In a paper published in Nature Communications, Niels Rattenborg from the Max Planck Institute and colleagues from several other institutions have offered the first proof showing that flying birds can sleep with either one half of their brains active, or with both hemispheres shut down at the same time. Remarkably, these birds can retain their navigational ability while in REM sleep. In other words, they can literally fly with their eyes closed.
This technique of sleeping is called unihemispheric slow-wave sleep (USWS), which allows one half of the brain to enter into deep sleep while the eye corresponding to this half is closed and the other eye remains open. Unihemispheric sleep allows an animal to get some rest, while also allowing it to maintain awareness of its surroundings. In fact, dolphins also use this technique to avoid drowning while they rest.
