How long should a nap be?
Why twenty minutes and ninety minutes both work, and the length in between does not
There are two nap lengths that tend to leave you better than you started, and a long stretch in the middle that tends to leave you worse. Knowing which one you are taking is most of the decision.
The short one is around twenty minutes. The long one is a full cycle, around ninety. Everything between roughly thirty and seventy minutes is where the fog lives.
Why the middle is the problem
Sleep comes in cycles, and each cycle runs through lighter and deeper stages before surfacing again. Wake someone out of the deep stage and they come up slow, heavy and disoriented, and it can take a while to shake off.
A twenty-minute nap keeps you in the lighter stages, so you surface on your own before reaching the deep part. A ninety-minute nap lets you complete the whole cycle and come up at the other end of it. The gap between the two is where you get caught in the deep stage and dragged out of it.
The twenty-minute nap
This is the one to use in the middle of a day when you still have things to do. It buys alertness without the hangover, and it does not eat the afternoon.
The trick is to set the alarm for twenty-five minutes and lie down with the intention of resting rather than sleeping. If you fall asleep, you will come up before the deep stage. If you do not, twenty-five minutes of lying still is worth more than it sounds.
The ninety-minute nap
This is a proper sleep cycle, and it does more: it can make up some of a short night and leave you clearer for longer. It is the right choice when you are genuinely short on sleep and the afternoon is free.
It needs more room than people expect. Ninety minutes of sleep, plus the time it takes to drop off, plus the slow wake, is closer to two hours of your day. If that does not fit, take the short one instead.
Timing it so tonight still works
The later in the day you nap, the more it leans on the sleep you were going to have at night. As a rough guide, finish a nap by mid-afternoon, and leave at least six hours between the end of a nap and your usual bedtime.
If you are napping because last night was bad, keep it short. A long nap on a day you did not sleep well tends to take the edge off the tiredness you needed to build up for tonight.
If you cannot nap at all
Not everyone can fall asleep in the day, and lying there trying is its own frustration. Ten minutes of slow breathing with a longer exhale, eyes closed, gets a useful share of the benefit without needing sleep to arrive.
The same applies at your desk. Closing your eyes and slowing the breath for a few minutes is not a nap, but it is not nothing either.
Pick the short nap when the day is still going, and the full cycle when you are genuinely behind and have the room. Skip the forty-minute one, which is the one that costs you.