some of it unspent, and that leftover pressure makes the next sleep deeper in its first hours²³3 Van Dongen et al., 2003. After that, your sleep goes back to its usual pattern.

Even when short nights come in a run, the body makes good use of the little time it's given. In a US laboratory, a group of healthy adults spent five nights in a row with only four hours in bed, and they slept more efficiently than people given ten hours⁵5 Banks et al., 2010, sleeping through more of the time they had. That trial followed a run of short nights, and a single ordinary one hasn't been studied in the same way. Taken together, the mechanism and the trial point toward a small debt, which the nights that follow begin to repay.

Eight hours falls well inside what sleep experts in the US advise, though what they advise is a range. The National Sleep Foundation puts it at seven to nine hours¹1 Hirshkowitz et al., 2015 a night for adults, and the American Academy of Sleep Medicine recommends seven hours or more on a regular basis⁷7 Watson et al., 2015.

The academy's figure came from a panel of experts who read through the studies and voted on them, round after round. They agreed that six hours or less was too little and that seven to nine was appropriate. About six to seven hours, they could not agree⁸8 Consensus Conference Panel et al., 2015. If that's where your nights usually fall, it's fair to know the experts themselves were unsure.

Most of the studies they looked at had asked people once how long they usually slept⁸⁹9 Cappuccio et al., 2010, and an answer like that can reflect time in bed as much as sleep. So the range is a careful judgement about large groups of people, made without measuring anyone's own need. The words “on a regular basis” matter here, because they describe how you sleep across weeks and months. No single night decides it.

People differ in how much sleep they need, and that need is shaped by genes, habits, health and surroundings⁷7 Watson et al., 2015. The biology behind any one person's need isn't well understood yet⁷, and so there's no test that can tell you yours.

The clearest picture of that difference comes from a small study in Japan. Fifteen healthy young men, aged 20 to 26, lived for nine nights in a laboratory with twelve hours in bed each night, far more than anyone needs. Every one of them believed he was sleeping enough at home, yet on the first night they slept about 10.6 hours, more than three hours longer than usual¹⁰10 Kitamura et al., 2016. Night by night their sleep came down, and from about the fourth night it leveled off¹⁰, each man at his own place. These were men of the same age, sleeping in the same rooms, each with the same twelve hours in bed. Still, the shortest sleeper and the longest ended up about two hours apart¹⁰.

Thirteen of the fifteen had been sleeping less than they needed¹⁰ without knowing it, and across the group the shortfall came to about an hour a night. When the researchers looked at how sleepy each man was, hours alone told them little¹⁰. What mattered more was the gap between the sleep a man got and the sleep he needed, which is how the same seven hours can be plenty for one person and a little short for the next.

Your days are the most faithful guide you have. If you wake reasonably clear, get through the afternoon without fighting to stay awake and feel like yourself on most days, that tells you more about your own need than a count of hours can, because the same hours can be plenty for one person and short for another¹⁰10 Kitamura et al., 2016. No one has tested a daytime checklist like this, so it's a rough guide, but it looks at the part of your life that sleep is for. If your days are hard going week after week even with plenty of time in bed, Why am I still tired after a full night's sleep? looks at what else might be going on.

If you knowingly cut your sleep night after night, feeling fine can mislead you. In a US laboratory, researchers kept healthy adults to six hours in bed for two weeks. They built up thinking deficits as large as after up to two nights without sleep³3 Van Dongen et al., 2003, while rating themselves only slightly sleepier. It may be worth giving yourself more time where you can, even while you feel you're coping.

Perhaps your tracker has shown a red score after a night you felt fine about. In one laboratory comparison, 34 healthy young adults wore seven consumer devices for three nights. Most matched research-grade movement monitors on total sleep¹³13 Chinoy et al., 2021. They were poorer at noticing time awake, uneven on sleep stages and weaker on disrupted nights¹³, so a restless night can look like more sleep than it was.

Sleep clinicians in the US wrote up three patients for whom the score itself became the worry. One man's goal was “at least 8 h of deep sleep each night”¹⁴14 Baron et al., 2017. Chasing a better number, the patients spent longer in bed, which may have made their insomnia worse¹⁴, and one woman still doubted her sleep after a laboratory test showed plenty of deep sleep¹⁴. Their clinicians suggest using a tracker to watch broad patterns and time in bed, and noticing how its scores line up with how you feel, which is often less closely than people think¹⁴.

