Birds actually sleep while flying, according to an international team of researchers. However, they only sleep for a very short amount of time while using only one hemisphere of their brain.

It is known that many bird species can fly great distances without stopping, so scientists have been speculating that birds nap in flight. To determine if the birds engage in this behavior, the team, including researchers from the Max Planck Institute in Seewiesen, Bavaria, placed brain activity monitors and movement trackers to 14 great frigate birds from the Galapagos Islands. These birds are famous for travelling for months without stopping.

In their study published in the journal Nature Communications, they revealed that the birds require less sleep while flying over the ocean than flying on land.  The researchers determined that birds have two types of sleep: slow wave sleep (SWS), which lasts several minutes, and rapid eye movement (REM) sleep, which only lasts a few seconds.

Overall, the birds only require up to 42 minutes of sleep each day while flying. By comparison, birds sleep more than 12 hours when they are back on land.

To avoid any threats from predators or collisions, with ducks for example, while flying, the frigate birds birds keep one hemisphere of their brains awake. In a way, the birds are usually half asleep.

However, there are also times when the birds were able to keep both brain hemispheres asleep without experiencing any problems. Despite these discoveries, the research team asserts that further investigations are required.

The researchers hope that they could find out why the great frigate birds were still able to perform well despite sleeping for a short duration. In other animals, especially mammals, high amount of sleep is required to function properly.

“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,” points out the study’s lead researcher Niels Rattenborg from the Max Planck Institute.