In October of 2000, biologists on a research boat off the coast of Chile happened upon what seemed to be a group of sleeping whales.   The sperm whales were completely still and hanging vertically in the water with their noses up toward the surface, and tails disappearing down into the depths. The researchers had switched off the boat's engines to record the whales' sounds, but ended up drifting silently through the whale pod.

Although the boat passed within a foot of several whales, they didn't seem to notice or respond. But when the boat accidentally brushed against the side of one whale, it suddenly awoke and swam away, rousing its neighbors.

The biologists decided to investigate further. They tracked the behavior of fifty-nine more sperm whales across the world. They found that sperm whales in the wild switch off completely for short periods of time, having "cat naps" while performing slow rhythmic dives.

Biologists had long thought that whales and other air-breathing marine mammals survived without any sleep at all. But studies of captive dolphins and small whales later revealed that they did sleep, but only with one side of the brain at a time.

Essentially one half of their brain shuts off while the other half stays alert, with one eye open. This kind of "uni-hemispheric" sleep allows the animals to swim, breathe, avoid predators and stay in contact with neighbors while still catching "forty winks."Although more research is needed to know for sure, sperm whales seem to have evolved a different strategy for sleeping at sea -- sleeping with their whole brain more like terrestrial mammals, rather than sleeping with half a brain and one eye open.