Sleep cycle
A roughly 90-minute progression through the sleep stages, repeated four to six times a night.
A sleep cycle is one pass through the stages of sleep: light non-REM, deep slow wave sleep, and REM. It averages about 90 minutes in adults and repeats four to six times a night, though real cycles run anywhere from roughly 70 to 120 minutes.
The composition shifts as the night goes on, which is the part most people miss. Early cycles are heavy on deep sleep; later ones trade it for progressively longer REM periods. So the first half of the night is weighted towards physical restoration and the second half towards REM and its role in emotional and procedural processing.
That asymmetry means cutting sleep short is not a uniform loss. Waking two hours early removes mostly REM, while going to bed two hours late removes mostly deep sleep. Both matter, and neither is fully replaceable by a nap.
The 90 minute figure has produced a popular rule: set an alarm on a multiple of 90 minutes to wake between cycles. The physiology behind it is real, since waking from deep sleep produces the worst grogginess, but the arithmetic is shakier than it sounds because cycle length varies between people and between nights.
Treat it as a tiebreaker rather than a timetable. If two possible bedtimes are otherwise equal, the one that lands your wake time near a cycle boundary is a reasonable bet. Choosing six hours over seven and a half because it divides neatly is not.
Wearables report the stages and report them poorly. Validation studies find total sleep time reasonably accurate and stage estimates unreliable, so cycle counts from a wrist device are an entertaining approximation rather than data.
In practice, our sleep calculator works backwards from your intended wake time in cycle lengths, and the more valuable half of its advice is having a fixed wake time at all.