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Fact

Frigatebirds sleep in mid-air, about 40 minutes a day

Great frigatebirds on flights of up to 10 days over the ocean slept 2.89% of the time (0.69 h, about 41 minutes, per day), sometimes with both brain hemispheres asleep at once. Back on the nest, they slept 53.28% of the time.

Confidence: Single studyOne study so far. Treat it as a strong lead, not a law.
Share of time spent asleep, great frigatebirdsTime asleep (%) (%)
Show the data as a table
labelTime asleep (%)
In flight (n=14)2.89%
On land after returning (n=9)53.28%

Results, "Sleep loss in flight", mean ± s.e.m. 2.89 ± 0.58% vs 53.28 ± 4.82%. The 0.69 h/day figure is stated in the abstract (2.89% of 24 h ≈ 0.69 h).

This is the first direct evidence that birds sleep while flying. Frigatebirds can sleep with one hemisphere or with both at once, and both kinds of slow-wave sleep occurred on the wing. Asymmetric sleep made up a larger share in flight (71.57% of slow-wave sleep vs 47.64% on land), and the "awake" side was usually the one facing the direction of a circling turn. In flight, sleep came almost entirely at night, in episodes averaging about 11 seconds (longest 348 s), mostly while the birds circled upward on rising air. Caveats: this is one study of 14 birds, all breeding females from a single colony. The on-land recordings were made right after long flights, when recovery sleep is likely, so the 53% is not a normal day's sleep on land.

Source and method

  1. Rattenborg NC, Voirin B, Cruz SM, Tisdale R, Dell'Omo G, Lipp H-P, Wikelski M, Vyssotski AL (2016). Evidence that birds sleep in mid-flight. Nature CommunicationsSample: 15 breeding female great frigatebirds instrumented; EEG in flight from 14, on land from 9

In August 2014, researchers on Genovesa Island in the Galápagos fitted breeding female great frigatebirds (Fregata minor) with head-mounted loggers. The loggers recorded EEG from both brain hemispheres and 3D head acceleration, and a separate GPS logger tracked position and altitude. The birds then flew foraging trips of up to 10 days and about 3,000 km over the Pacific. Sleep was scored in 4-second epochs as wake, slow-wave sleep or REM sleep. Each bout of slow-wave sleep was classed as bihemispheric (both halves asleep), asymmetric or unihemispheric (one half asleep) from the difference in slow-wave activity between hemispheres. Land comparisons use recordings averaging 16.4 h made after the birds returned to the nest.

Published October 14, 2026.

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