You can, though it takes more than one long sleep-in. After lost sleep, the next sleep runs deeper at first²³2 Dijk, 20093 Van Dongen et al., 2003, and the rest of the debt is repaid over the nights that follow.

In the US trial where people spent five nights with four hours in bed, each of them was then given a single recovery night, anywhere from none to ten hours in bed. The longer that night, the more their alertness and performance came back, in step with how long the recovery sleep was⁵5 Banks et al., 2010. Even after ten hours in bed, some sleepiness, fatigue and lapses of attention were still there⁵, and the researchers suggested that several nights may be needed⁵¹⁰10 Kitamura et al., 2016.

In a small laboratory trial, researchers kept young adults to five-hour nights through the working week, then let them sleep as long as they liked at the weekend. When the short nights came back, their weight rose and their insulin sensitivity fell¹⁵15 Depner et al., 2019 all the same. The trial tested a repeating week of short nights, and it didn't look at an occasional late one. The panel behind the seven-hour figure also likened short weekdays and long weekend sleep-ins to flying a few time zones west on Friday and back on Sunday night⁸8 Consensus Conference Panel et al., 2015, as 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. is pushed back and forth.

So the old saying that you can't catch up on sleep is only partly right. You can make up a good deal of lost sleep over a few nights, which the saying leaves out. Where it's right is that one long sleep-in won't clear a debt built over weeks.

This part is best read in daylight, if you can, because it's about habits kept up over years in large groups of people, and a hard week is well outside what it measures. Regularly sleeping less than seven hours is linked in population studies⁷7 Watson et al., 2015 with weight gain, diabetes, high blood pressure, heart disease and stroke, depression and earlier death, and with more errors and accidents. When researchers pooled large studies that followed people for years, those who usually slept short had about a 12% higher risk of dying⁹9 Cappuccio et al., 2010 over that time than people sleeping seven to eight hours. That is a relative difference, and on its own it can't tell you how large the risk is for any one person.

Researchers don't yet agree how much of this short sleep causes and how much it simply marks, and they disagree most about six hours a night. The mortality analysis found no evidence of harm from habitually sleeping six to eight hours⁹. It singled out five hours or less as the group at higher risk. The academy's panel also looked at laboratory studies, like the two-week one above. Most of its members tended to call six to seven hours too little⁸8 Consensus Conference Panel et al., 2015, without reaching agreement. Neither side called six hours ideal.

So a steady habit of five hours or less is where the concern is clearest, and seven or more is where both sides are comfortable. Between them, the experts are honestly split on six to seven, which is a reason to look after your sleep where you can, with no need for dread. If your sleep usually runs far outside the range, very short or very long, and it worries you, the expert statements suggest talking it over with a doctor, since that can be a sign of a health problem¹⁷1 Hirshkowitz et al., 2015.

Perhaps you've noticed that you wake earlier than you used to, or that you lie in bed longer than sleep seems to fill. The National Sleep Foundation's range for older adults is seven to eight hours¹1 Hirshkowitz et al., 2015, a shade below the seven to nine it gives younger adults. Its panel noted that need varies from person to person, and somewhat outside the range can be fine¹. In a US hospital study, healthy volunteers aged 60 to 76 who could sleep as long as they liked leveled off about an hour and a half below younger adults¹¹11 Klerman & Dijk, 2008, and the researchers couldn't tell whether they needed less sleep or found it harder to get. Children and teenagers need more, and have pages of their own: How can I help my child sleep? and Why can’t my teenager fall asleep before midnight?

If your nights have grown shorter and more broken over the years, that matches what recordings show: across adulthood, sleep tends to get shorter, lighter and more broken¹⁶16 Ohayon et al., 2004. In healthy people, though, most of those changes level off after 60¹⁶. Only the share of time in bed spent asleep keeps falling. And when researchers screened their volunteers more carefully for illness, the changes with age came out smaller¹⁶.

What about those long hours in bed? In that study, the older volunteers slept about 6.5 hours of 8.1 hours in bed¹¹. The researchers suggested that time in bed beyond what sleep now fills may feed lying awake and waking early¹¹, the kind of early waking you might have noticed. Even so, they found these same volunteers had some sleep debt¹¹.