Every spring and fall, billions of songbirds migrate thousands of miles. Most fly by night, yet are active during the day as well, raising the question of how they cope with little sleep. A new study, published online in the open-access journal PLoS Biology, suggests such nocturnally migrating songbirds simply skimp on sleep--but without the disastrous effects of sleep-deprivation seen in other animals. If researchers discover the mechanisms behind the birds' feat, it could prove useful for people that need to stay awake for long periods, such as pilots, and could shed light on mood disorders that disrupt sleep.
In the study, led by Ruth Benca of the University of Wisconsin, Madison, captive white-crowned sparrows were studied over the course of a year. These songbirds normally migrate approximately 2,700 miles each spring and fall between Alaska and Southern California, with flights typically occurring at night. In laboratory cages, during migratory seasons the birds get restless, with lots of hopping around and wing flapping. The researchers tracked the birds' movement in the cages and placed sensors on their brains to monitor their sleep patterns across the seasons. During the times the birds would normally be migrating, they slept about a third as much as usual and entered more quickly into REM sleep, the stage of sleep typically associated with dreaming in humans. At night, when the birds were active, the brain recordings showed they were fully awake. The researchers also put the birds through tests of their learning ability. During the migration periods, the birds performed normally on little sleep, but during the times of year when they were not migrating, sleep deprivation seemed to hurt the birds' performance.
These results suggest that migrating songbirds simply slash their sleep time rather than "sleepwalking" through their migrations. The mechanisms underlying the birds' ability to forego sleep are unknown, but further studies could shed light on sleep processes in general as well as the neurobiology of some human disorders, particularly mood disorders. "Like migrating sparrows," the authors note, "both depressed and manic patients show reduced latency to REM sleep, loss of slow-wave sleep, and reduced amounts of total sleep." Understanding the mechanisms that power the sleepless flight of songbirds promises to unravel one of the longstanding mysteries of their improbable journey. But it may also shed light on the origins of sleep-related seasonal disorders and the much-debated role of sleep itself.
Citation: Rattenborg NC, et al. (2004) Migratory sleeplessness in the white-crowned sparrow. PLoS Biol 2(7): e212 DOI: 10.1371/journal.pbio.0020212
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