Counting Sheep: The Science and Pleasures of Sleep and Dreams. Paul Martin

      Sleeping organisms also adopt a characteristic posture. Sloths and bats, for example, sleep hanging upside down from a branch. The Mediterranean flour moth sleeps with its antennae swivelled backwards and the tips tucked under its wings. If you are careful, you can gently lift the sleeping moth’s wing without disturbing it – a trick that will definitely not work when it is awake. A lizard will settle on a branch during the hours before sunset, curl up its tail, close its eyelids, retract its eyeballs and remain in that distinctly sleep-like posture all night unless it is disturbed. A partridge, like many birds, will rest its weight on one leg while it sleeps. It is said that some gourmets can tell which leg, from its taste.

      Monkeys and apes, including humans, usually sleep lying down. Indeed, we are built in such a way that we find it difficult to sleep properly unless we are lying down. People can and sometimes do sleep after a fashion while sitting, notably in aeroplanes, business meetings and school classrooms. If you are really exhausted, you might even manage to snatch some sleep standing up. But sleep taken while standing or sitting upright is generally fitful, shallow and unrefreshing. The non-horizontal sleeper may repeatedly nod off, but as soon as they descend beyond the shallowest stages of sleep their muscles relax, they begin to sway and their brain wakes them up again. That is why we ‘nod off’. If you travel frequently on trains or buses, you might have had the dubious pleasure of sitting next to a weary commuter who has nodded off all over your shoulder. Recordings of brain-wave patterns show that people sleeping in an upright sitting position achieve only the initial stages of light sleep, not the sustained, deep sleep we require to wake up feeling truly refreshed. The reason is simple. Our muscles relax when we are fully asleep and we would fall over if we were not already lying down. Our brains therefore do not permit us to enter sustained, deep sleep unless we are in a physically stable, horizontal (or near-horizontal) posture.

      Despite the virtual impossibility of sleeping deeply while sitting upright, we are sometimes forced to try. In Down and Out in Paris and London, George Orwell describes a particularly unwelcoming form of overnight accommodation that was known to the homeless of prewar London as the Twopenny Hangover. At the Twopenny Hangover the night’s residents would sit in a row along a bench. In front of them was a rope, and the would-be sleepers would lean on this rope as though leaning over a fence. In that posture they were supposed to sleep. At five o’clock the next morning an official, wittily known as the valet, would cut the rope so that the residents could begin another day of wandering the streets.

      Nowadays, tourist-class airline passengers travelling long distances can enjoy an experience similar to the Twopenny Hangover, albeit at vastly greater expense. George Orwell’s autobiographical account of grinding poverty in the late 1920s is also a sharp reminder that lack of money is often accompanied by lack of decent sleep. Rough sleepers rarely get a good night’s sleep.

      Sleep has several other distinctive characteristics besides immobility and posture. In many species, including humans, individuals return to the same place each night (or each day, if they are nocturnal) in order to sleep. More generally, all members of a given species will tend to choose the same sorts of sleeping places. The distinctive feature of those places is often their security. Birds usually sleep on inaccessible branches or ledges. Many small mammals sleep in underground burrows where they are safer from predators. Fishes lie on the bottom, or wedge themselves into a crevice or against the underside of a rock. We humans prefer to sleep in relatively private and secure places. Given the choice, we rarely opt to sleep on busy streets or in crowded restaurants.

      One obvious feature of sleep is a marked reduction in responsiveness to sights, sounds and other sensory stimuli. To provoke a response from a sleeping organism, stimuli have to be more intense or more relevant to the individual. For example, the reef fish known as the slippery dick sleeps during the hours of darkness, partly buried in the sand. While it is in this state, the sleeping slippery dick can be gently lifted to the surface by hand without it waking up and swimming off.

      A sort of perceptual wall is erected during sleep, insulating the mind from the outside world. You would still be able to sleep if you had no eyelids, because your sleeping brain would not register what your eyes could see. This sensory isolation is highly selective, however. You can sleep through relatively loud noises from traffic or a radio, but a quiet mention of your name can rouse you immediately. Your brain is not simply blocked off during sleep. Moreover, this reduced responsiveness is rapidly reversible – a characteristic that distinguishes sleep from states such as unconsciousness, coma, anaesthesia and hibernation. A suitable stimulus, particularly one signifying immediate danger, can snap a sleeping person into staring-eyed alertness in an instant.

      Another diagnostic feature of sleep is its regular cycle of waxing and waning. Living organisms sleep and wake according to a regular 24-hour cycle, or circadian rhythm. All members of a given species tend to sleep during the same part of the 24-hour cycle, when their environment is least favourable for other activities such as looking for food. For most species this means sleeping during the hours of darkness, but some species do the reverse. Many small mammals, which would be more vulnerable to predators during daylight, sleep by day and forage at night. Aside from a few nocturnal specialists such as owls, birds cannot easily fly in the dark, and most reptiles find it hard to maintain a sufficiently high body temperature to be active during the cool of night. Most birds and reptiles therefore sleep at night. Predators tend to sleep when their prey are asleep and hunt when their prey are up and about.

      Sleep, then, is characterised by a special sleeping place and posture, prolonged immobility, a selective and rapidly reversible reduction in responsiveness to stimuli, and a 24-hour cycle. According to these and other criteria, all mammals, birds, fish, amphibians, reptiles and insects that have been inspected have been found to sleep.

      Take the humble fruit fly, Drosophila melanogaster, for example. This small insect displays all the key features of sleep, and more. Fruit flies alternate between periods of activity and rest according to a 24-hour cycle. They go to sleep in a preferred location where they remain immobile for two hours or more at a time, usually at around the same time of day. While they are asleep, they are much less reactive to sights, sounds and other sensory stimuli. If they are prevented from sleeping they show an increasing propensity to sleep. And when they do sleep after a period of deprivation, they are harder to wake up and they sleep for longer, as if catching up. Young fruit flies sleep more than old fruit flies, just as babies sleep more than adult humans. And, also like humans, their sleep is assisted by sleep-inducing drugs and disrupted by stimulants such as caffeine.

      Many small mammals spend more than half their lives asleep. Think of the dormouse, for example. Lewis Carroll’s description in Alice’s Adventures in Wonderland is biologically authentic, apart from the animals’ ability to talk grammatical English:

      There was a table set out under a tree in front of the house, and the March Hare and the Hatter were having tea at it: a Dormouse was sitting between them, fast asleep, and the other two were resting their elbows on it, and talking over its head. ‘Very uncomfortable for the Dormouse,’ thought Alice; ‘only, as it’s asleep, I suppose it doesn’t mind’…

      ‘You might just as well say that “I see what I eat” is the same thing as “I eat what I see”!’

      ‘You might just as well say,’ added the March Hare, that “I like what I get” is the same thing as “I get what I like”!’

      ‘You might just as well say,’ added the Dormouse, who seemed to be talking in his sleep, ‘that “I breathe when I sleep” is the same thing as “I sleep when I breathe”!’

      ‘It is the same thing with you,’ said the Hatter.

      Dormice (which actually comprise 20 different species of nocturnal rodents) really do spend most of their time asleep, as do many other species of small mammals. The volcano mouse spends more than 17 hours a day asleep. Even the naked mole rat devotes 12 hours a day to sleep, despite the fact that it lives underground and has become so adapted to subterranean life that it has lost the power of sight.

      The champion sleepers are two-toed sloths, which dedicate an average of 20 hours a day, or more than 80 per cent of their entire lives,