One of the many extraordinary traits birds have is their sleep pattern. Yes, birds do sleep, but they don't sleep like mammals do. Birds share with mammals the cycles of Non-rapid Eye Movement sleep and Rapid Eye Movement sleep; however there are differences. The first difference is that both cycles are shorter; Non-rapid Eye Movement sleep averages around two and a half minutes and Rapid Eye Movement sleep about nine seconds. Birds also sleep with one-half of their brain awake! It's called unihemispheric slow-wave sleep and keeps birds alert to potential predators while still catching some Zs. Other animals sleep this way, but only birds have the ability to control it. A sleeping bird can adjust how much of its brain is asleep by how wide it opens or closes its eye.
Another difference in a bird's sleep pattern is that it will not lose much muscle tone when in deep sleep. It doesn't matter if the bird is a species that stands, perches, roosts, lays down on the ground, swims, or hangs upside down (yes, these are all real sleeping positions). The down jacket design was patterned after sleeping birds. A bird fluffs up its feathers to better cover its body when sleeping in order to keep its body temperature high. The bird will also experience thermogenesis, and some birds take this one step farther in cold temperatures by making themselves undergo a controlled hypothermia called "nocturnal torpor."
Researchers still don't have the complete answer to how migrating birds sleep, though several studies have tried. One focused on Swainson's Thrushes and discovered that Swainson's Thrushes take hundreds of daytime naps to make up for no nighttime sleep. Birds like ducks and geese that fly in J and V formations will use unihemispheric slow-wave sleep when not in the head of the group.