A wrinkle to add to one of my favourite neurobiology factoids: Dolphins may not sleep with one side of their brains at a time after all [1]. Or, rather, they do sleep one side at a time (unihemispherically), but they may also sometimes let both sides sleep at once, albeit very briefly.

It’s not just dolphins who sleep with one side of the brains at a time. Other sea-sleeping mammals (whales, seals and manatee), many birds and maybe reptiles, like crocodiles, sleep unihemispherically too [1].

Posture assumed by northern fur seals during unihemispheric slow-wave sleep in water. In this example, the fur seal is lying on its left side while the left flipper (connected to the awake (right) hemisphere) constantly paddles. This posture allows the fur seal to keep its nostrils above the water?s surface, while the left hemisphere sleeps. When the fur seal switches to lying on its right side, the left hemisphere remains awake while the right hemisphere sleeps. (Figure and text shamelessly stolen without permission)

So: possibly our reptilian ancestors slept unihemispherically and we lost the habit as we tripped up the phylogenetic scale. Secondly, dolphins probably wouldn’t sleep one hemisphere at a time if they weren’t sleeping in the water. If we could just convince those guys to move onto land…I mean they’re pretty smart already, think what they could do if they could let one of their hemispheres drop the functions needed for wakefulness [3]. With one hemisphere relived of the need to be self-sufficient it could develop a deeper, complementary role for the other, waking-responsible hemisphere. They could be even smarter than they already are. Lets face it, we could do with some help up here- we need the dolphins on our team!

Hmm. Anyway. I wonder if there asymmetries in the human hemispheric response to sleep? It would suggest that a similar process had happen in mammalian evolution. And did dolphins evolve from a land-based mammalian line, and when?


1. Sam H. Ridgway (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, Behavior and Evolution 2002;60:265-274

Abstract: Studies of sleep in cetaceans (whales, dolphins, and porpoises), substantiated by electrophysiological data, are rare with the great majority of observations having been made by one group from Russia. This group employed hard-wired recording with low-noise cables for their EEG observations, whereas our report describes behavioral and EEG observations of dolphin sleep using telemetry. Marked asymmetry of the EEG was observed during behavioral sleep posture. At different times synchronized slow waves appeared in both left and right brain hemispheres concurrently with lower voltage, faster, desynchronized EEG activity in the opposite hemisphere. On the other hand, during one brief period of sleep behavior, sleep-like EEG activity appeared on leads from both hemispheres. When the animal was exposed to a loud sound, it woke with lower voltage, faster, relatively symmetrical, desynchronized EEG activity appearing from both hemispheres. Additionally, the EEG appeared relatively desynchronized and symmetrical between the two hemispheres when the animal was awake during recovery from pentothal-halothane anesthesia as well as during waking periods when one or both of the animal’s eyes were covered by an opaque rubber suction cup.

2. N.C. Rattenborg, C.J. Amlaner, S.L. Lima (2000). Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep. Neuroscience & Biobehavioral Reviews. Volume 24, Issue 8 , December 2000, Pages 817-842

3. Incidentally dolphins do have some hemispheric specialisation – they have a left dominance for visuo-spatial cognition (ie the opposite to humans): Kilian, A., von Fersen, L., G?nt?rk?n, O. (2000). Lateralization of visuospatial processing in the bottlenose dolphin (Tursiops truncatus). Behav Brain Res 116: 211-215