A night of sleep is not one continuous state but a sequence of distinct stages that repeat. The composition of those cycles changes systematically between the first hour and the last.
The stages are defined by what instruments record
Sleep staging comes from a sleep study that records brain electrical activity, eye movement and muscle tone simultaneously. Each of those three signals contributes to the classification.
Light sleep shows a slowing of brain rhythms and drifting eye movement. Deeper sleep is defined by large, slow waves dominating the recording.
Rapid eye movement sleep is identified by fast eye movements, a brain trace resembling wakefulness, and near-total loss of tone in most skeletal muscle.
A cycle runs roughly ninety minutes
Sleep progresses from light stages into slow-wave sleep and then into a period of rapid eye movement sleep, before returning towards lighter stages again.
One full pass takes something in the region of ninety minutes, so a typical night contains four or five of them.
The cycles are not identical copies. What changes between them is the ratio of deep sleep to dreaming sleep.
Deep sleep is concentrated early
Slow-wave sleep dominates the first cycles of the night and diminishes with each subsequent one. By the later cycles there may be very little of it.
This front-loading is why a night cut short at the end loses proportionally more rapid eye movement sleep than deep sleep.
It also explains why a very short sleep can still feel physically restorative while leaving the person foggy.
Dreaming sleep expands towards morning
Rapid eye movement periods start brief and lengthen across the night, so the longest ones occur in the hours before waking.
Waking naturally in the morning therefore often happens close to a dream period, which is part of why morning dreams are the ones people recall.
Body temperature and hormonal rhythms are climbing during those same hours, and the two patterns are linked rather than coincidental.
Brief awakenings are part of normal structure
Short arousals occur between cycles in ordinary sleep, and most are never remembered because a wakening has to last long enough to be encoded.
A person who notices waking several times has not necessarily slept badly. Loud snoring, gasping or persistent daytime sleepiness are different signals, and those warrant assessment by a clinician rather than self-interpretation.