When Sleep Is for the Birds

Birds Can Sleep While Flying, With One Eye Open


Have you ever wondered about how animals sleep? In particular, do birds slumber and dream? How do they accomplish this when migrating over long distances for days or even weeks at a time? Learn about our understanding of sleep as it occurs in birds.

Birds Share Sleep Traits with Mammals

It is somewhat surprising that birds – that are more closely related to reptiles – share many sleep characteristics with mammals.

In fact, the members of the avian class are the only animals outside of mammals that are noted to have both non-REM and REM sleep. Despite these similarities, they have a few unique manifestations of sleep behaviors.

When the sleep of birds is studied with EEG to measure the electrical activity of the brain, specific changes are noted that reflect various changes of sleep. With non-REM sleep, the EEG demonstrates a high voltage, low frequency tracing. This similarly occurs in other animals, including humans.

Birds Can Sleep with an Eye Open or While Flying

Birds are interesting in that they often keep one eye open during non-REM sleep. Reptiles do as well. How is this accomplished? It may relate to the fact that birds, and other migrating animals, are able to sleep with one half of their brains at a time. This phenomenon is called hemispheric sleep.

By closing just one eye, the bird is able to stay aware of its environment and the potential presence of predators.

The wiring works in such a way that the eye closed is opposite to the side of the brain that is asleep. Research demonstrates that birds that are sleeping in riskier situation spend more time with one eye open while sleeping. (Incidentally, humans may sleep with eyes open for different reasons.) They will also keep the eye open that is closer to the potential threat.

Being able to sleep with half of the brain at a time has some noteworthy advantages. Beyond protection from predators, other activities can be enhanced by maintaining brain productivity over prolonged periods. In particular, this phenomenon allows birds to fly continuously for days, weeks, and longer while sleeping during flight. It is not necessary for them to land to rest. Instead, half the brain powers down while the other half maintains the course.

What Do Birds Dream About?

Many species of animals demonstrate rapid eye movement (REM) sleep, and birds are no exception. As in humans, REM sleep appears similar to wakefulness when observed with an EEG. In addition, features of the state are observed: rapid eye movements, twitching of muscles, and reduced temperature regulation. There are a few observed differences, however.

During typical REM sleep, the muscles are relaxed to prevent the enactment of dreams from occurring. If you dream about jumping out of a window, you don’t want to actually do it while remaining unconscious. Abnormalities of this normal sleep paralysis can occur in REM behavior disorder. When observing the REM sleep of birds, the measure of muscle activity via EMG rarely shows relaxed or paralyzed muscles (a condition called atonia).

Nevertheless, reduced muscle tone can be noted. For example, head drooping is often observed when birds are in REM sleep.

In addition, REM sleep is often much shorter in birds compared to mammals. Each episode is brief, often lasting less than 10 seconds. When the entire period of sleep is assessed, less REM occurs overall. Interestingly, songbirds are observed to have more REM sleep. It is fascinating to speculate on the role of REM sleep in learning and whether these birds have more utility for this stage of sleep.

In many birds, but not all, the periods of REM sleep become more prolonged towards morning, just as occurs in other animals.

There is also some evidence that the amount of REM sleep increases following sleep deprivation.

No one really known for sure what birds dream about, but one might surmise that they are re-experiencing their daytime activities, just as humans often do.

Sleep seems to be important for all known animals, and birds are no exception.


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