A team from Germany has found great frigatebirds sleep mid-flight after fitting them with small brain activity monitors and movement trackers.
The red-breasted birds can fly continuously for up to two months, without landing to take a nap.
Now scientists have found that they actually sleep for an average of 41 minutes per day – even though they are in flight – after monitoring 14 of the birds.
The naps happen in short episodes of about 12 seconds each, usually using only one half of their brain but sometimes using both.
Niels Rattenborg, from the Max Planck Institute for Ornithology in Germany, said: “Some people thought that all their sleep would have to be unihemispheric otherwise they would drop from the sky.
“But that’s not the case – they can sleep with both hemispheres and they just continue soaring.”
The short sleeps usually took place while the birds are circling in rising air currents, meaning they do not need to flap their wings.
The brain hemisphere connected to the eye facing the direction of the turn was more likely to stay awake, but not always.
Dr Rattenborg told the New Scientist: “This may be to avoid collisions with other birds that are sleeping in the same air mass.
“For me, that was interesting because it’s similar to what we have shown in mallard ducks. When they’re sleeping in a group, the ones on the edge will keep one eye open that’s facing outwards.”
The birds’ ability to survive on such small amounts of sleep is likely to be studied further, given how badly sleep deprivation affects other species.
Dr Rattenborg added: “If we can determine how they’re able to manage on such little sleep, that might inform our understanding of the consequences of sleep loss in humans and other animals.”
The birds are unable to bob in the ocean because they do not have waterproof feathers, so their sleep abilities may have evolved out of necessity.
By contrast, the birds usually slept for more than 12 hours per day on land.