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Why do wake-up periods use so much energy for hibernating animals?

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The image shows a group of Virginia big-eared bats sleeping upside down in a dark cavern. — Dormancy Virginia big-eared bats (*Corynorhinus townsendii virginianushibernating*) are true hibernators, entering a state close to death for much of the winter.

Wake-up periods, or arousal, are energy-intensive for hibernating animals because they require a rapid and complex physiological transition from a state of minimal metabolic activity back to full, warm-blooded function.12

During hibernation, an animal’s body temperature drops significantly, and its heart rate and respiration slow to a near-standstill to conserve resources. To reverse this, the animal must generate a massive amount of heat to warm its body, particularly the head and heart, which warm faster than the rest of the body to ensure vital functions are restored quickly. This process involves a rapid increase in heart rate and complex circulatory adjustments, such as differential vasodilatation, to maintain blood pressure while the body temperature rises.321

Because of these extreme physiological demands, a single period of arousal can consume more heat and energy than many days spent in deep hibernation. In fact, approximately 90 percent of an animal's total heat production and weight loss during the hibernation season occurs during these brief wake-up intervals, rather than during the long periods of dormancy itself.2