Masako Tamaki
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Many animals need sleep. Even brainless jellyfish enter sleep-like states where they pulse less and respond more slowly to food and movement. But all of the threats and demands animals face don’t just go away when it’s time to doze. That’s why a range of birds and mammals experience some degree of asymmetrical sleep where parts of their brain are asleep and other areas are more active. This is even true for humans. So how does it work?

All vertebrate brains consist of two hemispheres: the right and left. Brain activity is usually similar across both during sleep. But during asymmetrical sleep, one brain hemisphere can be in deep sleep while the other is in lighter sleep. And in an extreme version called “unihemispheric sleep,” one hemisphere may appear completely awake while the other is in deep sleep.

Take bottlenose dolphins. Their breathing is consciously controlled, and they must surface for air every few minutes or they’ll drown. When they have a newborn calf, they must actually swim nonstop for weeks in order to keep it safe. So dolphins sleep unihemispherically, with just one hemisphere at a time. This allows them to continue swimming and breathing while snoozing.

Other marine mammals also need asymmetrical sleep. Fur seals might spend weeks on end migrating at sea. They slip into unihemispheric sleep while floating horizontally, holding their nostrils above the surface, closing their upward-facing eye, and keeping their downward-facing eye open. This may help them stay alert to threats from the depths.

Similar pressures keep birds partially awake. Mallard ducks sleep in groups, but some must inevitably be on the peripheries. Those ducks spend more time in unihemispheric sleep, with their outward-facing eyes open and their corresponding brain hemispheres more active.

Other birds have been shown to catch z’s in midair migration. While undertaking non-stop transoceanic flights of up to 10 days, frigatebirds either sleep with one or both hemispheres at a time. They do so in seconds-long bursts, usually while riding air currents. But the frigatebirds still sleep less than 8% of what they would on land, suggesting a great tolerance for sleep deprivation.

It’s currently unclear whether asymmetrical sleep packs the same benefits as sleep in both hemispheres and how this varies across species. In one experiment, fur seals relied on asymmetrical sleep while being constantly stimulated. But in recovery, they showed a strong preference for sleep across both hemispheres, suggesting that it was more restorative for them. Dolphins, on the other hand, have been observed to maintain high levels of alertness for at least five days. By switching which hemisphere is awake, they get several hours of deep sleep in each hemisphere throughout a 24-hour period. This may be why unihemispheric sleep alone meets their needs.

So, what about humans? Have you ever woken up groggy after your first night in a new place? Part of your brain might’ve spent the night only somewhat asleep. For decades, scientists have recognized that participants sleep poorly their first night in the lab. It’s actually customary to toss out that night’s data. In 2016, scientists discovered that this “first night effect” is a very subtle version of asymmetrical sleep in humans. They saw that, during the first night, participants experience deeper sleep in their right hemisphere and lighter sleep in their left. When exposed to infrequent sounds, that lighter sleeping left hemisphere showed greater bumps in activity. Participants also woke up and responded to infrequent sounds faster during the first night than when experiencing deep sleep in both hemispheres during nights following. This suggests that, like other animals, humans use asymmetrical sleep for vigilance, specifically in unfamiliar environments.

So, while your hotel room is obviously not trying to eat you and you’re not going to die if you don’t continue moving, your brain is still keeping you alert. Just in case.