The way a cetacean sleeps is surprising. Quite unlike us, whales "sleep" by resting one half of their brain at a time. While one half of the brain stays awake to make sure the whale breathes and alerts the whale to any danger in its environment, the other half of the brain sleeps.
We humans are involuntary breathers, meaning we breathe without thinking about it and have a "breathing reflex" that kicks into gear when we're sleeping or are knocked unconscious.
Whale sleep is complex and still being studied. One interesting finding, or lack thereof, is that whales do not appear to have REM (rapid eye movement) sleep that is characteristic of humans. This is the stage in which most of our dreaming occurs.
Where cetaceans sleep differs among species. Some rest at the surface, some are constantly swimming, and some even rest far below the water surface (for example, captive dolphins have been known to rest at the bottom of their pool for a few minutes at a time). I've seen large baleen whales such as humpbacks resting at the surface for sometimes half an hour at a time. These whales take slow breaths that are less frequent than a whale that's active. They are so relatively motionless on the surface that we refer to this behavior as "logging," because they look like giant logs floating on the water.
Sources:
- Lyamin, O.I., Manger, P.R., Ridgway, S.H., Mukhametov, L.M., and J.M. Siegal. 2008. "Cetacean Sleep: An Unusual Form of Mammalian Sleep." (Online). Neuroscience and Biobehavioral Reviews 32:1451–1484.
- Mead, J.G. and J.P. Gold. 2002. Whales and Dolphins in Question. Smithsonian Institution.
- Ward, N. 1997. Do Whales Ever...? Down East Books.
