[wordup] How do dolphins and whales sleep without drowning?

[Adam Shand]

 From http://www.water-consciousness.com/must/must_article13.htm

How do dolphins and whales sleep without drowning?
01.08.2002

According to Scientific American, observations of captured bottlenose 
dolphins, and of whales and dolphins in the wild, show two basic 
methods of sleeping: resting quietly in the water, vertically or 
horizontally, or sleeping while swimming slowly next to another animal.

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 surface.

When marine mammals sleep and swim at the same time, 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, 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, the calf will begin to 
sink; it is not born with enough body fat or blubber to float easily.

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 watches for predators, obstacles and other animals. 
It also signals when to rise to the surface for air.

After about two hours, the animal will swap to the other eye and 
brain-half.

Bottlenose dolphins, based on electroencephalogram (EEG) readings, 
spend an average of a third of their day asleep. Rapid Eye Movement 
(REM) - a characteristic of deep sleep - is hard to discern but a pilot 
whale was clocked having six minutes of REM in a single night.

Although still a matter of debate, most researchers believe a dolphin 
or whale must be conscious and alert to recognise that its blowhole is 
at the surface.