Dolphins, killer whales, and beluga whales are all voluntary breathers. This means that unlike humans, their breathing is not automatic. That is why they have to consciously come up to the surface to breathe.
That means their way of sleeping also differs from that of humans and other animals that are voluntary breathers. Studies have now shown that they only shut down one hemisphere of their brains at a time. In this way they can rest and still control their breathing.
In killer whales, resting behaviour depends on whether a whale is alone or part of a pod. If alone, the whale will rest with its blowhole, part of its head and dorsal fin above the water's surface. Reportedly killer whales can sleep like this for as long as eight hours.
In pods, they form a tight circle and synchronize their breathing and movements. They can take up to ten breaths at the surface before submerging and resting for about ten minutes at a time. Resting pods will avoid boats and other animals during this period.
Killer whale calves are active at all times, and the calf and its mother never sleep during the first few months of its life.
