Due to their small size and lack of insulating downy feathers hummingbirds rapidly lose body heat to their surroundings. Because of this hummingbirds must regularly consume large quantities of nectar and insects to feed their rapid metabolism. To survive when sleeping at night hummingbirds must enter a state of reduced physiological activity called torpor. This process involves the bird lowering its internal temperature to a level barely able to maintain life, becoming hyperthermic. Doing so allows the bird to reduce its metabolic rate by as much as 95%, causing the bird to consume up to 50 times less energy. Alexander Wilson first described torpid hummingbirds in his book, American Ornithology; “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.” Awaking from a torpid state takes approximately 20 minutes; the bird gently vibrates its wing muscles to warm the blood supply. This process leaves the hummingbird with just enough energy supplies to survive the first feeding bouts of the morning.