A bird can sleep while flying.

Researchers fitted 15 adult female great frigatebirds with EEG, motion, and GPS loggers. During ocean flights lasting up to 10 days, the birds entered slow-wave sleep with either one brain hemisphere at a time or both at once. In flight they slept about 0.69 hours per day, or 2.89% of flight time, compared with 53.28% on land. 1

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