Study

Evidence that birds sleep in mid-flight

Rattenborg, Niels C.; Voirin, Bryson; Cruz, Sebastian M.; Tisdale, Ryan; Dell’Omo, Giacomo; Lipp, Hans-Peter; Wikelski, Martin; Vyssotski, Alexei L.

Nature Communications, 2016

Peer-reviewedfield observationwild

What they did

Miniature devices recording brain activity, head movement and GPS position were fitted to great frigatebirds on the Galápagos, which then flew continuously over the ocean for up to ten days. Sleep was scored from the electroencephalogram in flight and compared with sleep recorded on land after the birds returned.

Studied in: Fregata minor
Sample size: 15 great frigatebirds carrying EEG and movement loggers

What they found

Frigatebirds slept in flight, in both hemispheres at once and one hemisphere at a time, usually while circling in rising air. Flight sleep totalled around 42 minutes a day against more than 12 hours a day on land — a reduction of roughly 95% — and the birds showed brief episodes of REM sleep in flight without falling.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

Birds can sleep in flight but do so far less than on land, indicating that the demands of flight, rather than an inability to sleep, limit sleep aloft.

How NatureHQ reads it

The headline is that birds sleep while flying. The more interesting number is 42 minutes: they *can* and they almost do not, which reframes the question from "how do they manage it" to "how do they survive without it". It is also a clean demonstration that unihemispheric sleep is not exclusive to marine mammals, and that REM sleep does not require lying down.

Limitations

One species, and one that soars rather than flapping continuously — a swift or a warbler faces a different problem.
Fifteen birds, all female, carrying instruments that may themselves affect behaviour.
Whether the flight sleep is restorative in the way ground sleep is remains unknown.