the.com/rem

means REM, or Rapid Eye Movement, is the sleep phase where your eyes dart around under closed lids while your brain hums at near-wakeful intensity and dreams get vivid.

from Coined in the 1950s by sleep researchers who noticed, while watching sleeping subjects' eyes, that they twitched and darted beneath the lids in bursts. They named the phenomenon literally — Rapid Eye Movement — and the acronym REM escaped the lab to become shorthand for the whole dream-rich stage of sleep itself.