Migrating birds fly non-stop for days at a time, sometimes crossing long stretches of ocean where they have no opportunities to land and rest. People have long speculated that birds still find a way to sleep, but for the first time, now scientists have observed migrating birds taking mid-air naps, proving that they don’t pull all-nighters to finish their days-long or weeks-long journeys.

A study in Nature Communications analyzed the brain activity of nine frigatebirds from the Galápagos Islands, a group that spends weeks or months flying over the ocean without stopping to take a load off. The researchers attached to each bird a sensor designed for homing pigeons that detects brain-wave activity using an electroencephalogram test, as well as an accelerometer to measure the bird's head movements. They then tracked the birds for 10 days. 

Like dolphins, birds can sleep with one eye open and half their brain awake, a fact that scientists have known for more than a decade. This study found that birds can partially shut off their brains while flying, too.

While they often slept with just one hemisphere at a time, the birds did occasionally allow both hemispheres of the brain to sleep while still flying. When they slept mid-flight, they tended to do so at higher altitudes and in rising air currents, so they were actually ascending as they snoozed. However, just because they can sleep doesn’t always mean they do.

On land, the birds tend to sleep about 50 percent of their day (during daylight and nighttime hours). While flying they only slept at night, for less than an hour, and slept less deeply than they did on land.That’s probably because while migrating, they need to be on the lookout for threats and for potential food sources, and prioritize those activities over getting rest.

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