Scientists measured the brain activity of frigatebirds
Researchers at the Max Planck Institute for Ornithology have discovered that birds can sleep in flight.
In the first study of its kind, Dr Niels Rattenborg and an international team of colleagues measured the brain activity of frigatebirds.
They found that the birds sleep in flight with either one cerebral hemisphere at a time or both hemispheres simultaneously.
Furthermore, despite not being able to engage in all types of sleep in flight, the birds slept less than an hour a day.
How frigatebirds are able to perform on such little sleep remains a mystery, the researchers say.
Scientists have long known that some bird species are able to fly non-stop for several days, weeks or even months. Given the adverse effect sleep has on performance, it is assumed that these birds must fulfil their need for sleep while flying.
In the study, the team developed a small device to record electrical changes in brain activity and head movements of flying birds. They attached the device to the head of nesting female frigatebirds in the Galapagos Islands, which subsequently carried the recorder non-stop for flights lasting up to 10 days.
During this period, the device registered the EEG activity of both brain hemispheres and movements of the head, while a GPS on the birds' back recorded location and altitude.
The team found that frigatebirds stay awake during the day, searching for foraging opportunities. But as the sun set, the EEG pattern switched to a slow wave sleep (SWS) pattern for periods of up to several minutes while the birds were in flight.
What is most surprising, the team say, is that the SWS could occur in one hemisphere at a time or both hemispheres together. This suggests that unihemispheric sleep is not required to maintain aerodynamic control.
Another major finding was that despite being able to engage in all types of sleep while flying, frigatebirds slept on average just 43 minutes per day. In comparison, they slept for 12 hours a day when back on land.
"Why they slept so little in flight, even at night when they rarely forage, remains unclear," says Rattenborg.
Looking ahead, the team hope to learn how frigatebirds are able to sustain adaptive performance on such little sleep.
The Veterinary Medicines Directorate (VMD) is inviting applications from veterinary students to attend a one-week extramural studies (EMS) placement in July 2026.
