Birds spend most of their lives flying. Previous researchers have speculated that birds snooze during these long-hour flights, but there has been no evidence to show it, until recently, when an international team of researchers tracked the brain activity of frigatebirds in-flight.

In the study titled "Evidence that birds sleep in mid-flight" published in the journal Nature, Niels Rattenborg from Max Planck Institute and his colleagues from other institutions showed evidence that birds snooze in-flight and they do so by engaging in different types of sleep.

For the study, the researchers attached electroencephalograms on the heads of nesting female frigatebirds to track their brain activity while they were in-flight for 10 days. They also attached a GPS device on each of the birds' back to track its location and altitude.

Once the birds landed, the researchers gathered the records and found out that during those 10 days on air, the birds engaged in two types of sleep -- slow wave sleep (SWS), also known as deep sleep, and rapid eye movement (REM), characterized by random movement of the eyes and low muscle control.

It has been known before that to avoid danger when flying, birds rely on unihemispheric sleep or the ability to shut down half of the brain while the other half remains active. Gizmodo notes that it also allows them to get some rest while maintaining their aerodynamic control.

But the study of Rattenborg and colleagues revealed that birds do not need unihemispheric sleep to track their surroundings.

Impressively, birds not only engage in SWS with one hemisphere active but with both hemispheres shut down as well.

Another surprising fact is that even though birds are able to have aerodynamic control in SWS and REM, they did not sleep too much on their flight. While the frigatebirds sleep on land at an average of 12 hours, they only sleep in flight for 42 minutes per day.

"Why they sleep so little in flight, even at night when they rarely forage, remains unclear", says Rattenborg in a press release published in

Rattenbord hopes that in the future, they may be able to find out why birds, despite having less sleep, are able to perform their activities while whereas humans with little sleep cannot function in full performance.

"Why we, and many other animals, suffer dramatically from sleep loss whereas some birds are able to perform adaptively on far less sleep remains a mystery," he added.