Rattenborg, Niels C.
For centuries, people have wondered whether birds sleep on the wing during long, non-stop flights. However, until recently there was no direct evidence for sleep in flight. Measuring the brain activity of frigatebirds in the wild showed that these birds can sleep with either one cerebral hemisphere at a time or both hemispheres simultaneously. Despite being able to engage in all types of sleep in flight, the birds slept less than an hour a day, a mere fraction of the time spent sleeping on land.