Rapid eye movement
From Memory Alpha, the free Star Trek reference
Rapid eye movement, or REM, is the stage in the sleep cycle where dreams occur in many humanoid species.
Betazoids reach REM sleep at a different frequency from other humanoids. (TNG: "Night Terrors")
When a humanoid is unable to enter into REM sleep for a prolonged period of time, they may suffer from paranoia or hallucinations. (TNG: "Night Terrors")
While raising the mimetic simbiot Sim, Doctor Phlox explained in a letter to Dr. Lucas how he was at a loss to understand how Human adults could endure the REM cycle interruptions that accompany the teething stage of a Human infant. (ENT: "Similitude")
When Commander William T. Riker wasn't able to sleep properly, due to tampering by solanogen-based lifeforms, Doctor Crusher first assumed, his problems were caused by a lack of REM sleep. (TNG: "Schisms")
