Many animals need sleep. Even brainless jellyfish enter sleep-like states where they have less pulse and responds at a slower pace to food and movement.
But all the threats and demands that animals face do not disappear when it is time to sleep. This is the reason why a range of birds and mammals experience some degree of asymmetrical sleep where parts of their brain are asleep while other regions are more active.
This is true even for humans. What are the mechanisms for doing this? All vertebrate brains are made up of two hemispheres, the left, and the right. Brain activity is usually the same in both while sleeping. But during asymmetric sleep, One hemisphere hemispheres of the brain can be in a deep sleep while the other is in a lighter sleep.
And in an extreme type called "unihemispheric sleep", one hemisphere may appear to be awake totally while the other is in a deep sleep.
In the case of bottlenose dolphins, for example, their breathing is consciously controlled and they must surface every few minutes to breathe or else they will drown. When they are accompanied by a newborn calf, they swim non-stop for weeks to maintain their safety. So dolphins sleep unilaterally, only one cerebral hemisphere at a time. this allows them to continue swimming and breathing while napping
Other marine mammals also need asymmetric sleep. Fur seals may spend weeks or longer migrating in the sea. They fall asleep with one hemisphere of the brain while swimming horizontally, keeping the nose above the surface of the water. They keep their upward-facing eye closed eye while the downward-facing eye remains open.
This might helps them stay alert from threats from the depths.
Such pressure also makes the birds stay partially awake. Mallard ducks sleep in groups, but some must inevitably be on the peripheries. Those duck spends more time half asleep with their outward-facing eyes open and corresponding cerebral hemisphere more active.
It is proved that other birds sleep in air migration while taking non-stop ocean voyages for up to 10 days.
It is currently unclear whether asymmetric sleep brings the same benefits as sleep in the cerebral hemispheres and how does that differs for different species.
So what is the case of humans? Many people tend to wake up more groggy when they sleep in a new environment. Scientists have established that this new place effect is a form of asymmetry which hints at some parts of the brain being active during sleep.
This indicates that just like in the case of other animals, humans also use asymmetric sleep for vigilance especially when in unfamiliar environments
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