In dream or REM sleep, brain activity is more like awake than non-REM activity, and muscles are paralyzed. Various regions of the brain affect REM sleep, but neuroscientists have found a group of neurons in the medulla that seem to play a key role. Activating them makes mice go immediately into REM sleep, while inactivating them abolishes it. REM control could be useful for researchers studying the function of sleep in animals.