Nature Notes – June 2015
“To sleep, perchance to Dream;…”*
I fell asleep whilst writing this contribution for Hilltop News, but it was all in the cause of scientific research of course. I was encourage to delve further having read an interesting article on sleep in animals by Robert Burton who wrote endearing articles on the countryside for the Daily Telegraph.
As adults we spend around the third of our lives asleep. It is estimated that very young babies need 14-17 hours rest a day, taken in the form of several short sleep bouts and also some naps. This is probably a trait we share with our evolutionary mammalian ancestors which persists today as a crucial part of childhood development. Another element that has changed little has been the biological clock which has been set at somewhere just over 24 hours. This is understandably a complex and as yet not fully understood process but it is known that an important factor is exposure to daylight which triggers receptors in the brain.
Over the millennia this biological clock, or to be more correct the circadian clock, of Homo sapiens has been a driver behind man’s emergence as the dominant species on the planet. This dominance required a highly developed brain and it has become essential that there was a solid period of downtime (sleep) for the repair of neural pathways throughout the body. Our bodies are continually the subject of synchronisation to this diurnal cycle through the production of chemical signals regulated ultimately by our genes. We humans are not alone in enjoying a set period of rest each 24 hours sharing this habit of daily sleep with the other ‘Great Apes’, the Chimpanzee, Gorilla, Bonobo and Orangutan.
Through evolving larger brains and the increased intelligence the Great Apes developed inherited instincts enabling them to spend less time foraging or capturing food and more time socialising etc. However, this for other mammals though programmed to the 24 hour clock, their daily asleep / awake patterns differs from us. Herbivores like rabbits have to remain active and may feed for up to 15 hours each day to secure sufficient nutrition from grazing plant materials with low nutritional value. If elephants roamed the Chiltern Hills they would be seen or heard feeding for 18 hours a day and our native bats sleep for 20 hours each day. Consequently, opportunities to renew or repair the neural pathways are minimal and brain size remains relatively small which is perhaps also the reason why rabbits don’t rule the world. It is part of country lore that horses ‘sleep’ or rather nap standing up. It would be more correct to say they spend time resting on the hoof and only lie down to sleep for short periods. Napping rather than sleeping is the order of the day (and night) for their cousin the African wild horse, not least because there is a high risk of predation by carnivores. Domesticated horses it seems have retained this trait not for avoiding predators but perhaps because it instead affords them other advantages.
Carnivores on the other hand appear to spend long periods inactive, though sleep duration is very variable, mainly because they are opportunist hunters. It’s all about conserving energy for those rare opportunities when their prey is caught off-guard. Good examples here are the otters, stoats and weasels, known for their speed, strength and aggression.
Perhaps the most extreme example of ‘sleep’ predominating in mammals are dormice. Not only do they hibernate for over six-months of the year but even in summer they are inactive for large periods of time. It is not correct to call hibernating sleeping. Hibernation is a period of extreme inactivity which is best described as being in a state of extended torpor when heartbeat and breathing drop to the extent they are almost imperceptible. in fact, dormice do sleep from time to time during hibernating. They do this by actually moving out of their state of hibernation for a short period. When dormice are active they live their lives in the fast lane, feeding and foraging for nesting materials at breakneck speeds. As a consequence dormice use large parts of this long period of inactivity to undertake repairs to the body, particularly their muscles which take a heavy toll during their short bursts of frenetic activity. If we damage a muscle, the pain we experiences forces us to rest up in order to allow healing. For dormice there is no chance to rest so they carry injuries only mitigating the damage when they hibernate. As a consequence such damage eventually takes its toll which accounts for why dormice and their ilk are very short-lived. If you’ve ever had hamsters as pets you will know they spend a short period of time in frenzied activity interspersed with a lot of time dormant. In fact the dormant period is split between a deep sleep when energy is conserved and shallow sleep when restoration and renewal of tissues takes place.
One dilemma for mammals that live in the sea is how to continue to return to the surface to breathe when needing to sleep? As we know mammalian brains are divided into two halves or hemispheres. Whereas in humans each half has become specialised and carries out different functions with whales and seals it is believed such differentiation is limited. Instead each half can control basic body functions independently, so whilst one half ‘sleeps’ the other maintains a watching brief on breathing as well as keeping a wary eye looking out for threats such as predators. A variation on this solution has recently been discovered to occur with water birds such as mallards. In this case whilst birds in the centre of a community afloat on the water are sleeping normally those towards the edge keep one eye open and presumably the associated part of the brain on alert for possible danger. This has led to theories about how birds, such as swallows that fly long distances, manage to rest on the wing. It is suggested such birds have the ability to switch off those parts of the brain not concerned with flapping wings and navigation. No one yet knows whether they can fly with their eyes shut or just keep one eye open!
Just in case you are wondering sleep, or something akin to sleep, occurs across much the animal kingdom. Whether the eyes are shut or open is often a clue to sleep. Because fish lack eyelids this has led to disputes as to whether they sleep. It is suggested something at least akin to sleep occurs with some solitary fish, but apparently, does not apply to those that swim in shoals, or live in total darkness such as the blind fish in caves. Apparently, certain types of worms sleep, for example nematodes that live in the soil or ponds. Amongst insects; wasps, bees, moths, butterflies and flies are all known to sleep. Reptiles such as lizards or snakes and amphibians like newts or frogs being air-breathing, cold-blooded animals are thought not to ‘sleep’ as such, but rather shut-down their systems when temperature fall below a certain level, either overnight or during the winter months.
By the way despite their name, sloths sleep the same amount of time per day as us humans. If you’ve reached this point without nodding off, well done!
Despite what we may like to believe the flickering eyelids and the twitching feet of our pets there is as yet no evidence that aside from humans any other animal dreams…
* ay, there’s the rub.”
* from Hamlet Scene 3 Act 1
PS. Do you know what a sheep suffering with insomnia counts?
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