Migratory birds travel vast distances as they move between their summer and winter homes. Songbirds make the feat all the more impressive, traveling mostly at night yet remaining active during the day, thereby cutting back on opportunities for sleep. Research published today in the journal PLoS Biology indicates that during migration season the creatures manage to survive on less shuteye without deleterious effects often observed in other sleep-deprived animals.

In the wild, white-crowned sparrows travel some 2,700 miles between Alaska and Southern California twice annually. Ruth Benca of the University of Wisconsin at Madison and her colleagues observed captive animals for a year, tracking their sleeping patterns and movements. During migratory season, the caged birds became restless and exhibited increased hopping and wing flapping. In addition, the creatures slept only about a third as much as usual and entered REM sleep--the type associated with dreaming in humans--more quickly than they did during other times of the year. At night, while their free-living counterparts were flying, the captive animals were fully awake and performed normally on learning tests, which suggests that the birds don't sleepwalk during their migrations.

The scientists determined that lack of sleep during nonmigration seasons does negatively affect the birds, however. This suggests that they have an unprecedented ability to reduce sleep specifically during migration without deficits in their cognitive function. Just how the creatures accomplish this remains unclear. The authors note that understanding the mechanisms that mediate migratory sleeplessness may provide insights into the etiology of changes in sleep and behavior in seasonal mood disorders, as well as into the functions of sleep itself.