Seasonal dormancy (e.g. diapause, hibernation) is widely considered an adaptation for surviving life-threatening conditions during part of the year. However, the selective pressures acting on dormancy are poorly studied. An underestimated common aspect is the high survival rate during dormancy compared to the active period, perhaps due to the reduced risk of predation and competition. We hypothesize that dormancy phenology is influenced by a trade-off between the reproductive benefits of being active and the survival benefits of being dormant. Using a phylogenetic comparative method and more than 20 hibernating mammals, we find that the sex difference in hibernation phenology is explained by sex differences in physiological constraints that may influence this trade-off. Consistent with the trade-off hypothesis, the sex that spends more time in an activity directly associated with reproduction (e.g. testicular maturation, gestation) or indirectly (e.g. recovery from reproductive stress) spends less time in hibernation. Some of the tested parameters such as testes maturation or a late mating period during the active season also influence the sex difference in dormancy phenology among ectotherms (e.g. reptiles, invertebrates). We then gathered evidence that dormancy in non-life-threatening periods that are unfavorable for reproduction may be more widespread than previously thought.