Some swifts and frigatebirds stay aloft for months. But for a long time, scientists did not know if the birds might be sleeping on the wing. A 2016 study provided answers. Tiny devices attached to the heads of frigatebirds revealed fascinating information: the birds did sleep while aloft, most often one half of the brain at a time. But they also fell into normal, whole-brain sleep and sometimes, even deeper REM sleep. But this deepest sleep came in bursts of just a few seconds — an inflight power-nap.

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