How do we know birds can sleep while flying?
Because researchers measured their brain activity.
For years, scientists suspected that birds making extremely long nonstop flights might sleep on the wing. But a bird gliding quietly is not proof of sleep. The brain itself had to be measured.
A 2016 Nature Communications study tracked Great Frigatebirds during flights over the ocean lasting up to ten days. Small devices recorded their brain activity, called EEG, while they were airborne.
The recordings showed unmistakable sleep.
What kind of sleep happens in the air?
Two forms showed up.
Sometimes one hemisphere of the brain entered slow-wave sleep while the other stayed more alert. That is called unihemispheric slow-wave sleep. At other moments, both hemispheres slept at the same time.
Birds already use one-hemisphere sleep on land in situations where they need to remain partly watchful. Frigatebirds appear able to carry that ability into the sky.
Do birds really sleep with one eye open?
They can.
Each eye is strongly connected to the opposite brain hemisphere. During one-sided sleep, a bird can keep the eye linked to the more awake hemisphere open and directed toward something important.
In the frigatebird study, the birds tended to keep that open eye facing the direction of flight while circling. That makes sense. Even a sleeping bird does not want to bump into its neighbours.
How much do birds sleep while flying?
Much less than you might expect.
The Great Frigatebirds averaged about 0.69 hours of sleep per day while flying, or roughly 42 minutes. That was only about 7 percent of the amount they slept on land.
So yes, they slept in the air. No, they were not getting a normal night's sleep while cruising along.
When did the frigatebirds sleep?
Most airborne sleep happened at night.
The birds often slept while circling in rising air currents rather than during active flapping. Soaring gives them a more stable flight situation, with less muscular work and less need for constant wingbeat control.
Even then, most of the time they stayed awake.
Why can frigatebirds stay in the air for so long?
Great Frigatebirds are built for soaring over open ocean.
Their long narrow wings let them use rising air efficiently. They also avoid landing on the water for long periods because their plumage is less waterproof than that of many seabirds and taking off from the sea can be difficult.
That lifestyle creates a strange problem: if you spend days or weeks over water, where do you sleep?
Part of the answer is apparently: up there.
Do swifts sleep while flying?
Swifts are famous for spending enormous stretches of time in the air, and researchers have tracked Common Swifts remaining airborne for months.
That makes sleep in flight seem plausible, but long airborne duration alone does not prove sleep. Direct brain recordings like the frigatebird study are much stronger evidence.
I would not turn "swifts sleep while flying" into a certainty unless the sleep itself has been measured.
Do migrating songbirds sleep in the air?
Maybe some do, but we should not assume it.
Many songbirds migrate at night, then stop during the day to feed and rest. They can also reduce sleep during migration. Those strategies may get them through the journey without needing to sleep in active flight.
Different birds have different solutions.
Can birds fly safely with half their brain asleep?
At least frigatebirds can manage it for short periods.
One hemisphere remains more alert, and the bird can keep one eye open. The study also found that airborne sleep was shallow, brief, and much less common than sleep on land.
It is more like grabbing tiny bits of sleep when conditions allow than switching off for the night.
Do birds make up for lost sleep after landing?
The study found that frigatebirds slept much more on land than in flight, and their sleep changed after they returned.
That suggests the birds still have normal sleep pressure even though they can suppress it dramatically during long flights.
How they maintain performance on so little sleep remains one of the fascinating parts of the research.
Is sleeping while flying the same as dreaming?
No.
Sleep is a measurable brain state. Dreaming is a subjective experience, which is much harder to prove in an animal. Birds do have REM sleep and other sleep patterns that raise interesting questions about dreams, but we cannot ask a frigatebird what it was experiencing.
That is why the separate question do birds dream? gets a more cautious answer.
Frequently asked questions
Can birds sleep while migrating?
Some birds can sleep in flight, but many migrants also stop to sleep and refuel. The strategy varies by species.
Can a bird sleep with both eyes closed while flying?
Great Frigatebirds showed brief periods of sleep involving both brain hemispheres, but one-sided sleep was an important part of their airborne sleep behaviour.
Can birds sleep standing up?
Yes. Many birds sleep while perched or standing, using leg and tendon arrangements that make maintaining the position easier.
Where do most birds sleep at night?
Most birds are not sleeping in the air. They use branches, dense vegetation, cavities, water, cliffs, buildings, and other sheltered places. See where birds sleep at night and how sleep timing changes for night-active birds.
Forty-two minutes changes the question
The amazing thing is not simply that a frigatebird can sleep in the air. It is that it can fly for days while sleeping so little. That tells us bird sleep can be far more flexible than our own.
Continue with which birds migrate at night, compare migration distances, or see whether birds get tired of flying.