Nap Calculator
Pick the right nap length — power nap, NASA's 26-minute nap, 60 or 90 minutes — and get your exact wake-up time, plus the latest safe time to nap.
If you lie down now, set your alarm for:
- Power nap—:——No grogginess
- NASA nap—:——Minimal grogginess
- 60-minute nap—:——Some grogginess on waking
- Full-cycle nap—:——Low grogginess (wake in light sleep)
Avoid 30–50 min naps: long enough to hit deep sleep, too short to finish the cycle — the worst of both worlds.
Nap length decides how you wake up
The reason naps feel wildly inconsistent is that different lengths end in different sleep stages. A 10–20 minute nap stays in light NREM sleep, so waking is easy and the alertness boost is immediate. Somewhere past the 30-minute mark you slide into slow-wave (deep) sleep, and an alarm there triggers sleep inertia — the groggy, slow-thinking state that can wipe out the benefit of the nap for 15–30 minutes. Go all the way to ~90 minutes and you complete a full sleep cycle, waking back in light sleep. The Sleep Foundation's napping guide recommends 10–20 minutes for most people for exactly this reason.
The famous middle option comes from NASA. In the fatigue countermeasures research program led by Mark Rosekind and colleagues, long-haul flight crews were given a planned 40-minute cockpit rest period and slept an average of about 26 minutes. Relative to the no-rest group, the nappers improved reaction-time performance by 34% and physiological alertness by 54%. That 26-minute average is what people mean by the "NASA nap" — short enough to mostly dodge deep sleep, long enough to measurably restore performance.
Timing matters as much as length. Napping late in the day burns off homeostatic sleep pressure — the drive to sleep that builds while you're awake — which is why the Sleep Foundation advises finishing naps by early-to-mid afternoon (around 3 PM) and keeping a healthy gap of roughly 8 hours before bedtime. This calculator applies whichever of those two limits comes first for the bedtime you enter, so the "latest safe nap" line is personal to your schedule.
A good nap routine is a habit like any other: same window, same length, most days. If you want the afternoon nap (or the no-naps-after-3 rule) to stick, track it in HabitBox — a free, on-device habit tracker for iOS and Android where checking off "20-minute nap before 3 PM" takes one tap.
Frequently asked questions
How long should a nap be?+
It depends on what you want. For a quick alertness boost with zero grogginess, keep it to 10–20 minutes — you stay in light sleep. NASA's in-flight study found ~26 minutes worked well for pilots. A 60-minute nap adds deep sleep, which helps memory but causes 15–30 minutes of grogginess on waking. A 90-minute nap completes one full sleep cycle, so you wake back in light sleep feeling refreshed. The one length to avoid is 30–50 minutes: long enough to enter deep sleep, too short to finish the cycle.
What is the NASA nap?+
In a 1990s NASA study led by Mark Rosekind, long-haul pilots were given a planned 40-minute rest opportunity in the cockpit and slept an average of about 26 minutes. Compared with a no-nap group, the nappers showed a 34% improvement in reaction-time performance and a 54% improvement in physiological alertness. That's why '26 minutes' is quoted as the NASA nap — it's the average sleep the pilots actually got, not a magic number.
What is the latest time I should nap?+
Sleep Foundation guidance is to avoid napping after about 3 PM, and more generally to keep naps at least 8 hours away from your bedtime. Late naps reduce your homeostatic sleep pressure — the biological drive that builds the longer you're awake — which can make it harder to fall asleep at night. This calculator takes the earlier of those two limits based on the bedtime you enter.
Why do I feel worse after a long nap?+
That heavy, disoriented feeling is sleep inertia. Naps of roughly 30–60 minutes tend to end during slow-wave (deep) sleep, and being woken from deep sleep impairs alertness and thinking for 15–30 minutes, sometimes longer. The fix is either to keep naps short (under ~30 minutes, before deep sleep starts) or long enough to finish a full ~90-minute cycle so you wake from light sleep.
Does napping make up for lost night sleep?+
Partially. A nap can restore alertness and performance after a short night, which is exactly what the NASA study measured. But naps don't fully replicate consolidated overnight sleep, and relying on long daytime naps can push your bedtime later and feed the cycle. If you're chronically short on sleep, treat naps as a patch and fix the night schedule — a sleep debt calculator can show how far behind you are.
Make the good nap a habit
A well-timed 20-minute nap beats a third coffee — if you actually take it. HabitBox is a free, on-device habit tracker (iOS + Android, no account) where a one-tap check-in keeps your nap-before-3-PM streak alive.
Free · Local-only data · No account required