Definition
- Sleep cycle
- A sleep cycle is one complete pass through the stages of sleep, from light sleep into deep sleep and then into REM. A cycle lasts roughly ninety minutes on average, and most people complete four to six of them per night, with cycle composition shifting as the night progresses.
The stages
Each cycle moves through light sleep, deep sleep, and REM sleep. Early cycles contain proportionally more deep sleep; later cycles contain more REM. This is why the last two hours before waking feel different from the first two, and why cutting sleep short disproportionately removes REM.
The ninety-minute rule, honestly
The popular advice to time your alarm to a multiple of ninety minutes is based on real physiology but overstated in practice. Cycle length varies between individuals and between nights for the same person — anywhere from roughly seventy to one hundred and twenty minutes — so the arithmetic is far less precise than the advice implies.
Waking from light sleep does feel better than waking from deep sleep. But total sleep duration matters considerably more than cycle timing, and shortening your night to hit a supposed cycle boundary is a poor trade.