About the Nap Calculator
This nap calculator tells you exactly when to set your alarm. Choose the kind of nap you want — a 10- or 20-minute power nap, the 26-minute "NASA nap" or a 90-minute full sleep cycle — enter when you lie down and how long you usually take to fall asleep, and it gives the wake-up time, the alarm duration and how groggy you are likely to feel.
Nap length matters because of sleep stages. In the first 20–30 minutes you stay mostly in light sleep, so a short nap refreshes you without much grogginess. Beyond that you sink into deep, slow-wave sleep, and waking from it causes sleep inertia — the heavy, foggy feeling that can last up to half an hour. A full cycle of roughly 90 minutes brings you back toward light sleep before the alarm.
It is useful for shift workers, new parents, students and anyone hitting an afternoon slump. The early-to-mid afternoon is the easiest time to nap; long or late naps can make it harder to fall asleep at night. Sleep cycles vary between people, so treat the timings as a starting point and adjust.
How to use the nap calculator
- 1Enter the time you will lie down.
- 2Pick a nap type — 20 minutes for a quick boost, 90 minutes for a full cycle.
- 3Enter how long you usually take to fall asleep.
- 4Set an alarm or timer for the result and get up when it goes off.
Formula and method
The alarm has to cover both the minutes you spend drifting off and the sleep you actually want, so the calculator adds your usual time to fall asleep to the chosen nap length. Times past midnight wrap around to the next day.
The grogginess rating follows the typical structure of sleep: up to about 30 minutes of sleep you remain mostly in light stages (low inertia); between roughly 30 and 80 minutes you are likely to be in deep slow-wave sleep (high inertia); around 80–100 minutes you approach the end of a first cycle of about 90 minutes (lower inertia again). Individual cycles range from about 70 to 120 minutes, so these windows are approximate.
- Lie-down time
- When you start trying to sleep
- Fall-asleep time
- Minutes you usually take to drift off
- Nap length
- Minutes of actual sleep you want
Worked examples
20-minute power nap at 1:00 PM
Lying down at 1:00 PM and taking 10 minutes to fall asleep, a 20-minute nap needs a 30-minute timer, so the alarm goes off at 1:30 PM while you are still in light sleep.
Full 90-minute cycle at 2:15 PM, slow to fall asleep
15 minutes to fall asleep plus a 90-minute cycle is 105 minutes, so starting at 2:15 PM you would wake at 4:00 PM, near the end of a cycle.
NASA nap on a night shift, starting 11:50 PM
A 26-minute nap plus 5 minutes to drop off is 31 minutes; from 11:50 PM that rolls past midnight to 12:21 AM.
Custom 45-minute nap at 12:30 PM
10 + 45 = 55 minutes, so the alarm rings at 1:25 PM. Forty-five minutes of sleep usually means waking from deep sleep, so expect some grogginess for a while.
Frequently asked questions
How long should a power nap be?+
About 10 to 20 minutes of sleep. That is long enough to improve alertness but short enough that you stay in light sleep, so you wake without the heavy grogginess that follows deeper sleep.
What is a NASA nap?+
It comes from a NASA study, published in 1994, of long-haul airline pilots: given a planned 40-minute rest period in the cockpit, pilots fell asleep in about 6 minutes and slept for about 26 minutes on average, and showed better alertness and reaction-time performance than pilots who did not nap.
Why do I feel worse after a long nap?+
Naps of roughly 30–80 minutes often end during deep slow-wave sleep. Waking from that stage causes sleep inertia, a groggy, slow feeling that can last 15–30 minutes or more before it clears.
What is the best time of day to nap?+
For most people the early-to-mid afternoon, around 1–3 PM, when there is a natural dip in alertness. Napping late in the afternoon or evening makes it harder to fall asleep at night.
Is a 90-minute nap good?+
A 90-minute nap lets you complete roughly one full sleep cycle, including deep and REM sleep, and wake near light sleep. It suits catching up on lost sleep but takes a large chunk of time and can affect night sleep if taken late.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.