Is is well known fact that marine mammals and some birds can sleep with one brain hemisphere at a time, since it's essential for their survival.

However, at least in my opinion, such mechanism would give significant advantage to many other species (e.g. to antelops) because it helps avoiding predators.

While answers to this question explain why sleep is necessary, animals with unihemispheric sleep mode does not seem to suffer from any of mentioned drawbacks of non-sleeping (except maybe increased energy consumption).

Are there any particular disadvantages of unihemispheric sleep versus "normal" sleep?

EDIT: I've stumbled upon the following paper: Rattenborg N., Amlaner C., Lima S.. 2000. Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep. Neuroscience & Biobehavioral Reviews 24: 817–842, which discusses several possible reasons for absense of unihemispheric sleep in most mammals, but does not present any evidence to support their hypothesis. The main reason (from authors' point of view, at least) is the need for very loose coupling of both hemispheres to allow them sleep independently, which might negatively affect overall brain effectiveness.

Along these lines, sleep may play a role in integrating the functions of different brain regions. Thus, BSWS1 in terrestrial mammals may be superior to the BSWS in animals with USWS2 because it allows integration within and between hemispheres, rather than just within a particular hemisphere.


Specifically, the reorganization of the central nervous system required for USWS may interfere with other adaptive brain functions, such as integrating the functions of both hemispheres.

1 Bihemispheric Slow Wave Sleep; 2 Unihemispheric Slow Wave Sleep

They also argue that the unihemispheric sleep might have been present in first mammals, but wasn't giving any advantages on first stages of evolution.

However, this paper was published more that 10 years ago, so there might have been some advances in the field since then.

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    $\begingroup$ I would assume that to give your brain the rest it needs you would have to spend double the time sleeping when resting one hemisphere at a time. For humans at lest that would be kind of disastrous. $\endgroup$ Aug 19, 2012 at 10:57
  • $\begingroup$ @UltimateGobblement I agree that it might be bad for humans, but I doubt that most low savannah animals have more complex lifestyle than dolphins, so using one hemisphere most of the time might be enough, and enhanced alertness during night increases chances of survival during attack of nocturnal predators. Giraffes and elefants, e.g., sleep approx. 4 hours a day, so reduced sleep time is likely very beneficial to them. $\endgroup$
    – aland
    Aug 19, 2012 at 17:16
  • $\begingroup$ that would make it easier to work as a night watchmen or some other jobs like working in a toll booth. $\endgroup$
    – shigeta
    Sep 4, 2012 at 4:53
  • $\begingroup$ I am pretty sure one hemisphere of my brain is asleep when I watch TV... ;) $\endgroup$
    – Bitwise
    Oct 12, 2012 at 23:46
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    $\begingroup$ Interesting note, seals which can sleep both on water and on land, employ unihemispherical sleep in water but whole brain sleep on land, so this suggests unihemispherical sleep is non-optimal $\endgroup$
    – dayuloli
    Jun 2, 2013 at 9:14

1 Answer 1


I thought this was an interesting subject and I had some time to do a little search around today. I can't find anything better than the review you point out. All I can do is add a few interesting notes since 2001. Nothing really turns over Rattenborg et al.

Sleep is itself a poorly understood process and most of the theories of the necessity of sleep can teeter precariously with discoveries like this report that baby dolphins and killer whales and their mothers simply don't sleep at all for the first few months of life.

The Rattenborg review is only guessing, but I think its clear that there is a cost to unihemispheric sleep, even if we don't know what it is. Cetaceans and birds and perhaps reptiles having unihemispheric sleep are all cases where there are perpetual and strong selection forces that maintain the ability in these animals. Without constant predation experienced by tasty ducks or the vagaries of being an air-breathing underwater mammal, animals quickly lose their ability to sleep with one half their brain, one assumes because the ability to maintain it is particular and easily lost with variation. Modelling work of the hemispheres in sleep indicates that the connections between the hemispheres are modulated between the two modes of sleep - there may be advantages to such brain structures for instance.

This blog post is a good read though and a bit more recent, though most of the references are from before 2001. Dolphins do use both hemispheres to sleep, but just not very often (about 15% of the time), indicating there is some advantage even for these animals. Seals may alternate between bi and uni-hemispheric sleep when they are on land and at sea.

If you were wondering if there was something you were missing - I'd say no. At the time of this writing THIS web page is the sixth hit on a google search for the term 'unihemispheric sleep'....


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