Cold and hunger induce diurnality in a nocturnal mammal

Significance The circadian system drives daily rhythms in physiology and behavior. Mammals in nature can change their daily activity rhythms, but causes and consequences of this behavioral plasticity are unknown. Here we show that nocturnal mice become diurnal when challenged by cold or hunger. Negative energy balance changes hormonal, physiological and behavioral rhythms without modifying the rhythm of the circadian pacemaker in the suprachiasmatic nucleus. This response is adaptive because activity during daytime warmth and resting in a buffered environment during the cold nighttime generally reduces energy expenditure. This mechanism may explain why nighttime activity in humans generally evokes higher energy uptake and subsequent obesity and metabolic syndrome, as seen in late chronotypes and night shift workers.