Several species of seals and dolphins sleep in a very unique sort of way. Like humans, their brains are divided into two hemispheres. These animals have the ability to allow one hemisphere of their brains to sleep while the other halves are awake. One hemisphere will show sleeping patterns of slow brain waves while the other will show signs of wakefulness. Whales and manatees are also believed to use unihemispheric sleep, as do even mallard ducks, as reported in 1999 news articles about sleep research. (1)
This is the only way that bottle-nosed dolphins can sleep. Professor Lev Mukhametov from Moscow discovered this by doing EEGs to study them. During sleep, the dolphins tested in the aquarium at Moscow swam mechanically in circles. (2)
Fur seals also use this type of sleep for the purpose of breathing. They lie on one side, with one front flipper paddling underwater to keep their nostrils above the water's surface. Half of the brain remains awake to operate the the moving fin. After a while, the seal will flip over to give the other hemisphere of the brain a chance to sleep. (3) Most seals can have both sides of the brain sleep simultaneously as well. (4)
Mallard Ducks were the subject of recent sleep research by Neils Rattenborg, Indiana State U. graduate student. When ducks sleep in the water in a row, the outer ducks on the row actually sleep less and keep an eye open facing the outside of the group, presumably to check for danger while sleeping.
"The EEG recordings confirmed that when one eye was open, the corresponding hemisphere was in a quiet waking state, while the opposite hemisphere was experiencing slow wave sleep. [When faced with a video of a predator's approach,] . . . the wakeful hemisphere was capable of predator detection: the ducks initiated escape behaviour. The ducks would also make sure to alternate which of their halves was getting shut-eye -- and shutdown brain activity -- by rotating their bodies to switch the side that was facing outwards." (1).
(1) Simon, Tamar. "Losing Sleep over Predators: Peeking Ducks" Discovery Channel Canada[online] Science section, February 3, 1999. URL: http://www.exn.ca/html/templates/htmlpage.cfm?ID=19990203-51. The research itself was reported in the February 4, 1999 issue of Nature.
(2) Lavie, Peretz. The Enhchanted World of Sleep. New Haven: Yale University Press, 1996. p. 103-106.
(3) Coren, Stanley. Sleep Thieves: an Eye-Opening Exploration into the Science & Mysteries of Sleep. New York: The Free Press, 1996. p. 40-41.
(4) Sleep cd
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