Sleep is not one state. It is a sequence of four, cycled four to six times a night, each with a distinct job. Deep sleep — stage three, slow-wave sleep — is where the most physical work happens, and it front-loads into the first half of the night.
What happens in slow-wave sleep
During slow-wave sleep, cortical neurons fire in synchronised waves roughly once per second. Growth hormone release peaks. Blood pressure drops, core temperature falls, and the glymphatic system — the brain’s waste clearance pathway — increases flow, moving metabolic byproducts out of neural tissue.
This is also when declarative memory is consolidated: the day’s facts are replayed from the hippocampus and filed into the cortex. Cut deep sleep short and the filing does not finish.
Why the first half of the night matters most
Slow-wave sleep is heavily weighted toward the first two cycles. REM sleep dominates later. This means a night that is short at the end costs you REM, but a night that starts late costs you deep sleep — and deep sleep is far harder to recover.
Practically: a consistent bedtime protects deep sleep better than a consistent wake time. Most people optimise the wrong end.
What actually disrupts it
Alcohol is the clearest offender. It shortens sleep onset but suppresses slow-wave activity in the second half of the night and fragments REM. Late-evening intense exercise, a warm bedroom above 20°C, and alcohol within three hours of bed each measurably reduce slow-wave time.
Caffeine’s half-life of five to six hours means an afternoon coffee is still circulating at bedtime. It does not always prevent sleep onset — it reduces sleep depth, which is harder to notice and easier to ignore.
How to protect it
Three interventions have the strongest evidence: a cool room, a dark room, and a fixed sleep window. Magnesium glycinate and glycine both show modest but replicated effects on sleep onset and subjective sleep quality. Neither is a sedative; both support the pathways that were already trying to work.
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