Great Frigatebirds spend weeks at a time flying over open ocean without landing — some tracked flights lasted up to 10 days without touching down once. For years, scientists assumed birds capable of this kind of endurance flight must be sleeping somehow while airborne, since no animal can function indefinitely without sleep. Nobody had actually proven it, because there was no way to measure a wild bird’s brain activity mid-flight. In 2016, a research team led by Niels Rattenborg at the Max Planck Institute for Ornithology finally found a way — and the answer was stranger than expected.
How Do You Even Test This?
Rattenborg’s team, working with great frigatebirds nesting in the Galápagos Islands, developed a small device that could be temporarily attached to a bird’s head to record electroencephalogram (EEG) brain activity along with head movement data, then retrieved later when the bird returned to land. This let researchers directly measure brain states — awake, slow wave sleep, and REM sleep — during real, extended ocean flights lasting up to 10 days, rather than relying on guesswork or behavioral observation alone.
Frigatebirds were a good fit for this kind of study for a practical reason too: the Galápagos nesting population had grown accustomed to years of human observation and was calm enough to allow researchers to safely catch and fit the birds with the recording equipment.
What They Found: Yes, They Sleep — But Barely
The confirmed result: frigatebirds genuinely do sleep while flying, using both of the sleep mechanisms available to them. Sometimes they slept with only one half of the brain at a time (unihemispheric sleep, with one eye staying open), and — more surprisingly to researchers — they also sometimes slept with both hemispheres simultaneously while still airborne, something not previously confirmed in a flying bird.
But the total amount of sleep was strikingly small. Across the full flight recordings, frigatebirds slept for an average of just 0.69 hours per day — roughly 41 minutes — occurring in short bursts, mostly after dark while the birds were soaring on rising air currents rather than actively flapping. That’s a small fraction of what the same birds get once back on land: roughly 12.8 hours of sleep per day, over 18 times more.
Sleeping With One Eye on the Turn
One of the more specific findings involved exactly when and how the unihemispheric sleep occurred. When frigatebirds circled within rising thermal air currents to gain altitude, the brain hemisphere connected to the eye facing into the turn tended to stay awake, while the opposite hemisphere slept. Researchers interpreted this as the bird effectively keeping watch on where it was turning, even while partially asleep — plausible evidence that this pattern exists specifically to avoid mid-air collisions with other birds, since frigatebirds have no aerial predators to watch for instead.
Why Both-Hemisphere Sleep Was the Real Surprise
Before this study, the working assumption in sleep science was that flying birds would need to sleep unihemispherically — keeping at least one half of the brain alert at all times — specifically to maintain enough awareness for aerodynamic control and collision avoidance. Finding that frigatebirds also slept with both hemispheres at once, apparently without immediately losing control or crashing, challenged that assumption directly. It suggested that at least during stable, low-effort soaring flight, a bird doesn’t necessarily need any part of its brain actively monitoring the environment to stay briefly airborne and stable.
How Do They Function on So Little Sleep?
This remains a genuine open question in sleep science. In most animals, including humans, significant sleep deprivation causes measurable declines in attention, reaction time, and decision-making. Frigatebirds apparently sustain multi-day foraging flights on a small fraction of their normal sleep need without any documented crash or major failure of flight control during the study, and researchers have explicitly stated that how they manage this remains unresolved.
The same research group has separately studied Pectoral Sandpipers, another species shown to function for extended periods on unusually little sleep during intense mating competition — suggesting some birds may have evolved genuine flexibility in sleep requirements under specific ecological pressure, a trait not well understood in mammals.
FAQ: Do Birds Sleep While Flying?
Can birds actually sleep while flying?
Yes, confirmed directly for the first time in a 2016 study on great frigatebirds using EEG brain recordings during real multi-day ocean flights — the first direct evidence of sleep in a flying bird.
How much do they actually sleep while airborne?
Very little — about 0.69 hours (roughly 41 minutes) per day during flight, compared to about 12.8 hours per day once back on land.
Do birds sleep with half their brain awake while flying?
Often, yes — unihemispheric sleep, with one side of the brain resting while the other stays alert, particularly while circling in thermals. Surprisingly, researchers also documented some sleep occurring in both hemispheres simultaneously during flight.
Why do frigatebirds sleep so little in the air?
Researchers believe the ongoing demands of flight — navigation, foraging opportunities, and avoiding collisions — leave little room for extended sleep, even though the birds clearly need and get much more sleep once they land.
For more on remarkable bird flight and endurance, see our guide on Chimney Swifts, birds that rarely land at all, and our guide on how perching birds sleep without falling. Read the original study in Nature Communications (Rattenborg et al., 2016).
MD Imdadul Haque is the founder of BirdzFly, a research-backed resource on North American backyard birds. Drawing on years of independent study into bird identification, behavior, and migration patterns, he writes in-depth guides designed to help US birders understand the species in their own backyards — from common feeder visitors to seasonal migrants passing through.
