Nap calculator

Short. In a well-known lab study, Brooks and Lack had 24 healthy young adults sleep about 5 hours at home, then nap at 3 p.m. for exactly 5, 10, 20 or 30 minutes of sleep, or not at all. The 5-minute nap did little. The 10-minute nap improved sleepiness, fatigue and performance straight away, with some of the benefit lasting about two and a half hours. The 20-minute nap helped too, but the benefit appeared about 35 minutes after waking. The 30-minute nap brought a groggy spell first, then a benefit.

The authors called 10 minutes the most effective length of the ones they tested. Keep in mind who was tested: young adults short on sleep, napping mid-afternoon. CDC NIOSH guidance for shift workers gives a wider range, saying short naps of 15 to 30 minutes and longer naps can both raise alertness.

The minutes in those studies are minutes of sleep, not minutes lying down. If you take ten minutes to drop off, a 20-minute nap needs a 30-minute alarm. The calculator adds your time to fall asleep before the nap starts, which is the step most nap timers skip.

The fog after waking is called sleep inertia. CDC NIOSH describes it as a temporary drop in performance and mood after waking, usually gone within 30 minutes, though it can last longer when you are short on sleep. It is more likely when a nap is long enough to reach deeper sleep.

If you need to be sharp right after waking, a short nap is the safer choice. If you take a longer one, leave time for the fog to clear before driving or anything that needs a clear head. NIOSH notes that bright light and washing your face helped people wake up faster in studies.

According to CDC NIOSH, pressure to sleep rises in the afternoon, around 2 to 4 p.m., which makes falling asleep easier then. It is harder to fall asleep around 11 a.m. to noon, and in the 1 to 3 hours before your usual bedtime. Enter your usual bedtime and the calculator flags a nap that falls in that late window.