Mammals, including humans, prepare for sleep by nesting and curling up, creating microclimates of skin warmth. To address if external warmth induces sleep through defined circuitry, we used c-Fos-dependent activity-tagging, which captures populations of activated cells, and allows
them to be reactivated to test their physiological role. External warming
tagged two principal groups of neurons in the MnPO/MPO hypothalamic
area. GABA neurons located mainly in MPO produced NREM sleep but no body temperature decrease. Nitrergic/glutamatergic neurons in MnPO
/MPO induced both body cooling and NREM sleep. This circuitry
explains how skin warming induces sleep, and why the maximal rate of core body cooling positively correlates with sleep onset. Thus, the pathways that promote NREM-sleep, reduced energy expenditure, and body cooling are inextricably linked, commanded by the same neurons. This implies that one function of NREM sleep is to lower brain temperature and/or conserve energy.