Sleep cycle
A roughly 90-minute progression through the sleep stages, repeated four to six times a night.
A sleep cycle runs from light sleep through deep slow-wave sleep and into REM, taking around 90 minutes on average — though the figure varies between people and between cycles. A normal night contains four to six of them.
It matters because the composition of cycles changes across the night. Deep slow-wave sleep dominates the early cycles, while REM periods lengthen towards morning. That is why cutting a night short does not simply remove a proportional slice of everything — it disproportionately costs you REM.
In practice, timing sleep to land on a cycle boundary can make waking feel easier, but the popular '90-minute rule' is far more approximate than sleep calculators imply, since cycle length varies. Total sleep duration and consistency of schedule matter considerably more than hitting a boundary.
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Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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