Nap calculator

A nap is a compressed version of the night's architecture. The first ~20 minutes are light N1/N2 sleep — enough to drop adenosine pressure and lift alertness, shallow enough to wake from cleanly. Around 30–60 minutes you enter deep slow-wave sleep: excellent for memory consolidation, but waking from it produces sleep inertia, the 15–30 minute window of feeling worse than before. At ~90 minutes the cycle completes through REM and you surface again. The calculator's three options are those three landing points — nothing else is a sensible place to stop.

The post-lunch dip sits about 7 hours after waking for most chronotypes — earlier for lions, later for wolves (your chronotype shifts it). The second constraint is tonight: napping within ~6 hours of bedtime measurably delays sleep onset, because you've burned off the pressure the night needs. The calculator shows both edges of the window and warns you when your chosen start time breaks the second one. If it does, protect tonight instead — the wind-down routine planner schedules the evening that sleep actually depends on.

Timing is the whole trick. Caffeine absorbed on an empty-ish stomach reaches peak blood levels in roughly 20 minutes — the exact length of a power nap. Sleep clears adenosine; caffeine then blocks the cleared receptors. In the driving studies, the coffee-nap group made fewer errors than either intervention alone. The failure mode is napping 40 minutes "because the coffee needs time" — that's a slow-wave nap with caffeine on board, and you'll wake wired and groggy at once.