Alertness and working memory boost without inertia
Best Window
1:00 PM to 3:30 PM — earlier is better if you have an early bedtime
A 15-minute nap is the most commonly recommended "power nap" duration, and for good reason: it's long enough to cross briefly into stage N2 (giving a measurable working-memory boost on top of the alertness benefit) but short enough that most sleepers wake before reaching slow-wave sleep (N3) — the stage that causes grogginess.
At 15 minutes you typically get 8–10 minutes of actual sleep after accounting for sleep onset. That's enough to engage sleep spindles — the brief bursts of neural activity in N2 that research has linked to memory consolidation and procedural learning. If you're napping between study sessions or before a skilled task, 15 minutes is the minimum dose that produces a learning benefit.
Practical notes: set the alarm for exactly 15 minutes from lying down, not from closing your eyes. If you have an average sleep onset of 5 minutes, you'll get 10 minutes of real sleep, which is ideal. If your onset is faster (say 2 minutes, which suggests sleep debt), you'll risk drifting into N3 and waking groggy — in that case, prefer 10 minutes.
The 15-minute nap is what most flight crews and high-performing shift workers use. NASA's classic 1995 study on cockpit napping used a 40-minute nap for long-haul pilots, but follow-up work on short-haul showed 15 minutes delivered ~80% of the alertness benefit with ~0% of the inertia penalty.
Warnings
Avoid naps after 4 PM unless you are night-shift. An afternoon nap this late will reduce sleep pressure and delay your nighttime sleep onset by the same amount.
Get yours measured
Calculate your personal cycle length.
Every number on this page assumes you\u2019re an average sleeper. You probably aren\u2019t. Our 2-minute calculator gives you the exact bedtime that matches your cycle length — not the generic 90-minute assumption.