Sleep is a necessary period of reduced responsiveness and lowered metabolic rate for rest and recovery. For birds, this period is highly complex due to the constant threat of predators and the demands of flight. Avian sleep involves remarkable physiological adaptations that allow them to balance the restorative need for rest with the immediate necessity for vigilance. Understanding how birds sleep reveals sophisticated biological strategies designed for survival.
When Birds Sleep
The majority of bird species are diurnal, meaning their activity cycle is primarily driven by the 24-hour light-dark cycle, known as the circadian rhythm. These birds typically begin to settle down and sleep shortly after dusk and resume activity shortly before dawn. The pineal gland secretes the hormone melatonin, which peaks at night and drops during the day, synchronizing their internal clock with the environment.
This natural timing is not always absolute, as external factors can influence their rest period. Exposure to artificial light at night (ALAN), often from human development, can disrupt the natural circadian timing. Light pollution can cause birds to wake up earlier or alter their internal clock, which may suppress the sleep hormone melatonin. The time a bird sleeps is a flexible response to both their internal biological rhythm and environmental light conditions.
How Birds Stay Safe While Sleeping
The behavior of settling down for the night is called roosting, which focuses on finding a location that provides both safety and warmth. Small songbirds often seek out dense foliage, thick shrubs, or tree cavities to avoid ground predators and aerial hunters. These locations offer insulation from harsh weather, which is important for maintaining body temperature during cold nights.
A physical adaptation allows perching birds, or passerines, to remain securely attached to a branch without expending muscular energy. Their feet possess a tendon-locking mechanism where the tendons involuntarily clasp and tighten around the perch as the bird squats down. The grip will not loosen until the bird consciously straightens its leg, preventing them from falling off a branch. For added protection, many species, such as ducks, engage in social roosting, sleeping in large groups where individuals on the periphery remain vigilant to detect danger.
The Physiology of Avian Sleep
Avian sleep is fundamentally different from mammalian sleep, characterized by the ability to rest only a portion of the brain at a time. This is Unihemispheric Slow-Wave Sleep (USWS), where one cerebral hemisphere enters deep rest while the other half remains awake and alert. The eye connected to the awake hemisphere stays open, allowing the bird to monitor its surroundings for potential threats.
The proportion of USWS a bird uses is facultative, meaning it can be voluntarily controlled and increases significantly in environments with high predation risk. For instance, mallard ducks positioned at the edge of a sleeping group exhibit a 150% increase in USWS compared to those in the center. When a bird is in a safe location, they can transition into Bihemispheric Slow-Wave Sleep (BSWS), where both sides of the brain rest simultaneously.
Birds also cycle through the two major sleep states observed in mammals: Slow-Wave Sleep (SWS) and Rapid Eye Movement (REM) sleep. Avian SWS, the deeper restorative phase, occurs in brief bouts lasting just a few minutes, repeated hundreds of times throughout a rest period. In contrast, a bird’s REM sleep is significantly shorter than in mammals, often lasting only a few seconds per episode, and is characterized by increased brain activity. This short REM duration prevents the temporary muscle paralysis that occurs during this phase, allowing the bird to remain ready to fly away instantly if danger is detected.
Sleeping During Flight and Migration
The need for continuous movement during long migratory journeys or extended periods over open water has led to the evolution of aerial sleep strategies. Long-distance fliers, like the Great Frigatebird and some albatross species, utilize their USWS capability to rest one half of the brain while maintaining aerodynamic control and visual navigation. This “napping” often occurs while the bird is soaring on air currents, minimizing the energy expenditure required for flapping.
By employing USWS during flight, the bird obtains partial rest, with the eye associated with the active brain hemisphere facing the direction of travel. However, this in-flight rest is not fully restorative. Studies on frigatebirds show they sleep far less deeply and for a much shorter duration—sometimes less than an hour a day—while airborne compared to when they are on land. Once these birds land, they compensate for the lost deep rest by spending more time in SWS to pay off the accrued sleep debt.