How the nap calculator works
Two good nap lengths — and a danger zone
~20 minutes total (alarm before deep sleep begins) buys alertness for 1–3 hours with no grogginess — NASA's fatigue research on pilots found ~26-minute naps improved performance by 34% and alertness by 54%. ~90 minutes is a complete cycle ending in light sleep: full physical and cognitive recovery, natural waking, no inertia. Everything between 30 and 60 minutes is the danger zone — you wake mid-deep-sleep with sleep inertia worse than no nap at all.
The window matters as much as the length
The circadian dip — the natural afternoon trough around 1–3 pm — is the easiest nap window: sleep pressure is high, sleep onset is fast, and night sleep is far enough away. Naps after ~3–4 pm spend the sleep pressure your night needs (the calculator checks your bedtime and warns). Shift workers run inverted rules: their "afternoon" is whenever their schedule puts the trough.
Execution details
- Set two alarms: one for the nap, one for the snooze you promised yourself you wouldn't take.
- Dark and cool beats comfortable: 20 minutes in a genuinely dark room outperforms 30 in daylight gloom.
- The coffee nap: espresso immediately before a 20-minute nap — caffeine arrives as its adenosine-competition target is being cleared. Sharpest nap in sleep research.
- Insomniacs, skip naps: if nighttime sleep is already broken, daytime naps spend pressure the night needs. Fix the night first.