Like virtually all animals, birds need to sleep, and although scientists don’t know for sure why sleep is so essential, they do know that the way birds sleep is different from how humans and most other mammals sleep.
One of the ways they differ is that birds exhibit something called unihemispheric slow-wave sleep (USWS), sometimes known as asymmetric slow-wave sleep. Slow-wave sleep is deep sleep and is the stage of sleep when your body is in its most restful state. Your brain waves are low, your heart rate and breathing rate drop, and your muscles relax. During slow-wave sleep, your body physically restores itself. Blood flow to your muscles increases, your cells are regenerated, tissues and bones are repaired, and your immune system is strengthened.
In USWS, only one half of the brain rests while the other half remains alert, in contrast to normal sleep where both halves of the brain are asleep, known as bihemispheric slow-wave sleep (BSWS).
Birds, along with aquatic mammals including dolphins, whales, and sea lions, and possibly some reptiles, are the only species known to exhibit USWS. For aquatic mammals, USWS allows them to swim to the surface of the water to breathe while asleep.
It is not as clear why USWS occurs in birds, but scientists think it gives them the ability to evade predators or undertake long migrations without stopping. A 2016 study showed that frigatebirds can stay aloft for up to two months by taking short bursts of USWS.
While engaging in USWS, birds intermittently open one eye in a behaviour known as ‘peeking’. Each hemisphere of the brain is connected to the opposite eye, so if the left hemisphere is awake, the right eye is open and vice versa.
During peeking, the eyelid opens to expose the pupil so they can monitor their surroundings and look out for predators. This is vital for their survival, as birds are at their most vulnerable when they are asleep. Scientists have also found that birds are able to control their sleep and wakefulness, so when they are sleeping in particularly risky situations, they increase the proportion of their sleep where they have one eye open and half their brain awake.
In 2020, a team of researchers wanted to find out whether human activity affected the sleep patterns of birds. They looked at Eurasian oystercatchers, which commonly sleep in large groups on the ground during the day at high tide. Sleeping on the ground puts them at risk from not only aerial predators such as birds of prey but also ground-dwelling predators like foxes.
Over 4 months the scientists from Bangor University monitored the eye movements of 300 sleeping oystercatchers at two sites on the Menai Strait in North Wales. They filmed the birds with a spotting scope fitted with a camera, and noted how they were affected by human activity, changes in weather, and the behaviour of nearby birds.
The results, which were published in the Journal of Zoology, found that the oystercatchers kept their eyes open for longer when walkers passed within 150 m of their roost and for significantly longer if the walker had a dog.
The team also found that the birds peeked more often but for shorter periods when boats passed by, suggesting that they felt safer because they are regularly disturbed by boats, so perceive them as less of a threat.
Additionally, the size of the sleeping group had an impact on how often the oystercatchers slept with one eye open. In larger groups, they peeked less often, but if birds surrounding the observed group were awake then peeking increased.
The findings backed up previous research by Niels Rattenborg, a sleep researcher at Indiana State University, which found that ducks that slept in a row also engage in USWS, but the individuals at the end of the row engage in single-hemisphere sleep more often than those in the middle.
To conduct his experiment, Rattenborg placed 4 mallards in a row of clear plastic tanks and waited for them to fall asleep. He noted that the ducks in the middle tanks would almost always sleep with both eyes shut, while the ducks at the end of the row kept an eye open for about a third of the time. An electroencephalogram which recorded the ducks’ brain waves showed the side of the brain with the open eye had almost the same level of activity as an awake bird, and with just one eye open the mallard could respond to a perceived threat in less than a fifth of a second.
It has been shown that birds have the ability to sleep with one eye open from the moment they are born. And in 2001, a team of scientists from the University of Padua in Italy found that light exposure during incubation has an impact on which eye is kept open.
During the last few days of incubation, chick embryos turn so that the right eye lies next to the eggshell and is stimulated by light entering the egg. The left eye is positioned next to the body so isn’t exposed to much light at all.
In their experiment, the team incubated two groups of 12 domestic chicks. During incubation, one group was exposed to light, while the other was kept in the dark.
Observing the chicks that were incubated in light conditions, they found that in the first couple of days after hatching, chicks tended to sleep more with the right eye open – the same eye that had been exposed to light during incubation. However, the chicks that were incubated in the dark slept with the left eye open more often once hatched.
The difference didn’t last long; after 5 days both light and dark-incubated chicks had a preference for sleeping with their left eye open. And the scientists found that over time, the preference changed again. For example, when chicks were reared with an imprinted object, during their second week they tended to sleep with their right eye open more often, except if the imprinted object is suddenly removed, and then they open their left eye more. The team believe this is due to the chick feeling threatened and evoking an antipredatory reaction.
The body of research around birds’ unihemispheric sleeping patterns has wider implications for the importance of sleep in general. For example, USWS may explain why humans often experience poor sleep in unfamiliar surroundings.