This article is from the Dolphin
FAQ, by Jaap van der Toorn
Dolphins have to be conscious to breath (Williams et al, 1990). This means that they cannot go into a full deep sleep, because then they would suffocate. Dolphins have "solved" that by letting one half of their brain sleep at a time. This has been determined by doing EEG studies on dolphins. Dolphins sleep about 8 hours a day in this fashion. Recent research confirmed that dolphins have only one eye closed when sleeping. The state (open or closed) of one eye remains constant for on average an hour, after which it switches state (Goley, 1999). REM (Rapid Eye Movement) sleep, usually associated with dreaming has been recorded only very rarely. Some scientists claim dolphins do not have REM sleep at all.
A dolphin's behavior when sleeping/resting depends on the circumstances and possibly on individual preferences. They can either:
- swim slowly and surface every now and then for a breath
- rest at the surface with their blowhole exposed
- rest on the bottom (in shallow water) and rise to the surface every now and then to breath.
P.D. Goley (1999) Behavioral aspects of sleep in Pacific whitesided dolphins (Lagenorhynchus obliquidens, Gill 1865) Marine Mammal Science 15(4): 1054-1064
S.H Ridgway (1990) The Central Nervous System of the Bottlenose Dolphin, in S. Leatherwood and R.R. Reeves: The Bottlenose Dolphin, pp. 69-97, Academic Press
Th.D. Williams, A.L. Williams & M. Stoskopf (1990) Marine Mammal Anesthesia. In: L.A. Dierauf (ed.): Handbook of Marine Mammal Medicine: Health, Disease and Rehabilitation, pp. 175-191 CRC Press, Boca Raton