Sleep is not one long, even state

Across a night, the brain moves repeatedly through non-REM and REM sleep. Early cycles usually contain more deep, slow-wave sleep; REM periods tend to lengthen later. What looks from the bedside like stillness is a changing architecture, with brain activity, eye movement, muscle tone and breathing shifting by stage.

Stage proportions vary with age, timing, health and the individual night, and consumer trackers infer rather than directly measure laboratory sleep stages. The bounded surprise is not that one stage performs one magical task. It is that “asleep” names a sequence of biological states, not a single switch left on until morning.