To save energy, hummingbirds lower their internal thermostat at night, becoming hypothermic. Their night time body temperatures are maintained at a point, called a set point , that is far below what is normal during the day. This physiological phenomenon is called torpor. There are several types of torpor, classified mostly by duration and season. For example, when torpor takes place for long periods of time during the winter, it is known as hibernation.
Hummingbird torpor can occur on any night of the year so it is called daily torpor or noctivation.
Even tropical hummingbirds have rigid metabolic budgets so they commonly use daily torpor to conserve energy, too. Torpid hummingbirds exhibit a slumber that is as deep as death. In 1832, Alexander Wilson first described hummingbird torpor in his book, American Ornithology . Awakening from torpor takes 20 minutes or more. During arousal, the hummingbird's body can warm up by several degrees each minute and the bird awakens with enough energy reserves to see him through to his next feeding bout.