Some birds possess the remarkable ability to rest their brains while remaining airborne for extended periods. This specialized behavior is an adaptation that allows certain species to survive grueling, non-stop journeys, such as long migrations or oceanic travel. The sleep birds get in the air is fundamentally different from human slumber. This aerial rest ensures birds can sustain continuous flight without being forced to land in unsuitable or dangerous environments.
How Birds Sleep One Hemisphere at a Time
The physiological mechanism enabling birds to sleep during flight is unihemispheric slow-wave sleep (USWS). This unique state allows one half of the brain to enter a deep rest while the other half remains awake and vigilant. During USWS, the resting hemisphere exhibits the slow, high-amplitude brain waves characteristic of deep sleep. Simultaneously, the active hemisphere displays the low-amplitude, high-frequency waves associated with wakefulness and alertness.
This asymmetrical sleep pattern is directly linked to the bird’s visual system, allowing the eye connected to the awake hemisphere to remain open. Keeping one eye open ensures the bird maintains awareness of its surroundings, such as watching for predators or navigating obstacles. The awake brain hemisphere processes sensory input from the open eye, providing a partial state of consciousness.
The vigilance provided by USWS is also crucial for maintaining aerodynamic control during flight. USWS ensures the bird can continuously monitor its trajectory and make subtle corrections necessary to navigate air currents. This ability to rest without losing environmental awareness makes continuous aerial travel possible for these species.
Scientific Evidence for Sleeping on the Wing
The suspicion that birds could sleep in flight was confirmed using sophisticated, miniaturized tracking technology. Researchers measured the brain activity of flying birds by implanting tiny electroencephalograms (EEGs). These devices record the electrical waves produced by both hemispheres, providing concrete data on the presence and duration of sleep states.
The EEG recordings revealed the distinct patterns of USWS, proving one hemisphere was resting while the other was awake. This evidence was paired with movement data collected by GPS trackers and accelerometers. The data showed USWS often coincided with soaring flight, where the bird was circling in rising air currents. In these instances, the eye linked to the awake hemisphere was often oriented toward the direction of flight.
Which Birds Rely on Aerial Sleep
The ability to sleep in flight is primarily found in birds whose lifestyles demand continuous, long-distance travel over environments where landing is not an option. Frigatebirds are a prime example, known to spend weeks on oceanic foraging trips without touching down. Since these seabirds are not waterproof, they cannot rest on the ocean surface, making aerial sleep essential for survival.
During their long journeys, Frigatebirds have been recorded sleeping for brief periods, sometimes totaling only about 45 minutes per day. This minimal sleep is sufficient to prevent the negative effects of sleep deprivation. They typically engage in rebound sleep upon reaching land. Alpine Swifts also exhibit extraordinary endurance, having been tracked flying continuously for nearly 200 days without landing.
Albatrosses undertake vast foraging loops over the open ocean, relying on dynamic soaring to minimize energy expenditure. For these species, USWS allows them to rest their brains while maintaining the necessary awareness to navigate the winds.