Dolphins Sleep With One-Half of Their Brain At A Time

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At the end of the day I all too frequently find myself in disbelief that the time to turn in for the night has arrived so soon. With much left on my to-do list, I struggle to fight the strong urge to close my eyes and drift into the state of non-productivity we call sleep, albeit, with great futility. Once sleep’s siren song starts, I really don’t stand a chance — I tend to fall into a deep sleep the instant I see my bed. However, this is not true of all people, and even more so untrue for other, highly specialized members of the animal kingdom.

Masters of multitasking, dolphins have evolved the ability to rest one hemisphere (i.e., half) of their brain at a time. The sleeping brain can be identified by the electroencephalographic (EEG) patterns emitted by the cortex throughout the various stages of sleep. These range from high frequency beta waves associated with wakefulness to the sweeping delta waves associated with deep — aka slow wave — sleep.

While for us humans, these are global, brain-wide phenomena that put us into a wonderfully vegetative state, dolphins only experience this in one-half of their brain at a time. While one hemisphere’s EEG progresses through the stages of sleep the other remains awake causing the animal to enter a strange somnambulistic state, surfacing only briefly to breathe.

This behavior likely evolved as a means for keeping vigilant enough to detect and avoid predation, as the awake, alert half of the brain remains capable of responding to external stimuli and coordinating behavior. For example, when a dolphin pod’s sleep session is over, the early risers slap their tails against the water rousing the other members into wakefulness.

Interestingly, dolphins in captivity can abandon this behavior and sleep with both hemispheres simultaneously, keeping their blowhole above water with a tail kick reflex (a behavior similarly seen in anesthetized dolphins).


While I’m sure dolphins have their fair share of tasks to accomplish throughout the day, this sleep strategy could be a potentially useful way for me (and other overworked humans) to divide my time and maybe clean my house while I sleepwalk. I can save the more important things for when both halves of my brain are online.

(Editor’s note: If you enjoy this article, consider a print or digital subscription!)

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