Sleep is experienced as a single continuous state broken by waking. Measured, it is a structured sequence that repeats several times, with the composition of each repetition changing as the night progresses.
The stages are physiologically distinct
Sleep is classified by patterns of brain electrical activity, eye movement and muscle tone, which separate cleanly into recognisable states.
Light sleep occupies the transition into and out of deeper states. Deep sleep shows large slow waves and is the hardest state to wake someone from.
Rapid eye movement sleep is different again, with brain activity resembling wakefulness while the muscles of the body are actively paralysed.
Each cycle runs through the sequence
A full cycle takes roughly ninety minutes in adults, progressing from light sleep into deep sleep and then into a period of rapid eye movement sleep.
The cycle then repeats, typically four or five times across a night, with brief arousals commonly occurring at the junctions between them.
The length of a cycle varies between people and shortens in children, which is part of why infant sleep is organised so differently.
The mix shifts as the night progresses
Deep sleep is concentrated in the first half of the night. By the later cycles, very little of it remains.
Rapid eye movement sleep does the reverse, occupying a small portion of the early cycles and lengthening substantially towards morning.
This is why cutting sleep short at either end removes different things. A late night mostly reduces deep sleep, while an early alarm mostly removes the later stage.
The stages appear to do different work
Deep sleep coincides with the strongest release of growth hormone and with the clearance processes that operate in brain tissue overnight.
Rapid eye movement sleep is associated with the consolidation of procedural and emotional memory, and selectively depriving people of it produces measurable effects.
Assigning single functions to stages oversimplifies an active research area, but the states are clearly not interchangeable.
Why total hours is an incomplete measure
Two people sleeping the same duration can have very different distributions across stages, particularly where sleep is fragmented.
Anything that produces repeated brief arousals returns the sleeper to lighter stages, so the architecture is disrupted while the recorded duration is unchanged.
Persistent unrefreshing sleep despite adequate hours is a recognised reason for clinical assessment rather than something to correct with routine adjustments.