How Do Whales Sleep?

The process of whale sleep is quite different and alien to that of humans. Whales have a respiratory system that allows them to survive beneath the waves for up to and beyond 30 minutes at a time, without the need for oxygen. This is of little aid when sleeping, though.
Humans are special in that they breathe without having to think about it; it is an involuntary process that can be performed unconsciously. Air intake is automatic, even when humans aren't paying attention or are asleep. Whales and other ocean mammals, such as dolphins, do not have this luxury. Because they are in an undersea environment and surrounded by water, they cannot afford to breathe indiscriminately. They must be conscious breathers and always be aware of their blowhole, which is tricky considering that they must sleep at times.

To compensate for the necessity for conscious breathing while sleeping, whales keep half of their brains awake at all times, even while catching a nap. The other half of the brain is completely out of commission. This ensures that they do not attempt to breathe while submerged. Whales tend to be in this mode several hours each day. The estimation is that the closest humans come to such a state is when they are semi-conscious and nearly asleep, but not yet unable to wake fully if prompted.

Whales may sleep wherever they like, but more often than not they sleep near the surface of the water. This way, open air and oxygen are only a short ways away in case of emergency.
They sleep either horizontally or vertically, and may also choose to sleep by slowly and semi-consciously swimming close to a fellow whale. Movement while sleeping is common with these animals, and young whales do it to an even greater extent.
Young whales rest, sleep and eat while on the move and during echelon swimming, when they follow the mother--who, at times, is sleeping as she slowly leads them along. This is necessary, as the mother cannot stop swimming at any time when she has a young one because they do not yet have the necessary amount of blubber to float on their own.

While whales must act as voluntary breathers, there are some positives to their respiratory systems and some advantages they hold over humans. Not only can they hold their breath for longer periods due to their larger lungs, whales also exchange more air between inhalations and exhalations, their body distributes oxygen more smartly to body parts that need it, and they possess a higher tolerance for carbon dioxide.