Sleep is a ubiquitous behavior in animal species. Yet, brain circuits controlling sleep remain poorly understood. Previous studies have identified several brain structures that promote sleep, but whether these structures are involved in sleep initiation or sleep maintenance remains largely unknown. Here we identified a population of glutamatergic neurons in the medulla that project to the preoptic area (POA), a prominent sleep-promoting region. Chemogenetic silencing of POA-projecting medulla neurons disrupts the transitions from wakefulness to Non-Rapid Eye Movement (NREM) sleep, whereas chemogenetic activation of these neurons promotes NREM sleep. Moreover, we show that optogenetic activation of medulla glutamatergic neurons or their projections in the POA reliably initiates long-lasting NREM sleep in awake mice. Together, our findings uncover a novel excitatory brainstem-hypothalamic circuit that controls the wake-sleep transitions.