Many animals need sleep. Brainless jellyfish also enter sleep-like states where they pulse less and respond more slowly to food and movement.
But all the dangers and demands animals face don't just end when it's time to sleep.
This is why a range of birds and mammals experience some degree of asymmetric sleep, where some parts of their brain are asleep and other parts are more active. This is true for humans as well.
So how does it work?
All vertebrate brains consist of two hemispheres: right and left.
Brain activity during sleep is generally similar in both. But during asymmetric sleep, one hemisphere of the brain may be in deep sleep while the other is in light sleep.
And in an extreme version called "unihemispheric sleep," one hemisphere appears fully awake while the other is in deep sleep.
Take the Bottlenose Dolphin.
Their breathing is consciously controlled, and they must surface for air every few minutes or they will drown. When they have a newborn calf, they must actually swim non-stop for weeks to keep it safe.
So dolphins sleep ungrounded, with only one hemisphere at a time. This allows them to swim and breathe while snoozing. Other marine mammals also require asymmetric sleep.
Fur seals can spend weeks at sea at the end of their migration.
They float horizontally, keeping their nostrils above the surface, closing their upward eye, and drifting into a restless sleep with their downward eye open.
This can 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 around.
Those ducks spend more time in undisturbed sleep, their outer eyes are open, and their respective brain hemispheres are more active. Other birds have been shown to catch zids in airborne migration.
During non-stop transoceanic flights of up to 10 days,
frigatebirds sleep with one or both hemispheres at a time. They usually do this in long bursts of seconds, riding air currents.
But frigatebirds still sleep less than 8 percent on the ground, suggesting a high tolerance for sleep deprivation. It is currently unclear whether asymmetric sleep has equal benefits for sleep in both hemispheres and how this varies across species.
In one experiment, fur seals relied on asymmetric sleep while being constantly active. But in recovery, they showed a strong preference for sleep in both hemispheres, suggesting it was more restorative for them.
Dolphins, on the other hand, have been observed to maintain a high level of alertness for at least five days. By switching which hemisphere is awake, they get several hours of deep sleep in each hemisphere in a 24-hour period.
This is why solitary sleep fulfills their needs.
So, what about humans? Have you ever woken up grumpy after your first night in a new place? Maybe some part of your brain spent the night just sleeping.
For decades, scientists have recognized that participants sleep poorly on their first night in the lab. In fact, it is customary to toss the data for that night. In 2016, scientists discovered that this "first night effect" is a much more subtle version of sleep asymmetry in humans.
They found that during the first night,
participants had deep sleep in their right hemisphere and light sleep in their left. When exposed to occasional sounds, this light-sleeping left hemisphere appears to have more disruptions in activity.
Participants also woke up and responded faster to unusual sounds during the first night compared to those experiencing deep sleep in both hemispheres during subsequent nights.
This suggests that, like other animals, humans use asymmetric sleep for alertness, especially in unfamiliar environments.
So, while your hotel room isn't obviously trying to eat you and you're not going to die if you don't keep moving, your mind is still keeping you alert. just in case.
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