Power nap: how twenty minutes beats an hour

Pilots on long-haul routes over the Pacific were once given permission to do something that sounds like a bad joke: fall asleep during the flight. It was a controlled study, the second pilot kept watch, and the results changed how short daytime sleep is seen.

A NASA team led by the sleep researcher Mark Rosekind published data in 1994 from flights where some crews were given a window for a nap during a quiet phase. The pilots who took it slept for about twenty-six minutes on average. Against colleagues who spent the same time resting in their seats without sleeping, they did measurably better in the final and most demanding stretch of the flight: performance improved by roughly a third and measured alertness by more than half.

So a short sleep in the middle of the day is not a sign of softness. It is a tool the aviation industry had properly tested, because it needed one. The name that stuck is power nap, and it describes the job accurately: a brief sleep meant to restore attention, not to replace a night. It works outside the cockpit too, but it comes with rules, and most people break them with the very first decision, which is how long to sleep.

Twenty minutes beats an hour

The reasoning that fails sounds perfectly sensible. If twenty minutes helps, an hour must help more. With sleep it is not the time that adds up, but how deep you get and which stage the alarm catches you in.

For the first twenty minutes or so you stay in the light stages of sleep. The brain surfaces from them quickly and without resistance, because it has nothing there left to finish. After roughly half an hour you start dropping into deep sleep, where the body slows the breath and lowers its temperature to switch into a mode you cannot leave on command. The alarm then pulls you out mid-task.

What you get is sleep inertia. Eyes that will not open properly, a heavy head, speech running half a second behind the thought. It usually lasts fifteen to thirty minutes and in the worst cases longer, which gives the hour-long nap its nasty side effect: plenty of people feel worse afterwards than before. They did not nap badly. They picked a bad length.

There is more behind that twenty-minute line than sleep stages alone. While you are awake, adenosine builds up in the brain, a substance that gradually dampens alertness, and even a short sleep knocks its level down. The light stages are enough for that. Deep sleep is busy with other jobs that twenty minutes cannot finish, so what you harvest from it is side effects rather than benefit. A power nap is therefore not a miniature night. It is a different operation with a different goal.

Ninety minutes is the odd case out. Anyone carrying a real sleep debt with a free afternoon can ride out a full cycle and wake at the end of it in a light stage. For an ordinary working day it is unusable. What remains is twenty minutes, or twenty-five if you count the time it takes to drop off.

When to take it so it does not steal your night

The afternoon drop in alertness is not caused by lunch, however often that gets repeated. The internal clock has a dip built in roughly seven to nine hours after you wake, and it turns up even if you eat nothing at all. The body slows, attention blurs, and you read the same paragraph for the third time. We take that mechanism apart in more detail in the piece on the afternoon slump.

That is exactly where the nap belongs. For most people it lands somewhere between two and half past three, earlier for morning types, sometimes as late as five for evening types. When do you find yourself on the third pass through the same paragraph? That moment is a better guide than any table.

One boundary holds, though. After four in the afternoon a nap usually stops paying off, because it draws on sleep pressure, the appetite for sleep you build up over the course of the day. Spend it at four and you have nothing left to pay with at eleven, so you lie in bed studying the ceiling. The short sleep steals the night, and the next day starts in the red.

Daniel, a sales rep working from home, ran the same script for years. The slump arrived around two, he made himself a third coffee and got through the afternoon calls at half power. When he swapped the coffee for twenty minutes on the sofa, two things caught him out. The calls were noticeably better, which he had expected. What he had not expected was falling asleep in the evening without an hour of turning over, for the first time in ages, because he had stopped pouring caffeine into himself at three in the afternoon.

Coffee right before a nap sounds like a contradiction

The combination that looks self-defeating is one of the best-tested tricks around short sleep. You drink a coffee and lie down straight away for twenty minutes. It goes by the name coffee nap, and the whole thing rests on timing.

Caffeine needs roughly twenty to thirty minutes to reach the blood at full strength. In the brain it blocks the receptors for adenosine, the substance that piles up while you are awake and produces the feeling of tiredness. And here is the point: a short sleep lowers the adenosine level. The caffeine therefore arrives in a tidied room where it has less competition, and it arrives just as your alarm goes off.

