Nap Calculator

What It Does

Enter the time you can start a nap, or the time you have to be awake by, plus how long your window is, and the calculator returns the power-nap end and the times that one, two and three sleep cycles would end at from the same start. It also names the option whose middle your window's end lands in — the stretch where sleep inertia is heaviest — so a nap that looks long enough can be seen to leave you groggy. Everything is arithmetic over cycle boundaries.

Real-World Usage

The midday dip is real for most adults, and a short nap is how many people take the edge off it without losing the afternoon. A 20-minute nap that ends before the first cycle reaches its deep stage restores alertness without the grogginess of a longer one, and a 90-minute nap that completes a full cycle also tends to end on a lighter stage. The nap that causes trouble is the one that stops in the middle, at 60 or 70 minutes, which is why this page shows the boundary rather than a single figure.

Local Context

Sleep need and nap tolerance vary from person to person: age, medication, shift work and how much you slept last night all change what a nap does for you. Some adults never nap and do not need to; for others a short nap is part of the working day. The 90-minute cycle is an average across adults, not a personal measurement — if your own cycles run nearer 70 or 120 minutes, enter that figure in the field and the options move with it.

Is a 20-minute nap better than a 90-minute one?

They do different jobs. A 20-minute nap ends before the first cycle reaches the deep stage, so there is little sleep inertia to shake off and you can go straight back to what you were doing; it is the safest length when you must be sharp immediately. A 90-minute nap completes a whole cycle, from light sleep through deep sleep to REM, and also tends to end on a lighter stage — useful when you have the time and slept badly the night before. What the two share is the boundary: both end at a point where waking is easier than it would be ten minutes earlier.

Why does the calculator use 90 minutes for a cycle, and can I change it?

About 90 minutes is the average length of one adult sleep cycle, and published ranges run from roughly 70 to 120 minutes. The field is editable precisely because that is an average: someone whose cycles are shorter or longer gets a better answer with their own figure, and the whole option list moves with it. The source is named on the page (Carskadon and Dement, Principles and Practice of Sleep Medicine) so you can read it yourself.

Why do I feel worse after a long nap than after a short one?

Sleep inertia — the grogginess on waking — is heaviest when you wake out of deep sleep, which sits in the middle of a cycle. Waking early in a nap is easy because the nap never reached that stage; waking at a cycle boundary is easier because the cycle has already turned. Waking in the middle, roughly an hour into a 90-minute cycle, catches the deep stage and can leave you slow for up to half an hour. This page names that stretch for your own window instead of leaving you to guess it.

When is napping the wrong answer?

A nap treats sleepiness, not the cause of it. If you are short every night, the shortfall is what needs measuring and repaying, so start with the Sleep Debt Calculator; if the problem is when your night starts and ends, the Sleep Calculator times a full night around your own schedule. If you are sleepy every day despite enough time in bed, or you cannot sleep at night because you nap, that deserves a clinician's assessment — and the guide on this site about when sleep restriction may not be appropriate sets out the situations that should not be attempted on your own.