HOW COME?: A birdbrained way of sleeping

Do birds sleep in the same way we sleep? asks a readerDuring a long day at school or work, do you ever feel half-asleep? Half-asleep for a human means "drowsy." But at night, that blue jay in your backyard may really be half asleep.

Unlike us, many birds can enter a state of true half-sleep. In a half-sleeping animal, one side of the brain is slumbering, while the other side is wide awake. Scientists call this kind of sleep "unihemispheric," because only one hemisphere of the brain is snoozing.

During half-sleep, one eye will often be open and one eye closed, corresponding to the sleeping and non-sleeping halves of the brain. Besides birds, aquatic mammals like whales, dolphins, and seals often do the half-sleep, as may some reptiles.

Why not simply drift off to sleep, both eyes blissfully shut? Unihemispheric sleep allows birds to stay on guard, keeping an eye (and half a brain) peeled for threatening predators.

Like mammals, birds sleep in cycles of so-called slow-wave sleep and rapid-eye movement (REM) sleep, where dreams occur. While birds spend part of their sleeping time only half-asleep, they also spend time with their whole brains asleep -- and both eyes closed. It's only when both hemispheres are snoozing that a bird's brain will enter short periods of REM sleep, and enjoy a few brief dreams.

Scientists say that both half-sleep and extra-short bouts of dreaming sleep help protect vulnerable birds from other animals. Unfairly, it's usually predators like cats who have the luxury of enjoying lots of deep, dream-filled sleep, two eyes closed on the world.

Like us, most birds sleep during the night. The only birds that sleep in nests are baby birds and the adults taking care of them. When not nesting, most birds sleep perched on branches in trees or bushes, or in handy tree trunk holes.

Unihemispheric sleep is especially useful when birds flock together to snooze. Studies of mallard ducks found that ducks sleeping on the edge of a row -- a particularly vulnerable spot -- spent most of their sleeping time half-awake, one eye open and facing out. Ducks in the more protected center, however, were more likely to sleep with their whole brains. Every so often, ducks on the perimeter were observed to turn 180 degrees and switch eyes, allowing the alert side of the brain to snooze, while the other side (and eye) took up guard duty.

But even birds can't resist the power of deep sleep if they're truly sleep-deprived. When scientists kept chickens awake long enough, they found that the birds spent less time with one eye open, half-asleep, and increasing amounts of time fully, deeply asleep, both eyes firmly shut.