REM sleep

A night of sleep runs in cycles, and REM is the stage each cycle builds towards. In a healthy adult it usually accounts for something like a fifth to a quarter of total sleep, and it is not spread evenly across the night. The first REM period can last only a few minutes. The last one before morning can run half an hour or longer.

Physiologically it looks nothing like the rest of the night. Brain activity resembles waking, breathing and heart rate turn irregular, and the muscles of the body go briefly slack, which is thought to keep you from acting out a dream. Researchers sometimes call it paradoxical sleep for exactly that reason. It sits at the opposite end of the night from deep sleep, which is concentrated in the first few hours and largely finished before REM periods get long.

Why your alarm so often lands in REM sleep

REM lengthens across the night while NREM sleep gets shallower, so the back half of your night is REM heavy. An alarm set for the morning is far more likely to interrupt REM or light sleep than slow wave sleep. That is usually the better outcome, because sleep inertia tends to be worst when you are pulled out of deep sleep rather than a lighter stage.

The same shape explains why a short night hurts in a specific way. Losing the last 90 minutes of sleep does not shave an even slice off every stage. It removes a large share of the REM you would have had, while the deep sleep you got early in the night stays mostly intact.

Cycle length is often quoted as 90 minutes, and that is a reasonable average, but it moves around between people and between cycles in a single night. If you want the full picture of how the stages stack up in order, sleep cycles explained walks through one whole night. The stage chart Tock draws in the morning is a hypnogram, and on a phone only night the stages behind it are estimates built from movement and sound rather than measured brain activity. Measured stages need Apple Health or a wearable.