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

sleeping for a mere 17 hours a day.) Close behind come armadillos, opossums and some species of bats, which sleep for 18–19 hours a day. Many lizards spend more than 16 hours a day in sleep. Nearer to home, cats, rats, mice and hamsters sleep for 13–14 hours a day. Birds too can be great sleepers, although their sleep is less obvious to the casual observer because it is fragmented into short episodes. Starlings, which are fairly typical, spend a total of more than nine hours a day asleep, but this is split into many short bouts, each lasting on average only seven minutes. At the other end of the sleep spectrum lurk the wakeful grazers – cows, goats, elephants, donkeys, horses, sheep, deer and giraffes – all surviving on a meagre ration of three or four hours a day. But that is the minimum.

      We humans occupy the low-to-middle ground of the zoological sleep spectrum, along with moles – or, at least, we do if we assign ourselves the proverbial eight hours a night. In reality, most people get substantially less than eight hours – a theme we shall be exploring in the next three chapters. On that same theme, it is notable that our closest biological relatives, the apes and monkeys, sleep more than us. Chimpanzees, rhesus monkeys, squirrel monkeys, vervets, patas monkeys and baboons sleep nine or ten hours a night, while the gorilla averages 12 hours. The sleepiest primate is the owl monkey, which clocks up 17 hours a day of sleep, accounting for more than 70 per cent of its life.

      Do plants sleep? This is almost, but not quite, as stupid a question as it may seem. In one loose and misleading sense plants do display some behavioural characteristics of sleep. Many plants alter their shape each night, as though curling up to go to sleep. Some species furl their leaves like an umbrella, some allow their leaves to droop as if they need watering, while others, including lettuces and radishes, point their leaves vertically upwards. They all display a distinct 24-hour circadian rhythm.

      Pliny the Elder noted this ‘sleep of plants’ in the first century AD. Eighteen centuries later, Charles Darwin investigated the ‘sleep of plants’ with a series of ingenious experiments in the garden of his home at Down House in Kent. Darwin was confident that the phenomenon was not true sleep. ‘Hardly any one,’ he wrote, ‘supposes that there is really any analogy between the sleep of animals and that of plants.’ As usual, Darwin was right. His experiments demonstrated that plants alter their shape at night to protect themselves from their physical environment. He found, for example, that if he left a plant outside at night, with its leaves tied up to prevent them from drooping, the result was a blackened, shrivelled and dead plant the next morning. Darwin also discovered that a plant will not ‘sleep’ if it is shaken violently.

Half asleep

      And the small fowl are making melody

      That sleep away the night with open eye

      Geoffrey Chaucer, Prologue to The Canterbury Tales (c. 1387)

      Sleep is such an overriding biological imperative that evolution has found ingenious ways of enabling animals to do it in the face of formidable obstacles. Nature, it seems, will do almost anything to ensure that animals sleep.

      Consider dolphins, for example. They are air-breathing mammals like us, so they must swim to the surface each time they want to take a breath. They would drown if they fell into deep sleep while deep underwater. One possible solution to this biological design conundrum would be to wake up each time a breath of air was required. However, evolution has produced a more elegant solution: only one half of the dolphin’s brain goes to sleep at a time.

      Dolphins are capable of what is known as unihemispheric sleep, in which one hemisphere of the brain submerges into deep sleep while the other hemisphere remains awake. The two halves of the brain take it in turns to sleep, swapping at intervals of between one and three hours. This cerebral juggling trick enables dolphins to sleep underwater without drowning, which is just as well considering that they spend a good third of their lives asleep. Unihemispheric sleep has been recorded in several species of dolphins, porpoises and whales, including bottlenosed and Amazonian dolphins, Black Sea porpoises and white whales.

      Despite the apparent convenience of being able to sleep and stay awake simultaneously, very few mammals are capable of unihemispheric sleep. The biological benefits of sleeping with only half of the brain at a time presumably outweigh the disadvantages only under unusual conditions, such as those encountered by air-breathing mammals living in the deep oceans.

      Unihemispheric sleep is widespread in birds, however. They do it for a. different biological reason. Sleeping with half the brain awake and one eye open allows them to sleep while simultaneously remaining vigilant for predators. In birds, each eye exclusively feeds the visual processing areas in the opposite half of the brain: thus, all the nerve fibres coming from the right eye connect to the left hemisphere of the brain and vice versa. When a bird is in unihemispheric sleep its open eye is the one corresponding to the waking half of the brain, while the closed eye is connected to the sleeping half. If a bird feels relatively safe, it closes both eyes, and both sides of its brain go to sleep.

      An experiment with mallard ducks demonstrated how unihemispheric sleep helps birds to stay safe from predators. Four ducks were placed in a row along a perch, the idea being that the ducks at either end of the row would feel more vulnerable to predators than the two in the middle. In the natural world it is generally a bad idea to be on the edge of a group if you might end up as some other animal’s dinner. As predicted, video recordings showed that the outer two birds were much more likely to sleep with one eye open than the two on the inside; their unihemispheric sleep increased by 150 per cent. The amount of unihemispheric sleep rose further when the ducks were shown frightening video images of an approaching predator.

      The relationship between unihemispheric sleep and vigilance was finely controlled. The exposed birds on the ends of the row preferentially opened their outward-facing eye – the one directed towards potential danger. From time to time, a bird would turn round and switch eyes, so that the open eye was still the one facing out. Simultaneous recordings of brain activity confirmed that the brain hemisphere corresponding to the open eye was always awake, while the hemisphere corresponding to the closed eye was the one in deep sleep.

      The one-eyed tactic was effective: when an attacking predator was simulated on a video screen, the birds sleeping with one eye open were able to react in a fraction of a second – far faster than if they had been in deep sleep with both eyes shut.

      Humans are not capable of unihemispheric sleep, although at least one writer has played with the fantasy. Damon Runyon wrote of how he once played cards with a fading champion card player who now lacked the stamina to stay awake during marathon games of gin rummy lasting eight or ten hours. When the man lost a game after making a bad play, the punters betting on him to win clamoured to remove their bets from the next game, on the grounds that he was asleep. Then someone pointed out that the allegedly sleeping player’s eyes were open, so he must be awake. ‘The one on your side is’, retorted one of the backers, ‘but the one on the other side is closed. He is sleeping one-eye.’

Nice, not naughty

      Like all other forms of pleasure, sleep may become a passion.

      Jean-Anthelme Brillat-Savarin, The Physiology of Taste (1825)

      Sleep is far more than just a biological necessity. It is also a neglected source of pleasure. Consider this. Activities that are biologically important for survival and reproduction tend to be enjoyable: think of sex, or eating, or drinking, or being successful. Pleasure is one of nature’s ways of ensuring that animals do enough of the right things. Whatever happened to sleep? It is clearly essential for survival, and yet for many people it is merely a maintenance activity that brings little positive enjoyment. They sleep because they have to rather than because they want to.

      In some respects, sleep has acquired the dismal status that eating had in post-war Britain, where austerity and a cultural blind spot reduced the culinary arts to a joyless act of refuelling. Bland, fatty food was daily shovelled in