Researchers have discovered that birds can sleep in flight without colliding with obstacles or falling from the sky.

Measuring the brain activity of frigatebirds, the researchers found that they sleep in flight with either one cerebral hemisphere at a time or both hemispheres simultaneously. 

Frigatebirds are large seabirds that spend weeks flying non-stop over the ocean in search of flying fish and squid driven to the surface by predatory fish and cetaceans. 

To actually determine whether and how birds sleep in flight, the 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.

Niels Rattenborg from Max Planck Institute for Ornithology in Seewiesen, Germany teamed up with Alexei Vyssotski from University of Zurich and Swiss Federal Institute of Technology who developed a small device to measure electroencephalographic changes in brain activity and head movements in flying birds.

In collaboration with the Galápagos National Park and Sebastian Cruz, an Ecuadorian seabird biologist, the team focused on great frigatebirds nesting on the Galapagos Island.

The researchers temporarily attached the small "flight data recorder" to the head of nesting female frigatebirds. 

The birds then carried the recorder during non-stop foraging flights lasting up to ten days and 3,000 kilometres. 

During this time, the recorder registered activity of both brain hemispheres and movements of the head, while a GPS device on the birds' back tracked their position and altitude. 

After the birds were back on land and had had some time to recover, they were re-caught and the equipment was removed. 

The flight data recorder revealed that frigatebirds sleep during flight, reported the study published in the journal Nature Communications.