Does Brain Ever Sleep?

Many animals need sleep. Even brainless jellyfish enter sleep where they pulse less and respond more slowly to food and movement. But all of the threats and demands of animals 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. Just wait, what is asymmetrical sleep? Is it a kind of math type of sleep? Well, the answer is no. Asymmetrical sleep is where part of the brain is asleep and other areas are more active. This is even true for humans. So how does it work? All brains consist of two hemispheres: the right and the left. Brain activity is usually similar across both during sleep. But during asymmetrical sleep, one 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. Let's take the example of dolphins, their breathing is consciously controlled, ad they must surface for air every few minutes or they will drown. When they have a newborn calf, they must actually swim nonstop for weeks to keep it safe. So dolphins sleep unihemispherically, with just one hemisphere at a time and this allows them to continue swimming and breathing. Other marine mammals also need asymmetrical sleep. Seals might spend weeks on end migrating at sea, they sleep into unihemispheric sleep while floating horizontally, putting 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.

Migratory birds have been shown to sleep in midair migration. While undertaking non-stop transoceanic flights of up to 10 days, they either sleep with one or both hemispheres at a time. They do so in seconds-long bursts, usually while riding air currents. But these birds sleep less than 8% of what they sleep on land.

So, now the question arises, how does the human brain sleep? Well, in 2016, scientists discovered that humans feel a subtle version of asymmetrical sleep. They saw that, during the first night in the lab, participants experience deeper sleep in their right hemisphere and lighter in their left. This suggests that, like other animals, humans use a symmetrical sleep, for vigilance, specifically in unfamiliar environments. So, while your hotel room is obviously not trying to eat you and you are not going to die if you don't continue moving, your brain is still keeping you alert.

" Even a soul submerged in sleep is hard at work"

       - Heraclitus 

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