Worked example: lying down at 2:30 pm
| Nap | Length | Alarm | Grogginess on waking |
|---|---|---|---|
| Power nap | 20 min | 2:57 pm | Low |
| Short nap | 30 min | 3:07 pm | Low |
| Slow-wave nap | 60 min | 3:37 pm | High |
| Full cycle | 90 min | 4:07 pm | Low |
Every alarm includes seven minutes to fall asleep, which is why the twenty-minute nap ends at 2:57 rather than 2:50.
Why the sixty-minute nap is the bad one
This is the useful thing on the page. Grogginess after a nap — sleep inertia — is not about how long you slept. It is about which stage you were in when the alarm went.
For the first twenty minutes or so you are in light sleep, and waking out of it is easy. Somewhere around the half-hour mark you descend into slow-wave sleep, and that is the stage that produces the thick, disoriented, worse-than-before feeling if you are woken from it. That state can last twenty or thirty minutes and it is the reason so many people conclude that naps do not work for them.
Sleep about ninety minutes and you come back out the other side into light sleep again, and waking is easy once more. So the two good answers are short or a full cycle, and the worst possible nap is the one you take when you have “about an hour”.
How late is too late
Sleep pressure builds through the day and a nap spends some of it. Napping close to bedtime means arriving at bedtime with less pressure than you need, which shows up as lying awake. As a working rule, keep naps at least six hours clear of your intended bedtime — for an 11:00 pm bedtime, nothing after 5:00 pm.
The other rule that matters more than length: if you are napping most days and still tired, the nap is a symptom rather than a solution. That is a sleep debt question, or a medical one, and neither is fixed by better nap timing.
Methodology
Alarm = start time + 7 minutes to fall asleep + the nap length. All clock arithmetic is minutes past local midnight, wrapped at 24 hours — a nap is a wall-clock event, so there is no date involved and no daylight-saving exposure.
The seven-minute onset allowance is shorter than the 14 minutes the sleep calculator uses for a night, because somebody who has decided to nap in the afternoon is usually already tired. Both figures are conventions and both are editable.
The grogginess column flags the 45 to 85 minute band as high, because that is where an alarm most reliably lands inside slow-wave sleep. It is a rule of thumb about stages, not a measurement of you.
Limitations
Sleep stages do not run to a timetable. The twenty-minute figure, the descent into deep sleep around the half hour and the ninety-minute cycle are all population averages. Real cycles run from roughly 70 to 120 minutes and vary with time of day, age, sleep debt and what you did last night. Someone carrying a large sleep debt will drop into deep sleep faster and can be groggy after a nap of twenty minutes.
The onset allowance is a guess about you. If you routinely take twenty minutes to fall asleep, the default of seven puts every alarm on this page thirteen minutes early. Change it.
Napping does not repay sleep debt on a one-for-one basis, and this page does not claim it does. A nap reduces sleepiness now; it is not a substitute for the night.
Persistent daytime sleepiness that naps do not fix is worth taking seriously. Sleep apnoea, narcolepsy, thyroid problems, depression, medication effects and shift-work disorder are all common causes and none of them are timing problems. This page does arithmetic on a clock. It is not medical advice and it does not diagnose anything — if you are falling asleep during the day when you do not intend to, that belongs with a doctor.