The brainstem is essential for non-rapid eye movement (NREM) sleep. Kashiwagi et al. identified a subpopulation of NREM-sleep-promoting neurons that express neurotensin in the sublaterodorsal tegmental nucleus (subLDT) in the mouse brainstem. Chemogenetic activation of these cells or their downstream targets increased NREM sleep, and infusion of neurotensin into the fourth ventricle rapidly induced slow-wave activity in the cortex that mimicked that seen during NREM sleep, suggesting that neurotensin itself participates in sleep regulation.