Science, Tech, Math › Animals & Nature Do Whales Sleep? Whales Sleep With One Half of the Brain at a Time Share Flipboard Email Print Rodrigo Friscione / Getty Images Animals & Nature Marine Life Marine Life Profiles Marine Habitat Profiles Sharks Key Terms Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Forestry Dinosaurs Evolution View More By Jennifer Kennedy Marine Science Expert M.S., Resource Administration and Management, University of New Hampshire B.S., Natural Resources, Cornell University Jennifer Kennedy, M.S., is an environmental educator specializing in marine life. She serves as the executive director of the Blue Ocean Society for Marine Conservation. our editorial process Jennifer Kennedy Updated February 13, 2019 Cetaceans (whales, dolphins, and porpoises) are voluntary breathers, meaning they think about every breath they take. A whale breathes through the blowhole on top of its head, so it needs to come up to the water surface to breathe. But that means the whale needs to be awake to breathe. How does a whale get any rest? The Surprising Way a Whale Sleeps The way a cetacean sleeps is surprising. When a human sleeps, all of his brain is engaged in being asleep. Quite unlike humans, 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. This is called unihemispheric slow-wave sleep. Humans are involuntary breathers, meaning they breathe without thinking about it and have a breathing reflex that kicks into gear when they are sleeping or are knocked unconscious. You can't forget to breathe, and you don't stop breathing when you are asleep. This pattern also allows whales to keep moving while sleeping, maintaining position in relation to others in their pod and staying aware of predators such as sharks. The movement may also help them maintain their body temperature. Whales are mammals, and they regulate their body temperature to keep it in a narrow range. In water, a body loses heat 90 times as much as it does in air. Muscular activity helps keep the body warm. If a whale stops swimming, it may lose heat too fast. Do Whales Have Dreams When They Sleep? 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. Does that mean that whales don't have dreams? Researchers don't yet know the answer to that question. Some cetaceans sleep with one eye open as well, changing to the other eye when the brain hemispheres change their activation during sleep. Where Do Whales Sleep? Where cetaceans sleep differs among species. Some rest on 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. Large baleen whales, such as humpback whales, can be seen resting on the surface for 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 this behavior is referred to as "logging" because they look like giant logs floating on the water. However, they can't rest for too long at a time, or they may lose too much body heat while inactive. 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.