For a long time now I have been, shall we say, gently encouraged by two of my friends to write about a subject that is both familiar, and yet also strangely alien and poorly understood. Sleep behaviour. We still know comparatively little about this subject: not only about the big stuff like its function, but even about its distribution within animals. I am not, by the way, about to tackle the big questions about sleep, nor am I going to discuss the different types of sleep (e.g., REM vs NREM sleep) and on how they differ from creature to creature. Instead I'm interested in the more superficial stuff, like how and where animals sleep, and on the postures they adopt.

What we do know about sleep is scattered widely in the literature and it isn't easy to obtain any sort of review of the subject. There is at least one book devoted to it though: Maurice Burton's Sleep and Hibernation in the Animal World (Odhams Books, 1969), one of twelve books written for the Animal World in Colour series. Burton is well known for amassing large amounts of anecdotal data on the natural world (he also wrote books covering cryptozoology and the emotional lives of animals), and while the books in the Animal World in Colour series include a lot of often bizarre and fascinating information, they are completely unreferenced and at least some of the content looks apocryphal and difficult to be confident about. Despite these problems I have borrowed heavily from his book on sleep here.

Needless to say, I'm only going to be looking here at sleep in tetrapods, though where relevant I will mention other animals. The first thing to mention is that some animals get by without virtually any sleep, or in fact may even go without sleep entirely. Among vertebrates, all identified non-sleeping species are fish (Kavanau 1998). The species concerned have little in common (they include blind cave-dwellers, various sharks including some triakids, lamnids, hammerheads and sevengills, and mackerels and scombrids), but all have lifestyles where 'visual input is greatly reduced or absent during lengthy periods', where 'schooling greatly reduces needs for sensory information, particularly visual', or where the animals lead a 'comparatively routine existence in essentially featureless, open waters' (Kavanau 1998, p. 269) [sleeping cow from here].

Perhaps because no tetrapod lives in this way, it seems that all indulge in at least some sleep activity. But some sleep very little, with one of the best examples being seabirds: some species are on the wing for weeks at a time and appear to be constantly active. It has been suggested that they might sleep on the wing, but it's considered equally plausible that they simply delay their sleep. And having mentioned 'sleep on the wing', while it's 'well known' that swifts sleep in flight, the evidence for this (which essentially consists of observations of swifts seen decreasing in altitude during the early hours of the morning) is poor and we still require detailed studies (Rattenborg 2006).

Some ruminants also seem to engage in very little sleep. This might be because they need to remain constantly alert for predators, but it might also be because their biology requires a virtually continuous cycle of eating and ruminating. Burton (1969) wrote of observations made on goats which suggest that, during a rest period that lasts about 8 hours, they never really relax fully, and it is even stated that some ruminants (like cattle) sleep with their eyes open (this must be true: before indulging in the nefarious activity known as cow-tipping you're supposed to shine a torch into the cow's eyes to see whether it reacts or not. No reaction = asleep). Ruminants do sleep with their eyes closed at least sometimes however, as verified by the sleeping cow shown above. Passive stay apparatus (special locking mechanisms in the limbs) mean that some hoofed mammals can remain standing virtually indefinitely, and such mammals (which include deer, rhinos and horses) are therefore capable of sleeping while standing. Having said that, they will still lie down to rest when they want to. I photographed the donkey shown here as I initially thought it was dead.

It used to be thought that the biggest extant hoofed mammals - the giraffes - did not sleep at all, but this was shown to be incorrect by Grzimek (1956) who demonstrated that adult giraffes slept recumbently on the ground for short periods (2.5-6 minutes), resting the head on the hindquarters or ground and holding the neck in an arced posture. This posture is not unique to giraffes but is also practised by okapis and some bovids. The sleep behaviour of giraffes was looked at in detail by Tobler & Schwierin (1996) who showed that giraffes slept both while standing and while recumbent for a total of 4.6 hours per 24 hours.

Evidently little known is that elephants lie down on the ground to sleep properly: even today it seems widely thought that elephants 'crush themselves under their own weight' if they lie down, but this is just not true. In the wild, elephants sleep for 1-4.5 hours (per 24) in a recumbent position, with individuals showing preferences for sleeping on their left or right sides. Tobler (1992) reported that captive Asian elephants slept in a recumbent position for an average of 72 minutes a night. Elephants have been photographed propping themselves up against termite mounds and other structures when lying down to sleep, and in captivity they have been reported to construct 'pillows' from straw. The photo at the top shows a sleeping captive elephant (from wikipedia): for images of African elephants sleeping recumbently in the wild go here.

