sleep stages

Sleep is not one flat, unchanging state. Across a night your brain travels through several different stages, and it repeats that journey over and over. Scientists sort these stages into two big families. The first is called NREM sleep, short for non-rapid-eye-movement sleep, which runs from a light drowsy doze down into very deep, slow, hard-to-wake sleep. The second is called REM sleep, named for the quick darting eye movements behind your closed lids, and it is the stage where most vivid dreaming happens. Think of it like descending a staircase into deep sleep and then climbing partway back up into a busy dreaming state, again and again.

One full trip through these stages, called a sleep cycle, lasts roughly ninety minutes, so a normal night holds about four to six cycles. The cycles are not all the same, though. Early in the night you spend more time in the deepest NREM stages, the kind that leave you feeling physically restored. As morning approaches the deep stages shrink and the REM periods stretch longer, which is why your most memorable dreams often come right before you wake. Researchers tell the stages apart by reading brain waves on an EEG, a recording of the brain's electrical activity, together with eye movements and muscle tone: deep sleep shows large, slow waves, while REM sleep shows fast, awake-like waves even though the body's muscles go nearly limp.