REM sleep -- the phase of night-time mammalian sleep physiology where dreams occur -- has long fascinated scientists, clinicians, philosophers, and artists alike, but the identity of the neurons that control REM sleep, and its function in sleep have been controversial due to a lack of precise genetic methods to study the sleeping brain. Now, in a remarkable demonstration of a recent brain technology, neuroscientists provide the first answers to both questions, identifying a neural circuit in the brain that regulates REM sleep, and showing that REM sleep controls the physiology of the other major sleep phase, called non-REM (NREM) sleep.