Rapid eye movement

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. 

When a humanoid is unable to enter into REM sleep for a prolonged period of time, they may suffer from paranoia or hallucinations. 

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. 

In 2367, the Tyken's Rift being tried to communicate with the crews of USS Brattain and the USS Enterprise (NCC-1701-D) during their sleep, and was responsible for the loss of the REM sleep, and eventual madness and death of the Brattain crew. 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. 

In 2377, Harry Kim asked Seven of Nine if the information she discovered about Unimatrix Zero was part of a dream. The Doctor disputed that, claiming she never reached REM sleep. 

External link
REM-Schlaf