For the first time, researchers have found that birds can sleep in flight without colliding with obstacles or falling from the sky.
Together with an international team of colleagues, Niels Rattenborg from the Max Planck Institute in Germany measured the brain activity of frigatebirds and found that they sleep in flight with either one cerebral hemisphere at a time or both hemispheres simultaneously.
Despite being able to engage in all types of sleep in flight, the birds slept less than an hour a day, a mere fraction of the time spent sleeping on land.
It is known that some swifts, songbirds, sandpipers, and seabirds fly non-stop for several days, weeks, or months as they traverse the globe, researchers said.
Given the adverse effect sleep loss has on performance, it is commonly assumed that these birds must fulfill their daily need for sleep on the wing, they said.
Researchers analysed how birds may sleep in flight without colliding with obstacles or falling from the sky. One way they do this may be to only switch off half of the brain at a time, as Rattenborg showed in mallard ducks sleeping in a dangerous situation on land.
When sleeping at the edge of a group, mallards keep one cerebral hemisphere awake and the corresponding eye open and directed away from the other birds, towards a potential threat.
Based on these findings and the fact that dolphins can swim while sleeping unihemispherically, it is commonly assumed that birds also rely on this sort of autopilot to navigate and maintain aerodynamic control during flight.
It is also possible that birds evolved a way to cheat on sleep. Researchers' recent discovery that male pectoral sandpipers competing for females can perform adaptively for several weeks despite sleeping very little raised the possibility that birds simply forgo sleep altogether in flight.
To actually determine whether and how birds sleep in flight, researchers needed to record the changes in brain activity and behaviour that distinguish wakefulness from the two types of sleep found in birds: slow wave sleep (SWS) and rapid eye movement (REM) sleep.
Rattenborg teamed up with Alexei Vyssotski from University of Zurich in Switzerland who developed a small device to measure electroencephalographic changes in brain activity and head movements in flying birds.
Researchers found that despite being able to engage in all types of sleep on the wing, on average frigatebirds slept only 42 minutes per day.
In contrast, when back on land they slept for over twelve hours per day. In addition, episodes of sleep were longer and deeper on land.
"Why they sleep so little in flight, even at night when they rarely forage, remains unclear," said Rattenborg.
The findings were published in the journal Nature Communications.
