Bruce Hecker, director of husbandry at the South Carolina Aquarium in Charleston, S.C., provides an answer that gives new meaning to the expression "half asleep."

Marine mammals such as whales and dolphins spend their entire lives at sea. So how can they sleep and not drown? Observations of bottlenose dolphins in aquariums and zoos, and of whales and dolphins in the wild, show two basic methods of sleeping: they either rest quietly in the water, vertically or horizontally, or sleep while swimming slowly next to another animal. Individual dolphins also enter a deeper form of sleep, mostly at night. It is called logging because in this state, a dolphin resembles a log floating at the water's surface.

Image: JOHN DAY Cetacean Behavior Labortatory UNDER TOW. A baby humpback whale eats and sleeps while being pulled along in its mother's slipstream.

When marine mammals sleep and swim at once, they are in a state similar to napping. Young whales and dolphins actually rest, eat and sleep while their mother swims, towing them along in her slipstream--a placement called echelon swimming. At these times, the mother will also sleep on the move. In fact, she cannot stop swimming for the first several weeks of a newborn's life. If she does for any length of time, the calf will begin to sink; it is not born with enough body fat or blubber to float easily.

Lots of swimming will tire an infant, producing a weak animal susceptible to infection or attack. Adult male dolphins, which generally travel in pairs, often swim slowly side by side as they sleep. Females and young travel in larger pods. They may rest in the same general area, or companionable animals may pair for sleeping while swimming.

While sleeping, the bottlenose dolphin shuts down only half of its brain, along with the opposite eye. The other half of the brain stays awake at a low level of alertness. This attentive side is used to watch for predators, obstacles and other animals. It also signals when to rise to the surface for a fresh breath of air. After approximately two hours, the animal will reverse this process, resting the active side of the brain and awaking the rested half. This pattern is often called cat-napping.

Image: BARBARA BILGRE Cetacean Behavior Labortatory NAPPING. Bottlenose dolphins rest quietly underwater while half of their brain remains alert, responding to their need to surface for air.

Dolphins generally sleep at night, but only for a couple hours at a time; they are often active late at night, possibly matching this alert period to feed on fish or squid, which then rise from the depths. Bottlenose dolphins, based on electroencephalogram (EEG) readings, spend an average of 33.4 percent of their day asleep. It is not clear whether cetaceans undergo dream sleep. Rapid Eye Movement (REM)--a characteristic of deep sleep--is hard to discern. But a pilot whale was noted as having six minutes of REM in a single night.

To avoid drowning during sleep, it is crucial that marine mammals retain control of their blowhole. The blowhole is a flap of skin that is thought to open and close under the voluntary control of the animal. Although still a matter of discussion, most researchers feel that in order to breathe, a dolphin or whale must be conscious and alert to recognize that its blowhole is at the surface.