There are a few bird species that can fly for numerous days, weeks on end without ever coming in touch with the ground. For long, scientists thought that they balance by taking in-flight naps, but haven’t ever observed it in reality.

In an email to The Christian Science Monitor, main author Niels Rattenborg said that most people supposed that birds sleep unihemispherically during their journey. However, Rattenborg said that the frigatebirds also at times take nap with the brain’s both halves at the same time.

The Max Planck Institute for Ornithology researchers followed the brain activity of 15 frigatebirds over a period of 10 days and 3,000 miles. Their research, which appeared in the journal Nature Communications on Wednesday, suggested that the tropical seabird can vary in degrees of sleep in flight.

In most cases, the birds rested only one hemisphere of their brain, and managed to maintain limited alertness. But, for a quite short span they may also fall into deep REM sleep without any effect on flight performance. Dolphins also sleep unihemispherically, and they are the sole another animal that sleeps while moving.

Co-author Alexei Vyssotski said, “A surprising finding was that majority of sleep was detected when animals were moving up by rising airflow in thermal, not during straight gliding downwards. It’s easy to explain by safety reasons: when sleeping animal is moving up, no danger of collision with water exists”.

But, probably the most astonishing disclosure was that, in spite of capable of doing so, frigatebirds slept ‘remarkably little’ in the air.

The concept of sleep can’t be grasped from a mammal-centric viewpoint, but it in reality differs widely across distinct species. For instance, humans deteriorate at a fast speed in the absence of enough rest. But numerous bird species, including the frigatebird, work on barely any sleep at all.

According to a report in CS Monitor by Joseph Dussault, "Some bird species can fly for weeks on end without ever touching ground. Scientists have long suspected that they compensate by taking in-flight naps, but none have actually observed it. Until now, that is."

“A surprising finding was that a majority of sleep was detected when animals were moving up by a rising airflow in a thermal, and not during straight gliding downwards,” says co-author Alexei Vyssotski in an email to the Monitor.

“Previous laboratory studies have shown that pigeons can stay awake for weeks without visible health problems, contrary to rats which will die under identical conditions,” says Dr. Vyssotski. “A surprising conclusion has been derived, that birds are lacking so-called ‘sleep-deprivation syndrome.’”

A report published in Seeker revealed, "Researchers from the Max Planck Institute for Ornithology put the "in-flight sleep" assumption to the test, outfitting frigatebirds with tiny, specially designed devices that measure their brain activity."

Once the birds returned themselves and their data recorders to their Galapagos nests and the numbers could be crunched, the scientists found that the birds did indeed sleep mid-flight. During the day, they stayed on the job -- awake and ready to forage as they flew.

The fact that the birds did, albeit infrequently, sleep with both hemispheres shut down told the scientists that unihemispheric sleep was not required for the animals to remain in control of their flight, aerodynamically speaking.