There are few things more peaceful than floating in a pool and closing your eyes for a bit. But if you are a whale, falling asleep in the sea is a much more complicated affair. This is because whales are voluntary breathers, meaning they must think about every single breath they take. If they were to fall completely unconscious while sleeping, they would drown. This is why whales have evolved a fascinating sleep pattern, where one half of their brain sleeps while the other remains awake and alert.
In this way, whales can rest and stay alert for predators, ships or other members of their pod. It also helps them regulate their body temperature, as the water loses heat much more quickly than air does. You might notice that whales, particularly sperm whales, tend to sleep motionless near the surface of the water, often lying vertically. This is a very common observation, and it has been suggested that whales sleep with half of their brain'shut off' and one eye closed in this way to maintain an awareness of what is around them.
However, it is possible that whales sleep in other positions too, as it has been observed that they will sometimes rest horizontally, with their belly facing down and their head tilted upwards. This is a behaviour known as ‘logging’ and it is thought that this is how whales rest when they are at the surface of the sea, during migrations.