Torpor is a state similar to hibernation. Hummingbirds must enter this state to ensure that the birds won’t actually starve to death before down. Torpor is a type of deep sleep where an animal lowers its hart and metabolic rate. In a state of torpor the hummingbird lowers its metabolic rate by as much as 95%. A torpid hummingbird consumes up to 50 times less energy than when awake.
The sleep of torpid hummingbirds is a sleep that is as deep as death. According to the book, American Ornithology, written by Alexander Wilson in 1832, torpor is described as follows: "No motion of the lungs could be perceived … the eyes were shut, and, when touched by the finger, [the bird] gave no signs of life or motion."
Hummingbirds reliably awaken from torpor one or two hours before dawn without any known cues from the environment. It appears that the bird’s internal circadian clock is responsible for signaling when it is time to awaken from the state of torpor.