Hummingbirds have incredibly busy days. Their heart is beating 1000 times a minute. This gets even mo repaid after the hummingbird starts flapping its wings 10-80 beats a second! In addition to buys days, hummingbirds need to keep their body temperature as high as 104 degrees Fahrenheit (or 40 degrees Celsius).
Hummingbirds need a significant amount of calories to fuel their busy bodies. One source of energy comes from their appetite of constantly sipping sugar-rich nectar from flowers, and catching insects for extra food. Few other creatures have to consume so much fuel compared to their body weight as does the hummingbird specie. When a hummingbird is busily flying, it would take only a few hours without food before they would starve to death!
So how can they possibly survive through a long winter night without starving?
The trick is that the bird goes into torpor. Each night, hummingbirds slow their heartbeat, and let their body temperature drop. Once their body cools (they sometimes cool their body temperature as low as 68 degrees F, or 20 degrees C,) the hummingbird becomes lethargic or even unaware of its surroundings. But when the bird is in such as state, they need fewer calories to keep warm and to fuel their metabolism.
The next morning, it takes a while (up to 60 minutes) for this super-cooled hummingbird to warm back up. But they have survived another night without starving, and are ready to seek out a breakfast of flower nectar.
Several other groups of bird species occasionally rely upon torpor as an energy saving strategy. For instance, tiny titmice and chickadees may let their bodies cool by 8-12 degrees Fahrenheit, if the night is particularly cold. But only the hummingbird needs such a drastic drop in its temperature each night to rest and conserve its energy.