Clocks slide rather than freeze during torpor in the mouse

Torpor is an energy-saving strategy used by animals in response to environmental challenges. Torpor is subject to circadian influence, but the effect of torpor itself on circadian rhythms is poorly understood. We investigated whether torpor alters circadian behavior by assessing locomotor activity after torpor in mice emerging into constant darkness (without Zeitgebers). Torpor was induced by food deprivation or by chemogenetic re-activation of "torpor-active" preoptic area neurons. In both conditions, torpor did not disrupt circadian rhythmicity, although a significant phase advance of ∼40 min was associated with arousal from fasting-induced torpor. Increased wheel-running activity occurred before torpor, likely reflecting food-seeking behavior. After emergence from fasting-induced torpor, locomotor activity decreased, while food/water intake increased to balance any nutritional deficit back to pre-torpor levels. Together, these findings suggest that torpor, and the associated hypothermia, does not disturb circadian function and that torpor (or arousal) acts as a non-photic phase advance stimulus.