The Mystery of Unihemispheric Slow-Wave Sleep (USWS) in Birds

The advice to ‘sleep with one eye open’ is often offered in situations where extreme vigilance is needed, to avoid being caught off-guard. For many birds, their survival depends on their ability to shut off half of their brain to rest, while the other half remains alert to threats, for example, from the weather or predators.

Read on to learn more about the fascinating concept of ‘half-brain sleep’, which is scientifically known as Unihemispheric Slow-Wave Sleep (USWS).

In regular sleep, both eyes are shut, and the brain enters a degree of unconsciousness. However, many birds cannot afford to shut down completely when they need rest, due to the extreme hazards they may face if they lose focus or vigilance, even for a matter of seconds.

They have evolved with a particular adaptation that helps them survive, balancing the need to rest with the importance of staying alert: Unihemispheric Slow-Wave Sleep (USWS).

By understanding the unique sleep patterns of birds, we can gain valuable insight into their survival mechanisms, and how they manage to undertake long and exhausting migration flights without stopping for a breather. We can learn a lot about a bird’s ability to survive common threats regardless of the time of day or night by studying their sleeping habits and behavior.

A Canada Goose. Many birds cannot afford to shut down completely when they need rest, due to the extreme hazards they may face if they lose focus or vigilance, even for a matter of seconds

What is Unihemispheric Slow-Wave Sleep (USWS)?

Unihemispheric slow wave sleep (USWS) is a sleep pattern observed in some birds where only one hemisphere (or half) of the brain enters a state of deep sleep, while the other half remains awake or in a shallow sleep state.

The phenomenon of USWS allows birds to rest while remaining responsive to their environment if needed. Half-brain sleep has been noted in various bird species and serves multiple purposes, including maintaining vigilance, avoiding predators while resting and helping survive long and arduous migration journeys.

During USWS, one eye stays open, and the ‘awake’ hemisphere of the brain remains alert and active. This allows birds to remain in control of their posture and balance, and able to detect and respond to any changes in the environment, including weather and the approach of predators.

The sleeping hemisphere of the brain enters a state of slow-wave sleep, during which synchronized slow electrical waves are released by the brain, lowering the heart rate and breathing rate. In this state, muscles relax and experience an increase in blood flow. Important repair of cells, tissues and bones takes place, and the immune system receives a boost.

In many mammals, the most common form of sleep is non-rapid eye movement (NREM) sleep, where both eyes are closed, and both halves of the brain slow down into a state of unconsciousness. This deeper sleep allows a fuller sense of rest and repair of bodily functions.

A Greylag Goose. During USWS, one eye stays open, and the ‘awake’ hemisphere of the brain remains alert and active

Why Do Birds Need USWS?

Many birds cannot simply switch off, sleep deeply, and know that they will wake up safe the following morning. Threats including predators, would certainly take advantage of their unconscious state, and pluck them from their roost without any warning or defense at all.

USWS contributes to the day-to-day survival of many bird species and also plays a major role in reaching the final destination of challenging annual migration flights safely.

Birds have evolved to take advantage of USWS, helping them adapt to threats in their environment and to match their behavioral needs. By engaging in USWS, birds can benefit from an increased rate of survival when migrating or avoiding predators.

Ground-dwelling birds are particularly vulnerable to the approach of predators while resting, and without the ability to remain at least partially vigilant while asleep, their survival rate would certainly be a lot lower. Shutting off half their brain allows them to catch some Zs at the same time as keeping a watchful eye and ear on their surroundings, and maintaining the ability to react immediately if a hazard or risk is detected.

Migration is a particularly challenging feat for many bird species and can be especially draining on a bird’s energy resources if regular or constant stops are needed for rest breaks. USWS makes long flights over water a possibility, as birds can rest on the wing, rather than coming into land to sleep. By keeping one eye open, they can use the navigational skills controlled by the half of their brain that is awake, and remain both on course and aware of hazards around them, without becoming too disoriented or exhausted.

Traveling in a flock also allows migrating birds to rest while others around them remain vigilant, and enables them to avoid predation, and obstacles and collectively reduce their vulnerability to other risks.

Retaining a degree of vigilance when resting helps birds notice the presence of predators nearby while roosting or incubating eggs. This enables them to escape attacks or raise the alarm to avoid being targeted by other birds or mammals.

A Common Swift in-flight. USWS makes long flights over water a possibility, as birds are able to rest on the wing, rather than coming into land to sleep

Bird Species Known to Exhibit USWS

The ability to shut off part of the brain while sleeping is not limited to one specific bird species. Many different birds, from ducks to pigeons, to sparrows, are able to practice USWS to various degrees and for different durations. Read on to find out more about birds that have perfected the art of sleeping with one eye open.

One of the world’s most common duck species, mallards are well practiced in USWS, using the technique during migration to conserve energy. They also rely on half-brain sleep to remain responsive to environmental hazards when roosting, both on land and on water.

White-crowned sparrows and Swainson’s thrushes are two smaller songbird species that rely on USWS during their annual migrations, benefitting from the ability to rest and not expend excessive energy while travelling to and from breeding grounds.

Black-capped chickadees are observed to switch off one-half of their brain when resting, particularly in winter months, to make their energy expenditure more efficient while remaining aware of any changes in their immediate environment.

