REM sleep

REM sleep is the stage of sleep where most vivid dreaming happens, named for the way your eyes dart quickly back and forth beneath closed lids — rapid eye movement. Here is the strange twist: your brain is buzzing with activity that looks almost like being awake, yet your body lies completely still. Picture a movie playing at full volume in a theater whose doors are locked from the outside: inside your head a rich, story-like dream unfolds, while your muscles are switched off so you cannot act it out. Each night you cycle in and out of REM several times, with longer stretches toward morning, which is why your most memorable dreams often come just before you wake.

That whole-body stillness is the headline feature, called muscle atonia — a near-total loss of muscle tone that briefly paralyzes your arms, legs, and trunk so you do not physically run, punch, or leap while dreaming. The brainstem, the stalk at the base of the brain, sends signals that actively clamp down on the muscles, while regions tied to vision, emotion, and memory light up. (Your eyes and breathing muscles are spared, which is why the eyes still move and you keep breathing.) Scientists think REM helps the brain sort and store memories, process emotions, and fine-tune its wiring, especially in early life — newborns spend far more of their sleep in REM than adults do. When this safety brake fails, people can physically act out their dreams, a condition called REM sleep behavior disorder.