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, to sleep. (Whereas three-toed sloths are much livelier, 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.
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
