Almost every animal ever studied spends a large part of each day sleeping.

Sleep is thought to play an essential role in the health of almost all animals. As a result, scientists have long wondered how cetaceans, such as whales and dolphins, are able to get enough sleep.

Since cetaceans must surface regularly to breathe, if they drift off to sleep at the bottom of the sea they could drown. Although whales and dolphins are sometimes observed resting for short times at the surface, others are known to swim continuously for weeks without stopping.

Is it possible that cetaceans have evolved the ability to survive without sleep?

A group of Russian scientists didn't think so. They noticed that bottle nose dolphins sometimes swam in slow stereotyped patterns with one eye closed. They investigated the behavior by measuring the dolphins' brain-wave activity during these times.

They discovered that half of the dolphin brain, the half controlling the closed eye, was apparently fast asleep, while the other side of the brain, the side controlling the open eye remained awake and alert!

By keeping one half of their brain alert and awake while the other half sleeps, dolphins can literally "keep an eye out" for danger or obstacles in the water, keep in contact with their group, and surface to breathe.

This kind of sleep, called uni-hemispheric slow wave sleep, has now been discovered in several other species of whales and dolphins.