I’ve got my eye on you (all the time)

As humans, have you ever thought of how other animals sleep? Unlike humans who have comfortable homes and beds to sleep in, animals have to sleep in the open and are exposed to the elements and this includes the threat of being attack by predators! Hence, different animal species have adopted different ways to protect themselves when they are asleep e.g. sleeping in burrows to hide from predators, sleeping in groups and taking turns to watch over the group.

After browsing the internet for interesting sleeping behaviours, I found that mallards, Anas platyrhynchos sleep with one eye open and occasionally ‘peek’ when asleep to monitor their environment when resting. (Indiana Public Media, 2012) Can you imagine sleeping with one eye open? This is actually very common among many birds and mammal species.

So how is it possible that the mallard can sleep with one eye open? Well the answer is  unihemispheric sleep (Rattenbord, Amlaner & Lima, 2000). It sounds complicated but what it really means is that when these mallards or birds fall asleep, only half of their brain goes into a deep sleep! The other half would maintain its alertness and observes the surroundings. The different halves of the brains actually take turns to sleep depending on fatigue levels of each individual half.

Another interesting factor is the difference in peeking frequency between males and females (Lendrem, 1983). As the male plume is more colourful than the females, they tend to attract more attention and thus have to devote more energy to peeking. Females actually take advantage of this and actually peek less in the presence of males because they know the males will look out for them, thus saving themselves the energy of peeking frequently. Now, isn’t that nice (and convenient)!


Lendrem D.W., 1983. Sleeping and vigilance in birds. I. Field observations of the mallard (Anas platyrhynchos), Animal Behaviour, Volume 31(2): 532-538

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

Sleeping With One Eye Open by Don Glass, Indiana Public Media, June 5 2012. URL: http://indianapublicmedia.org/amomentofscience/sleeping-with-one-eye-open/ (Accessed 08 Apr 2013)