In mammalian hibernation, the body temperature approaches that of the surroundings, allowing large savings in energy costs of basal metabolism and eliminating the need for heat production to compensate for heat loss. During entry into hibernation, heat production ceases while the body temperature set-point gradually decreases during slow-wave sleep. In the hibernating phase, the animal copes with problems concerning the maintenance of ion gradients, possible membrane phase transitions and the risk of ventricular fibrillation. In the arousal phase, the main part of the heat and practically all the necessary substrate comes from brown adipose tissue. The hibernation season is preceded by a preparatory phase. It may be concluded that hibernation is a practical, and perhaps even enviable, solution to a mammalian problem.