Large birds are also now known to recline on the ground for at least short periods. Burton (1969) reported how a scientist at Frankfurt Zoo monitored the sleep behaviour of ostriches: for 7-8 hours per night, they slept lightly with the neck erect. But for an average of 9 minutes each night they would lie on their sides, stretch their necks and legs out on the ground, and sleep deeply, during which time they could not be roused by lights or noises (except the very loudest noises). Emus, incidentally, are said to sleep deeply with the head and neck resting on the back, rather than lying on the ground.

Moving to other birds, terrestrial, ground-foraging birds like pheasants roost on the ground or in the low branches of trees, and simply huddle up and tuck in their appendages. I have been interested to learn, however, that small passerines (and perhaps others) don't sit out on branches during the night, but secrete themselves into small spaces. I know this because, while creeping through the woods at night once (don't ask), I discovered a Blue tit Cyanistes caeruleus tucked deep within a crevice in the bark of a tree, only its tail tip sticking out. It seems that most small birds climb deep into foliage, into crevices or cavities to sleep, and don't sit out in the open. Remember this next time you unnecessarily tear down the ivy from a tree or fence. Some passerines, including wrens, weavers and bush-tits, form large huddles within cavities (Perrins 1976). Burton (1969) wrote that birds which climb vertically on substrates, like nuthatches and treecreepers, also sleep in vertical postures, with nuthatches resting head-down and treecreepers resting head-up. Woodpeckers are also reported to sleep clinging vertically to vertical trunks (Perrins 1976). A few passerine species have been reported sleeping on their nests even when the nests were empty of eggs.

According to Burton (1969), birds do not put their heads under their wings when they sleep, but instead bury the bill into the scapular feathers. He also stated that penguins are the only birds that properly hide the bill under the wing: they can't submerge any part of their head within their scapular feathers, because their feathers are so short.

I wonder what the hesperornithines (which lacked long feathers and possessed strongly reduced wings) did [adjacent King penguin photo by J. P. Matth from here].

So far I haven't mentioned the fact that some animals sleep with only 'half of the brain': this is called unihemispherical sleep, and it allows the animal to stay partly alert while in a restful state. Animals capable of this may be able to keep one eye open and alert for danger (as in, for example, wading birds that rest while exposed in open habitats like mudflats), or may be able to regulate movement while also resting. The latter ability is important to marine mammals, which doze at or near the water surface and still need to regulate their breathing. Cetaceans sleep unihemispherically, and a number of pinnipeds are also reported to sleep floating in the water, hanging in a vertical posture in the case of Grey seals Halichoerus grypus, Northern fur seals Callorhinus ursinus and walruses Odobenus rosmarus. Weddell seals Leptonychotes weddelli are reported to do this while submerged at their breathing holes in the ice. It has also been noted that some pinnipeds sleep underwater in cetacean fashion, 'rising periodically, while still asleep apparently, to take a breath and then sinking again to continue their slumber' (Burton 1969, p. 27). This is correct, as verified by this photo (below) of a Harbour seal Phoca vitulina sleeping underwater (from Rock Paper Lizard - thanks to Vasha for the heads-up).

This has been nothing like a thorough review of the subject and, as I said at the start, it's been very superficial and I haven't looked at the big questions about sleep behaviour. For tetrapods at least, sleep would appear to be essential, even if members of some lineages can get by on very little. Speaking of which, at the time of writing it's gone 2 am. Good night.

Refs - -

Burton, M. 1969. Sleep and Hibernation in the Animal World. Odhams Books, London.

Grzimek, B. 1956. Schlaf von Giraffen und Okapi. Naturwissenschaften 17, 406.

Kavanau, J. L. 1998. Vertebrates that never sleep: implications for sleep's basic function. Brain Research Bulletin 46, 269-279.

Perrins, C. 1976. Bird Life: An Introduction to the World of Birds. Magna Books, Leicester.

Rattenborg, N. C. 2006. Do birds sleep in flight? Naturwissenschaften 93, 413-425.

Tobler, I. 1992. Behavioral sleep in the Asian elephant in captivity. Sleep 15, 1-12.

- . & Schwierin, B. 1996. Behavioural sleep in the giraffe (Giraffa camelopardalis) in a zoological garden. Journal of Sleep Research 5, 21-32.