Weird animal brain: Dolphins sleep one hemisphere at a time

Have you ever wondered how cetaceans (whales, dolphins, and porpoises) sleep? They need to breathe, avoid danger, and care for young just like us, but they have to do it all while living in the ocean. Thankfully, they don't have to sleep like we do. Dolphins do sleep mostly at night, although it is unclear whether they enter deep REM (rapid-eye movement) sleep. However, they need conscious awareness to breathe (unlike humans). In addition, when mothers give birth, their calves don't float very well and can't swim for long periods of time. Therefore, mothers must continue to swim for the first few weeks of her calve's life to keep it alive. When she swims, the calf gets pulled along in her slipstream. Because of these factors, dolphins sleep with only one hemisphere at a time while the other remains awake. They even sleep with one eye closed while the other stays open to watch for predators and obstacles.

For more information:

Hecker, B. (1998). How do whales and dolphins sleep without drowning? Retrieved from http://www.scientificamerican.com/article/how-do-whales-and-dolphin/

Ridgway, S. (2006). Dolphin continuous auditory vigilance for five days. Journal of Experimental Biology,209(18), 3621-3628. doi:10.1242/jeb.02405

Ridgway, S. H. (2002). Asymmetry and Symmetry in Brain Waves from Dolphin Left and Right Hemispheres: Some Observations after Anesthesia, during Quiescent Hanging Behavior, and during Visual Obstruction. Brain Behav Evol Brain, Behavior and Evolution, 60(5), 265-274. doi:10.1159/000067192