Dolphins cannot fall into a deep sleep like humans because they have to keep some activity. Why do they need it? These aquatic mammals must periodically rise to the surface to breathe air through their lungs because they cannot breathe underwater like fish.

From the outside, it seems that dolphins never sleep at all, since one of their eyes always remains open. Actually, it is not so. For having a rest, dolphins use the so-called "unihemispheric sleep" - this is when one half of their brain always remains active. Dolphins can even determine which half of the brain needs a rest. They change which side of their brain is sleeping and awake every 2 hours. Thus, each side gets the same amount of rest. 

Keeping one eye open is important not only for breathing, but also helps dolphins stay alert in case of danger.

Dolphins sleep in flocks, hovering in the water. These beautiful animals are social creatures, so they are very rarely seen alone. Sleeping with a group of relatives is not only fun, but also provides an extra layer of protection.

On average, dolphins spend about 8 hours a day sleeping and resting. However, this is not all at once. They can have a rest any time during the day.

It is very important for dolphins always to remain conscious. They don't have automatic breathing like humans do. Therefore, if a dolphin goes into a deep unconscious sleep, it would lose control of its breathing pattern and either drown or suffocate to death.