Ketsia Olangi — Sleep Hours by Age | ketsia-olangi.com
The 90-minute cycle repeats four to six times a night; the optimal nap is 10–20 minutes; slow-wave sleep begins at 30 minutes.
One cycle of alternate NREM and REM sleep takes an average of 90 minutes and repeats four to six times in a good night's sleep. The same cycle structure governs sleep of any length, including a nap.
The optimal nap is around 10–20 minutes. Kept inside that window, the nap stays in the lighter stages and ends before the deepest stage begins.
It takes at least 30 minutes to enter slow-wave sleep, the deepest stage. A nap that runs past 30 minutes crosses that threshold, and waking from it tends to feel heavier than waking from a shorter nap.
On narrow screens, swipe or scroll the plate sideways.
Between 20 and 30 minutes the nap sits in an awkward middle: long enough to start costing the light-stage benefit, short enough to cut off before slow-wave sleep is fully entered.
Physicians recommend muscle-stretching exercises on waking and relaxation routines before sleep. Apply the same split to naps: a short stretch when you get up, a calm wind-down when you lie down for the night.
Cycle arithmetic applies to naps as well as nights. A 90-minute nap is one full cycle and ends near a stage boundary, while a 10–20 minute nap deliberately stops early.
Further reading