This is not only a laboratory curiosity. Louise Reyner and Jim Horne tested tired drivers in a simulator in 1997 and found that caffeine combined with a short nap suppressed the signs of sleepiness at the wheel more than either of the two on its own. Drivers who had both made fewer mistakes than those who had only one of them.

A practical note: it has to be a coffee drunk quickly, not sipped for half an hour by the window. And if you are sensitive to caffeine, or it is already late in the afternoon, skip this one. The version without coffee works as well, it just takes longer to arrive.

When you cannot fall asleep on command

The most common objection is that it simply does not work for you. You lie down, your head keeps running, and twenty minutes later you get up just as tired, now with a sense of failure on top. The good news is that falling asleep is not actually required.

Lying quietly with your eyes closed and nothing coming in has an effect of its own, known in the literature as quiet rest. Stripped of visual input and notifications, the brain slows down even without sleep and part of the fatigue drifts off. It is not as strong as sleep, but it is a long way from nothing. Let yourself off that hook and you will fall asleep more often, oddly enough, because the question of whether you are asleep yet stops circling in your head.

A few things raise the chances:

• Set the alarm for twenty-five minutes. Five of them cover falling asleep, the rest is the nap itself.
• Phone on silent and out of arm's reach. One notification is enough to bring the whole attempt down.
• Go to the same spot, whether that is the armchair in the living room or the car in the car park. The body links it with rest faster than you would expect.
• A duvet and pyjamas send the signal for a long sleep, which is precisely what you are not after.
• Light keeps the internal clock in waking mode, so it pays to darken whatever you can.

With a racing head it helps to give your thoughts a simple job. Following your own breath is the easiest version. Some people do better walking through the body from the feet upwards, or replaying a dull familiar route, the commute stop by stop. This is not meditation and not a technique you have to master. The idea is to occupy attention with something so bland that your worries find nothing on it to catch.

The first attempt usually fails, and that is normal. Falling asleep quickly during the day is largely a learned skill, and the brain takes a week or two to get used to the new signal. If you are still lying there thinking after five tries, swap the nap for twenty minutes of walking outdoors. The effect on afternoon attention is a different one, but it is just as real.

Who napping does not suit

The honest answer is that daytime sleep is not for everybody, and it is certainly not compulsory. Some people have their night wrecked by the shortest of naps, even one taken at noon. Others wake up wrung out after fifteen minutes, because their sleep inertia simply runs longer.

How stable your rhythm is plays a part as well. Someone with a steady rhythm, who goes to bed and gets up almost to the minute and needs several days to recover from a shift in the schedule, tends to be more sensitive to sleep during the day. People with a flexible rhythm can usually nap almost whenever without their night paying for it.

Testing it is easy enough. Give the short sleep a run of five attempts at a similar hour and watch two things: how you feel an hour after waking, and how long it took you to fall asleep that evening. If the second question keeps coming out badly, you have your answer. A nap is no measure of discipline, it is one option for handling the afternoon drop, and a quiet walk or moving demanding work to the morning are full alternatives to it.

Where to find those twenty minutes

The usual obstacle is not the internal clock but the calendar. Most people have their afternoon spoken for, and twenty minutes on the sofa looks like a luxury reserved for somebody else. Yet the same twenty minutes often vanish into a phone right after lunch, and nobody gets up from that rested.

The timing also shifts with the way your internal clock runs. An owl who lies down at two in the afternoon may find the dip is only just starting and that the real window sits two hours later. A lark hits the low earlier, so by mid-afternoon a nap is already eating into the night. The gap between early birds and night owls is described in more detail in our guide to chronotypes. If you have no idea where you sit on that axis, a rough chronotype test will show you when to expect the dip for your type.

Try one thing tomorrow. When the moment comes where you would reach for another coffee, set an alarm for twenty-five minutes instead and lie down, even if all you manage is closing your eyes in the car. Where in your day would those twenty minutes sit that nobody else would claim?