Have you ever wondered if it’s possible to be awake and asleep at the same time? It turns out that many animals have a fascinating ability to do just that! Even creatures like jellyfish, which don’t have brains, enter a sleep-like state where they slow down and react less to things around them. But why do animals need this special kind of sleep?
Animals face dangers and challenges even when it’s time to rest. To deal with this, some birds and mammals have developed a unique way of sleeping called asymmetrical sleep. This means that parts of their brain are asleep while other parts stay awake. Surprisingly, humans can experience this too!
Our brains, like those of all vertebrates, have two halves: the right hemisphere and the left hemisphere. Normally, both sides of the brain show similar activity during sleep. But in asymmetrical sleep, one side can be in deep sleep while the other is more alert. In some cases, known as “unihemispheric sleep,” one side of the brain can be fully awake while the other is deeply asleep.
Let’s look at some amazing examples of animals that use asymmetrical sleep:
What about us? Have you ever felt tired after your first night in a new place? This might be because part of your brain was only partially asleep. Scientists have noticed that people often sleep poorly on their first night in a lab. In 2016, researchers found that this “first night effect” is a mild form of asymmetrical sleep in humans. During the first night, people had deeper sleep in the right hemisphere and lighter sleep in the left. When they heard sounds, the left hemisphere, which was in lighter sleep, became more active, and people responded faster to the sounds.
This suggests that, like other animals, humans use asymmetrical sleep to stay alert, especially in new environments. So, even if your hotel room isn’t dangerous, your brain stays on guard, just in case!
Research one of the animals mentioned in the article, such as bottlenose dolphins or mallard ducks. Create a short presentation explaining how they use asymmetrical sleep to survive in their environment. Include interesting facts and visuals to engage your classmates.
In a safe and quiet space, try an activity where you keep one eye open and the other closed for a few minutes. Reflect on how it feels and discuss with your classmates how this might help animals stay alert while resting.
Conduct a simple experiment by sleeping in a different room or environment for one night. Record your sleep quality and any unusual feelings or dreams. Share your findings with the class and discuss how this relates to the “first night effect” in humans.
Design a chart that compares the sleep cycles of humans and animals that experience asymmetrical sleep. Highlight the differences and similarities, and present your chart to the class to help them understand the concept better.
Participate in a class debate on the importance of asymmetrical sleep for survival. Take a position either for or against its necessity, using evidence from the article and additional research to support your arguments.
Many animals need sleep. Even jellyfish, which lack brains, enter sleep-like states where they pulse less and respond more slowly to stimuli. However, the threats and demands that animals face do not disappear when it’s time to rest. This is why various birds and mammals experience asymmetrical sleep, where parts of their brain are asleep while other areas remain active. This phenomenon is also observed in humans.
All vertebrate brains consist of two hemispheres: the right and left. Typically, brain activity is similar across both hemispheres during sleep. However, during asymmetrical sleep, one hemisphere can be in deep sleep while the other is in lighter sleep. In an extreme form known as “unihemispheric sleep,” one hemisphere may appear fully awake while the other is in deep sleep.
Take bottlenose dolphins as an example. They consciously control their breathing and must surface for air every few minutes to avoid drowning. When they have a newborn calf, they need to swim continuously for weeks to keep it safe. Therefore, dolphins sleep unihemispherically, allowing one hemisphere to rest while the other remains alert, enabling them to continue swimming and breathing.
Other marine mammals, like fur seals, also require asymmetrical sleep. They may spend weeks migrating at sea and can enter unihemispheric sleep while floating horizontally, keeping their nostrils above the surface, closing one eye, and keeping the other open to stay alert to potential threats.
Birds face similar pressures that keep them partially awake. Mallard ducks, for instance, sleep in groups, but some must remain vigilant on the edges. These ducks spend more time in unihemispheric sleep, keeping their outward-facing eyes open and their corresponding brain hemispheres more active. Other birds, such as frigatebirds, have been observed sleeping during long migratory flights. They can sleep with one or both hemispheres at a time, typically in brief bursts while gliding on air currents. However, frigatebirds sleep less than 8% of what they would on land, indicating a significant tolerance for sleep deprivation.
It remains unclear whether asymmetrical sleep provides the same benefits as sleep in both hemispheres and how this varies among species. In one study, fur seals relied on asymmetrical sleep while being constantly stimulated, but during recovery, they showed a strong preference for sleep across both hemispheres, suggesting that this type of sleep is more restorative for them. Dolphins, on the other hand, have been observed maintaining high levels of alertness for at least five days. By alternating which hemisphere is awake, they achieve several hours of deep sleep in each hemisphere over a 24-hour period, which may explain why unihemispheric sleep meets their needs.
What about humans? Have you ever woken up feeling groggy after your first night in a new place? Part of your brain may have only been partially asleep. For decades, scientists have noted that participants tend to sleep poorly on their first night in a lab, often discarding that night’s data. In 2016, researchers discovered that this “first night effect” is a subtle form of asymmetrical sleep in humans. They found that during the first night, participants experienced deeper sleep in their right hemisphere and lighter sleep in their left. When exposed to infrequent sounds, the lighter-sleeping left hemisphere showed increased activity. Participants also responded more quickly to sounds during their first night compared to nights when they experienced deep sleep in both hemispheres.
This suggests that, like other animals, humans utilize asymmetrical sleep for vigilance, particularly in unfamiliar environments. So, while your hotel room may not pose a threat, your brain remains alert, just in case.
Asymmetrical – Not identical on both sides of a central line; lacking symmetry – The human brain is asymmetrical, with the left hemisphere often controlling language and the right hemisphere managing spatial abilities.
Sleep – A natural state of rest for the body and mind, during which consciousness is suspended – Sleep is essential for the brain to process information and consolidate memories.
Hemisphere – One half of a spherical or roughly spherical structure, especially the brain – The left hemisphere of the brain is typically responsible for logical thinking and analytical tasks.
Dolphins – Intelligent marine mammals known for their playful behavior and complex communication – Dolphins are known to sleep with one hemisphere of their brain at a time, allowing them to stay partially alert.
Mammals – Warm-blooded vertebrates that have hair or fur and whose females produce milk to feed their young – Humans, dolphins, and elephants are all examples of mammals.
Birds – Warm-blooded vertebrates with feathers, wings, and beaks, most of which are capable of flight – Birds have a unique sleep pattern where they can rest one hemisphere of their brain while keeping the other awake.
Awake – Not asleep; conscious and aware of one’s surroundings – After a good night’s sleep, she felt fully awake and ready to learn.
Alert – Quick to notice any unusual and potentially dangerous or difficult circumstances; vigilant – Being alert during class helps students absorb and understand new information better.
Humans – Members of the species Homo sapiens, characterized by higher intelligence and the ability to use complex language – Humans have a highly developed brain that allows for advanced reasoning and problem-solving skills.
Animals – Living organisms that feed on organic matter, typically having specialized sense organs and nervous systems – Animals, including humans, rely on sleep to maintain their physical and mental health.