Starlings are another active, adaptable bird species that use USWS when roosting to ensure overnight survival and vigilance against any predators that may approach while they are asleep.

Frigatebirds are perhaps the world champions of USWS, able to remain airborne and in a state of half-sleep for up to two months. Tagged frigatebirds were recorded at sea for several weeks at a time, covering 300 miles per day, and dipping down to the water only briefly to feed on fish that had leapt into the air from the ocean surface.

A Swainson's Thrush is one of the smaller songbird species that relies on USWS during their annual migrations

The Evolutionary Significance of USWS

The evolutionary significance of Unihemispheric Slow-Wave Sleep is the topic of much ongoing research. Knowing more about how some birds have adapted and evolved to use USWS to conserve energy and avoid predation is crucial to having a better understanding of the ability and potential to survive the challenges and pressures of the world they live in.

The ability to continue with some basic activity levels while resting one half of their brain has evolved as a response to a number of environmental and ecological pressures, as well as lifestyle and behavioral factors.

Lack of nesting sites may result in birds setting up home in less suitable locations, meaning there is an increased risk of where they end up raising their young. In high-risk environments, heightened vigilance is necessary to avoid predators on a daily basis, and it is believed that, in part, the use of USWS has evolved as a response to these dangers.

The changing landscapes of the modern world may also have interrupted usual migration corridors of birds, meaning that previous stopover points are no longer available, and birds must find an alternative to breaking their journey.

Where pausing for a rest break and a period of deep sleep is not an option, or where it would require expending too much energy to pause and then resume a journey, many bird species have evolved with the ability to continue flying while half of their brain is asleep.

A Great Frigatebird in-flight. Many bird species have evolved with the ability to continue flying while half of their brain is asleep

Comparisons with Other Animals

Many birds that would otherwise be vulnerable when sleeping, such as ground-dwelling birds or those on migration flights, have adapted to use USWS to improve their chances of survival. But what about other animals? Find out more about mammals and even fish that may depend on USWS to rest while remaining vigilant.

Aquatic mammals, including dolphins, seals, sea lions and whales, are known to engage in USWS. As mammals, they rely on oxygen to breathe, but also survive by swimming underwater, so Unihemispheric Slow-Wave Sleep enables them to do both, without rest or breathing being compromised. No terrestrial mammals are able to engage in this phenomenon, including humans.

Some fish also possess the ability to switch off half of their brain while continuing to remain aware and in control of their movements. Small fish are able to survive while sleeping by facing into a current, and allowing the movement of the water around them to pass against their gills and meet their oxygen needs.

Larger fish, including rays and some sharks, need to make more significant movements to get enough water to pass their gills. They do this by entering a state of USWS and continue to swim while asleep.

The key difference in the manifestation of USWS in birds versus other animals is that for birds, shutting off half the brain is a voluntary process that they control to meet their own needs, depending on the situation.

In animals, periods of half-brain sleep may last for a lot longer than in birds, while in birds, the time spent in USWS is usually fairly brief and intermittent.

A Mallard resting on a icy lake. The key difference in the manifestation of USWS in birds versus other animals is that for birds, shutting off half the brain is a voluntary process that they control to meet their own needs

The Impact of USWS on Bird Behavior

We’ve discussed how birds use USWS to their advantage, avoiding dangerous situations and enabling them to rest while making progress on their long migration journeys. But are there any disadvantages associated with only half of the brain being asleep at any one time? Read on to find out.

The benefits and advantages of being able to shut off one part of their brain are well documented, with birds enjoying increased chances of survival by being able to avoid predation while catching up on valuable rest and cover long distances on migration between breeding grounds and wintering territories without constantly needing to break a journey to rest and reset, using up valuable energy resources.

Birds can navigate their way around their territories and also along familiar migration routes while only half of their brain is operational. Their senses remain switched on, so any changes to their immediate environment, including noises or visual disturbances, will also be detected, allowing them to quickly spring into action if needed.

However, there are also some downsides to spending a reduced amount of time in deep, restful sleep. Unihemispheric Slow-Wave Sleep means that a bird may spend a far shorter period each day or night in the restorative state that is needed for good health. With only half the brain functioning to full capacity, a bird will have reduced cognitive abilities, including problem-solving and decision-making, meaning that they are not functioning at an optimal level.

Although being half awake does bring the advantage of alertness and vigilance, these levels do not match those of a fully awake bird that is primed and ready for action.

A Grey Parrot. Unihemispheric Slow-Wave Sleep means that a bird may spend a far shorter period each day or night in the restorative state that is needed for good health

Summary

Being able to accomplish as remarkable a feat as crossing an ocean while not fully awake is one of nature’s greatest marvels. Birds that possess the ability to switch off part of their brain while remaining alert and vigilant have adapted to balance the demand for as much rest as possible to stay healthy with the urgent need to avoid being attacked or eaten by a predator sneaking up unnoticed.

Unihemispheric Slow-Wave Sleep is only one of a number of specialized behaviors that birds have evolved with to give themselves the greatest chance of survival in the face of threats of predation or completing a long-distance migration flight successfully.

Gaining a deeper understanding of the unique behaviors of the avian world can help you develop an appreciation of just how fascinatingly complex birds’ daily lives are.