The preoptic area (POA) of the hypothalamus is implicated in sleep generation, but deciphering exactly which POA neurons promote sleep has been difficult. The POA sends inhibitory projections to the wakefulness-promoting tuberomammillary nucleus (TMN). Through the use of virus-mediated retrograde tracing and optogenetics, Chung et al. identified a population of GABAergic POA→TMN neurons that promote sleep in mice. They also identified peptidergic markers for these neurons. This study reveals details of a population of sleep-promoting neurons that may be manipulated to probe sleep circuits.