When food is scarce and nights are cold, hummers enter a state of torpor at night. Their metabolic rate and temperature drops, which saves energy. Until recently, hummers were thought to go into a state of torpor every night. More recent research suggests they only do it when they arrive at a breeding or wintering site before flowers are abundant or when food supply becomes low for some other reason. At least some small bats enter torpor during the day to survive.
Estivation is a reduction in metabolism that occurs during the summer.