Why do some animals open one eye when they sleep? Can humans do that?

Sleep is a basic physiological need of all animals. Scientists say even creatures without brains like jellyfish and hydra can sleep. Jellyfish and hydras go into a state of sleep when they are less active and respond more slowly to food and their surroundings.

Sleep is therefore an act that has appeared billions of years ago, long when animals with brains were born. But there is a truth, when sleeping is also the time when animals fall into the most vulnerable state.

Predatory or predatory animals may attack an organism while it is asleep. So evolution has helped some animals develop the ability to sleep with only half a brain.

During sleep, one eye of these animals remains open, and the hemisphere of the brain that controls that eye remains active, while the other half remains in deep sleep. They can then reverse course, wake up half of the brain that's had enough rest, and watch the other half sleep.

This is called asymmetric sleep. Strangely, humans also sometimes experience this kind of sleep, especially when we sleep in a strange place, let's find out that phenomenon in the video below:

Sleep asymmetry in some animals and humans.

What is asymmetric sleep?

The brain of all vertebrates is divided into two hemispheres: the right and the left. During typical sleep, the activity of the two hemispheres of the brain is usually symmetrical and identical. But in asymmetric sleep, one hemisphere of the brain may be in a deep sleep while the other is in a more shallow state.

And in an extreme version of asymmetrical sleep known as "monospheric sleep," one hemisphere of your brain can be fully awake while the other is still in deep sleep.

Take the bottlenose dolphin as an example. Their breathing is consciously controlled and they must rise to the surface every few minutes or they will drown.

When a new baby is born, the mother actually has to swim nonstop for weeks to keep her babies safe. So dolphins sleep in a monohemispheric fashion, with only one hemisphere sleeping at a time. This allows them to continue to swim and breathe throughout their brooding.

Other cetaceans also have asymmetrical sleep. Fur seals make weekly migrations at sea. They fall into a monohemispheric sleep while hovering horizontally, keeping their nostrils above the surface of the water, eyes closed toward the sky, and eyes facing the sea remain open. This can help them stay alert to threats from the deep sea.

These ducks are sleeping with their beak hidden, but one of its eyes remains open to keep watch.

Similar pressures keep birds awake during sleep. Mallards sleep in flocks, but surely some have to stay awake for others. Those ducks spent more time in monohemispheric sleep, with their eyes facing outwards open and their respective brain hemispheres more active.

Other birds in mid-air migration are also half-sleeping and half-awake. During non-stop transoceanic flights lasting up to 10 days, sea cormorants can sleep using one or both hemispheres at the same time.

They often take advantage of the time they fly into the air to sleep for a while, maybe just a few seconds. But migratory sea cormorants still slept 8% less than when they were below ground, indicating their excellent tolerance for sleep deprivation.

A seal sleeps in monohemispheric sleep.

It remains unclear whether asymmetric sleep confers the same benefits as sleep in both hemispheres, and how this varies across species. In one experiment, fur seals relied on asymmetric sleep while under constant stimulation.

But during recovery, they prefer to sleep with both hemispheres of the brain, which suggests that complete sleep helps fur seals recover better.

On the other hand, there have been dolphins awake for at least 5 days. By switching which hemisphere is awake, they will sleep for several hours in each hemisphere over the course of 24 hours. This could be evidence that hemispheric sleep fulfills the dolphin's needs.

Humans also sleep asymmetrically

At this point, you might ask, what about humans? Can we also keep one hemisphere asleep and the other awake? The answer seems to be no, but humans also experience a very subtle asymmetrical version of sleep.

In 2015, scientists monitored volunteers who stayed awake for 22 hours using an fMRI functional magnetic resonance imaging machine. They found that part of the brains of these people was actually asleep during that time, while some parts of the brain were still awake.

Have you ever lost sleep because of a strange house?

Conversely, have you ever woken up nervous your first night in an unfamiliar place? Part of your brain may not be fully asleep yet. And while you might think you're having trouble sleeping at home, the truth is that part of your brain is already asleep. Scientists call it psychedelic insomnia.

For decades, sleep researchers have also realized that volunteers who go to a sleep measurement lab don't get a good night's sleep the first night there. As a rule, the data they tracked that first night needs to be discarded.

In 2016, scientists discovered that this "first night effect" is a very subtle version of asymmetrical sleep in humans. They found that, during the first night, the participants experienced deeper sleep in the right hemisphere and lighter sleep in the left hemisphere.

When exposed to irregular sounds, the left hemisphere of the brain that was sleeping more shallowly showed greater activity. Participants also woke up and responded to infrequent sounds more quickly the first night than when they experienced deep sleep in both hemispheres the following nights.

This suggests that, like other animals, humans use asymmetrical sleep to stay alert, especially in unfamiliar environments. So on your first night of travel, it's clear the hotel room isn't trying to eat you, but your brain will still keep you partially awake. Just a precaution!