Asymmetrical Sleep: Can you be awake and asleep at the same time?
Many creatures need sleep. Even brainless jellyfish enter sleep-like states where they’re pulseless and respond more slowly to food and movement. But all of the dangers and demands creatures faces don’t just go out when it’s time to doze.
That’s why a range of birds and mammals have some degree of asymmetrical sleep where parts of their brain are sleeping and other areas are more active.
This is even true for humans. So how does it work? All vertebrate brains consist of two hemispheres as the right and the left. Brain activity is generally 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 fully awake while the other is in deep sleep.
As an example, bottlenose dolphins’ breathing is knowingly controlled and they must surface for air every few minutes or they’ll drown. When they have a newborn calf, they must swim continuously for weeks, to keep it safe. So they 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. As an example, seals might spend weeks on end migrating in the ocean. They slip into unihemispheric sleep while floating horizontally. They hold their nostrils above the surface closing their upward-facing eye, and keeping their downward-facing eye, open. This may help them to stay alert to dangers from the depths.
Similar pressures keep birds partly awake. Mallard ducks sleep in groups, but some must inevitably be on the peripheries. Those ducks spend more time in unihemispheric sleep, with their outside-facing eyes open and their corresponding brain hemispheres more active.
Other birds have been shown to catch sleep in midair migration. While undertaking-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, generally while riding air currents. But the frigatebirds still sleep lesser than 8 of what they would on land, suggesting a great tolerance for sleep deprivation.
It’s now unclear whether asymmetrical sleep packs the same benefits as sleep in both hemispheres. And how this varies across species. In one test, fur seals depended 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 situations of alertness for at least five days. By switching which hemisphere is awake, they get several hours of deep sleep in each hemisphere throughout the day. This may be why unihemispheric sleep alone meets their needs.
So, what about humans?
Have you ever woken up weak after the 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 really subtle version of asymmetrical sleep in humans. They saw that, during the first night, participants witness deeper sleep in their right hemisphere and lighter sleep in their left.
When exposed to infrequent sounds, the lighter resting left hemisphere showed greater bumps in its activity. Participants also woke up and responded to infrequent sounds faster.
During the first night, when experiencing deep sleep in both hemispheres during nights. This suggests that, like other creatures, humans use asymmetrical sleep for alertness. Specifically in strange surroundings.
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.
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