Can birds truly sleep while flying? The idea of a creature resting while soaring through the air seems to defy natural limits. Birds exhibit remarkable adaptations for survival, and their sleep patterns are no exception. While in-flight rest might appear impossible, some avian species have developed unique ways to achieve a state of rest during sustained flight.
The Unique Science of Avian Sleep
The ability of some birds to sleep while flying is attributed to unihemispheric slow-wave sleep (USWS). This specialized sleep allows one half of the brain to enter deep rest, characterized by slow brain waves, while the other hemisphere remains awake and alert. The eye connected to the active brain stays open, enabling the bird to maintain awareness of its surroundings. This contrasts with bihemispheric sleep, the full, simultaneous sleep of both brain hemispheres, typical for humans and most other mammals.
USWS serves an evolutionary purpose, primarily allowing birds to detect predators or navigate during flight. Birds sleeping with one eye open can direct it towards potential threats, ready to react if danger approaches. This adaptation helps species undertaking long migrations or facing continuous environmental challenges, balancing rest with vigilance. While USWS allows for rest, it is less restorative than full sleep.
Aerial Sleepers: Masters of Mid-Flight Naps
Certain bird species have perfected sleeping while airborne, a necessary adaptation for their long-distance movements and foraging strategies. Great frigatebirds, for example, spend weeks flying over oceans, rarely touching land or water. Studies using electroencephalogram (EEG) recordings confirm these birds engage in USWS during flight, typically in short bursts of around ten seconds. This allows them limited rest while continuously soaring.
Frigatebirds often sleep while circling thermal updrafts, which helps them gain altitude without expending much energy. Although they can experience both USWS and brief periods of bihemispheric sleep, their total in-flight sleep is minimal, averaging about 45 minutes per day. This is significantly less than the 12 hours they might sleep on land, suggesting they accumulate a “sleep debt” during extended flights. Alpine swifts are another example, capable of staying airborne for months, even up to ten consecutive months without landing. They are believed to sleep in short bursts while gliding at high altitudes, using air currents to maintain flight.
Why Not All Birds Sleep On The Wing
Despite the adaptations of some species, the ability to sleep while flying is not universal among birds. For most avian species, it is more practical and safer to find a secure perch or nesting site for rest. The energy expenditure required to maintain flight, even during partial sleep, remains substantial for most birds. Factors like flight style, wing shape, and body mechanics make in-flight sleep inefficient or impossible for many.
Most birds rely on traditional bihemispheric sleep, which provides deeper, more restorative rest. They seek sheltered locations, such as tree branches, dense foliage, or rock crevices, to minimize predator exposure while fully resting. For these birds, attempting to sleep while flying would present an unnecessary risk of collision or predation, outweighing any benefit of continuous movement. The unique circumstances of species like frigatebirds and swifts, which spend extensive periods where landing is not an option, drove the evolution of their specialized in-flight sleep capabilities.