Free sleep calculator
Nap Calculator
The best nap length to avoid grogginess, and the latest time to nap without wrecking tonight's sleep.
Not medical advice. This tool gives a general estimate for informational purposes only — it does not replace professional judgment. If you're concerned, contact a doctor right away. This gives general nap-timing guidance based on population averages, not a measurement of your own sleep. If you're regularly exhausted despite napping, talk to a doctor or sleep specialist.
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Try to avoid naps between 30 and 60 minutes — that's usually just long enough to fall into deep sleep, so waking up mid-cycle leaves you groggier than before you napped (sleep inertia). Either keep it short (~20 min) or go long enough for a full cycle (~90 min).
Sources & how this is calculated
Most sleep researchers recommend keeping naps to about 20 minutes — long enough to feel more alert, short enough to avoid dropping into deep (slow-wave) sleep. Waking up from deep sleep mid-cycle causes sleep inertia, the groggy, worse-than-before feeling naps are supposed to prevent. If you have more time, a full 90-minute cycle is the other way to avoid it — you wake back up in light sleep instead of in the middle of one. NASA's own research on operational naps landed on 26 minutes as the length that maximized alertness gains, in the same range as the general 20-minute guidance. Both figures and the sleep-inertia mechanism are from the Sleep Foundation's "Napping: Benefits and Tips" and "NASA Nap" pages.
For timing, the same Sleep Foundation guidance recommends napping at least 8 hours before bedtime — for most people, that means before mid-afternoon — so the nap doesn't reduce how tired you are at your actual bedtime. This calculator works backward from your bedtime using that 8-hour buffer.
These are general guidelines, not a personal prescription — how naps affect your nighttime sleep depends on your own sleep debt, schedule, and sensitivity. If naps regularly leave you groggy or you can't fall asleep at night, that's worth raising with a doctor.