Sleep is a fundamental physiological need for all animal species. Scientists indicate that even brainless creatures like jellyfish and hydra understand sleep. These jellyfish and hydra enter a sleep state when they become less active and respond more slowly to food and their environment.
Thus, sleep has been an action present for billions of years, long before brain-bearing animals emerged. However, there is a fact that sleep is also when animals are in their most vulnerable state.
Predators or enemies can attack a creature while it is sleeping. Therefore, evolution has helped some species develop the ability to sleep with only half of their brain.
When sleeping, one eye of these animals remains open, and the hemisphere of the brain controlling that eye remains active, while the other half sleeps deeply. They can then switch, waking the rested hemisphere while allowing the other half to continue sleeping.
This phenomenon is known as asymmetrical sleep. Strangely, humans can also experience a form of this type of sleep, especially when we sleep in unfamiliar places. Let’s explore this phenomenon in the video below:
Asymmetrical sleep in some animal species and humans.
What is asymmetrical sleep?
The brains of all vertebrates are divided into two hemispheres: right and left. In typical sleep, the activities of both brain hemispheres are usually symmetrical and identical. However, in asymmetrical sleep, one hemisphere can be in a deep sleep state while the other hemisphere is in a lighter sleep state.
In an extreme version of asymmetrical sleep called “unihemispheric sleep“, one hemisphere of your brain can be fully awake while the other remains in deep sleep.
Take the bottlenose dolphin as an example. Their breathing is consciously controlled, and they must surface every few minutes; otherwise, they will drown.
When a newborn calf is born, the mother dolphin must swim continuously for weeks to keep her baby safe. Therefore, dolphins sleep in a unihemispheric manner, with only one hemisphere sleeping at a time. This allows them to continue swimming and breathing throughout the nurturing process.
Other marine mammals also exhibit asymmetrical sleep. Fur seals undertake long migrations lasting weeks at sea. They enter unihemispheric sleep while floating horizontally, keeping their nostrils above the water surface, with one eye closed towards the sky while the other eye remains open towards the sea. This may help them stay alert to threats from the deep sea.
These ducks are hiding their bills while sleeping, but one eye remains open for vigilance.
Similar pressures keep birds alert while sleeping. Wild ducks sleep in flocks, but certainly, some must stay awake to watch out for others. Those ducks spend more time in unihemispheric sleep, with their outward-facing eyes open and their corresponding brain hemispheres more active.
Other birds during migration also half-sleep, half-awake. While making non-stop transoceanic flights lasting up to 10 days, seabirds can sleep with one or both hemispheres at the same time.
They often take advantage of times when they fly into air currents to sleep for a moment, possibly just a few seconds. However, migratory seabirds still sleep 8% less than when they are on land, indicating their remarkable endurance in dealing with sleep deprivation.
A seal sleeping in unihemispheric sleep.
It remains unclear whether asymmetrical sleep offers benefits similar to sleep in both hemispheres and how this varies among species. In one experiment, fur seals relied on asymmetrical sleep while being continuously stimulated.
However, during recovery, they preferred to sleep with both brain hemispheres, suggesting that complete sleep helps fur seals recover their health better.
On the other hand, dolphins have been observed to stay awake for at least 5 days. By switching which hemisphere is awake, they can sleep deeply for a few hours in each hemisphere over a 24-hour period. This may be evidence that hemispheric sleep meets the needs of dolphins.
Humans also experience asymmetrical sleep
At this point, you might ask, what about humans? Can we also keep one hemisphere of the brain asleep while the other remains awake? The answer seems to be no, but humans do experience a very subtle version of asymmetrical sleep.
In 2015, scientists monitored volunteers who stayed awake continuously for 22 hours using functional magnetic resonance imaging (fMRI). They found that a part of these individuals’ brains had actually slept during that time while some parts remained awake.
Have you ever had trouble sleeping in an unfamiliar place?
Conversely, have you ever woken up anxious on the first night in a strange place? A part of your brain may not have fully slept at that time. While you think you are having trouble sleeping because of being in an unfamiliar place, the reality is that a part of your brain has been asleep. Scientists refer to this as “pseudo insomnia.”
For decades, sleep researchers have also recognized that volunteers coming to the sleep measurement laboratory do not sleep well on their first night there. Therefore, it has been customary to exclude the data tracked on that first night.
In 2016, scientists discovered that the “first night effect“ is a very subtle version of asymmetrical sleep in humans. They found that, on the first night, participants experienced deeper sleep in the right brain hemisphere and lighter sleep in the left brain hemisphere.
When exposed to irregular sounds, the left brain hemisphere, which is in lighter sleep, showed more activity. Participants also woke up and responded to irregular sounds more quickly on the first night compared to when they experienced deep sleep in both hemispheres on subsequent nights.
This suggests that, like other animals, humans use asymmetrical sleep to remain alert, especially in unfamiliar environments. So, on the first night when you travel, clearly, the hotel room isn’t trying to eat you, but your brain will still keep you partially awake. Just to be cautious!
