There seems to be several commonalities across the mammalian world, from the mighty elephant to the microscopic mouse. One apparent commonality is that every mammal sleeps. While this is true for the majority of mammals there are actually some that do not sleep in the same way that most others do.
Whales and dolphins do not sleep in the same way that others mammals do and some scientists argue that what they do could not really be called sleep at all. For land mammals normal sleep is comprised of certain key stages of neural activity like REM and slow wave sleep. These are standard across every land mammal.
Unusally, cetaceans do not appear to experience either of these stages though they do experience a different type of neural activity from their standard waking activity. This different activity is called Unihemispheric Slow Waves (USWs). The thing that marks USWs out from land mammal sleep patterns is that they only happen in half of the brain at any one time. To all outward appearances they are still awake. They do not do anything different, but from what scientists can see on brain scans, during USW half of the cetaceans’ brain is asleep.
The reason they have developed this unusual sleep behaviour is that they are an air breathing animal that lives in the ocean: they simply cannot afford to sleep.
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