A new study showed that during the migration, birds take hundreds of naps during the day, each lasting only few seconds, to compensate the night sleep loss.

Swainson's thrushes have a migration route of 3,000 miles from their breeding areas in northern Canada and Alaska to winter territories in Central and South America. They travel mostly during the night and often for long hours at a time, leaving little time for sleep. Scientists recorded caged thrushes for an entire year to see when and how long they slept. During autumn and spring - the migration periods - they reverse their typical sleep patterns, staying awake at night and resting during day. The difference is that, instead of sleeping for long stretches at a time, the birds took several naps a day, of only 9 seconds on average. The scientists discovered also two other forms of sleep beside shut-eye type.

The unilateral eye closure (UEC) means the birds rested one eye and one half of their brains while their other eye and brain hemisphere remained open and active, keeping them semi-alert to danger.

Other process, named drowsiness, is characterized by a partial shutting of both eyes that still allows for some visual processing. Drowsiness "is probably a state that, to some extent, grants the benefits of sleep while allowing for some of the benefits of wakefulness," said study team member Thomas Fuchs of Bowling Green State University in Ohio.

By alternating between naps, UEC and drowsiness, the thrushes and other migratory birds can reap some of the benefits of sleep while only marginally increasing their risks of being caught by prey birds. "In terms of quality, drowsiness and unihemispheric sleep may be less beneficial than [normal] sleep, but it may also be safer," Fuchs told LiveScience.

The idea that some birds can really sleep while in flight has not yet been proved. Sleep is nearly an universal must in the animal kingdom. It may be necessary to organize the memories we amass during the day and to give our bodies time to rest, but nothing is proven. "I think what's interesting about our findings is that even animals that should be highly adapted to sleep loss cannot go on indefinitely," Fuchs said.

"That a need for sleep cannot be eliminated even in these species underscores the importance of sleep for many, if not all, animals."