NREM-promoting neurons are distributed along the neural axis and also in the cortex. They are mainly GABAergic and inhibit wake-promoting systems. Cortical interneurons and the thalamo-cortico-thalamic feedback loop actively promotes cortical synchronization. Open circles, brain areas contributing to sleep control but not sleep-promoting per se. BF, basal forebrain; DR, dorsal raphe; LDT/PPT, laterodorsal and pedunculopontine tegmental nuclei; LH, lateral hypothalamus; LC, locus coeruleus; Nac, nucleus accumbens; NTS, nucleus of the solitary tract; PB, parabrachial nucleus; PZ, parafacial zone; RMTg, rostromedial tegmental nucleus; SLD, sublaterodorsal nucleus; TMN, tuberomamillary nucleus; vlPAG, ventrolateral periaqueductal gray; VLPO, ventrolateral preoptic area; VM, ventral medulla; VTA, ventral tegmental area; ZI, zona incerta.