Birds can sleep with half their brain while flying over oceans

Health · Published 18 October 2026 Swipe more like this →

In short: Imagine navigating a 3,000 km journey across the open sea without ever landing. For the great frigatebird, this isn't a myth. These birds utilize a unique biological hack called unihemispheric slow-wave sleep, allowing one side of the brain to rest while the other stays sharp enough to pilot the wings.

Unihemispheric slow-wave sleep (USWS) allows animals to maintain environmental awareness and motor control while resting. In terrestrial birds, this behavior is an adaptive response to ecological demands, enabling them to keep one eye open to monitor for threats while the corresponding brain hemisphere enters deep sleep. This phenomenon has also been observed in aquatic mammals like dolphins, who may use it to regulate body temperature or breathe while resting. Recent neurophysiological recordings of great frigatebirds (Fregata minor) provide the first evidence of this occurring during long-distance flight. These birds often spend weeks over the ocean because their long wings and poorly webbed feet make landing on water energetically costly. To survive, they engage in USWS, which is notably more asymmetric in flight than on land.

While navigating upward air flows, these creatures often slumber, ensuring the eye linked to the active brain half points toward the path of travel to preserve flight stability. However, the data suggests that sleeping on the wing is a compromise. While frigatebirds can sleep in flight, they do so in much shorter bouts and less deeply than they do on land. This indicates that the ecological demands of flight—requiring constant attention—cannot be fully met through unihemispheric sleep alone. Analysing these avian species allows scientists to question the prevailing theory that substantial nightly rest is vital for fitness, implying that mental processes can persist despite minimal slumber.

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