Birds sleep with half brain off and rest on

Steve Connor
Thursday 04 February 1999 00:02 GMT

BIRDS CAN catch 40 winks without being caught by a predator because they can, literally, stay half asleep with one eye open and the other closed, scientists have discovered.

A study of the unusual sleeping patterns of mallard ducks promises to help scientists to understand rare sleep disorders in humans.

Biologists at Indiana State University, led by Niels Rattenborg, say that birds can sleep with both eyes closed and their whole brain asleep, or they can rest half their brain by keeping one eye shut. "Birds have overcome the problem of sleeping in risky situations by developing the ability to sleep with one eye open and one hemisphere of the brain awake," the scientists report in the journal Nature.

Keeping one half of the brain at rest, called unihemispheric slow-wave sleep, is in direct contrast to the typical situation where sleep and wakefulness are mutually exclusive states of the whole brain, the scientists say. "We have found that birds can detect approaching predators during unihemispheric slow-wave sleep, and that they can increase the use of unihemispheric sleep as the risk of predation increases," they report.

"We believe this is the first evidence for an animal behaviourally controlling sleep and wakefulness simultaneously in different regions of the brain."

The scientists studied groups of mallard ducks while they rested side by side and found that those at the edge of the group were significantly more likely to keep one eye open, with the closed eye being on the side of its nearest neighbour.

"We have found that birds sleeping under risky situations spend more time with one eye open and half the brain awake, and choose to direct the open eye towards a perceived threat," added Dr Rattenborg.

The findings have important implications for sleeping patterns in humans, he said. "For example, the feeling of sleepiness and impaired performance experienced after losing sleep may occur because parts of the brain are falling asleep, while the other parts are staying awake.

"Furthermore, certain sleep disorders, such as sleep walking, are thought to occur when parts of the brain awaken while other parts remain asleep."

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