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Many types of birds migrate at night during certain times of the year. Flying at night reduces the risk of overheating and getting eaten by predators. However, these long night-time flights cause the birds to lose sleep because they normally sleep at night. Yet, the birds appear to do just fine even though they should be sleep deprived. How do they function so well without sleep? New research suggests that the answer is mini-naps!
To get to this answer, scientists recorded the brain wave patterns (EEG) of Swainson's thrushes (Catharus ustulatus), birds that migrate at night. These recordings showed that the birds rested with one hemisphere of their brain asleep and the other hemisphere awake. The birds sometimes slept with only one eye closed (the eye on the opposite side of the sleeping brain). The periods of sleep were very short, lasting an average of only about 14 seconds.
Short naps with one eye closed and one brain hemisphere asleep may allow the birds to watch for danger with the open eye and the awake hemisphere. Ducks and dolphins also sleep this way. Such sleep may be a way for an animal to get needed rest, but still keep moving and aware of its surroundings.
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