Sleep Stages

Sleep is not one state. A night runs through repeating cycles of roughly ninety minutes, each made up of light sleep, deep sleep and REM sleep, and the mix changes as the night goes on: deep sleep is concentrated in the first few hours and REM in the last few.

Deep sleep, sometimes called slow wave sleep, is when most physical recovery happens, and it is the stage that growth hormone release and immune function are tied to. REM sleep is when most dreaming happens and is associated with memory consolidation and emotional processing. Light sleep is the majority of the night and is not wasted time; it is the connective tissue between the others.

A typical adult night is around half light sleep, with the rest split between deep and REM. The proportions shift with age, with deep sleep falling steadily from your twenties onward, which is normal rather than a problem to solve.

What it is not

The stages a wrist device reports are inferred from movement and heart rate, not measured from brain activity, and they agree with a clinical sleep study only moderately well. Deep and REM in particular are often confused for one another. Treat the trend over weeks as meaningful and a single night's breakdown as a rough sketch.

Where you see it in FitnessSyncer

Every night that arrives with a stage breakdown is charted on your Dashboard, shown on the night itself in your Stream, and summarized in the built-in Sleep report alongside the rest of your sleep data.