A STUDY PUBLISHED IN THIS WEEK'S ISSUE OF NATURE gives new meaning to the phrase "sleeping on the job." Sleep researchers at Indiana State University say most birds have evolved the ability to sleep with one eye open and one half of their brains awake, a phenomenon called unihemispheric sleep. According to Niels C. Rattenborg, the study's lead researcher, this means the birds can get much needed rest while simultaneously watching out for lurking dangers.

To understand when, and why, birds sleep this way, Rattenborg's team videotaped resting mallard ducks. They found that when the ducks were arranged in a row, the end ducks spent three times longer in unihemispheric sleep than their center neighbors. Also, the end ducks controlled which side of the brain slept and which side stayed awake, orienting their open, wakeful eyes toward perceived threats and away from the other ducks. Measurements of brain-wave activity confirmed the behavioral studies, proving a one-to-one correspondence between open eyes and wakeful brains. Can humans sleep like this? Unfortunately not, although Rattenborg says that some sleep disorders, like sleepwalking, may have their roots in unihemispheric sleep.

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