Читать книгу Counting Sheep: The Science and Pleasures of Sleep and Dreams - Paul Martin - Страница 16

We rise with the lark and go to bed with the lamb.

Nicholas Breton, The Court and Country (1618)

The current predilection for staying awake all hours is very recent in historical terms, let alone when measured against the span of biological evolution. It really took root following the invention of the electric filament light bulb by Thomas Edison in 1879, a negligible fraction of an instant ago in evolutionary terms. Of course, people did stay up after dark in the days before cheap electric lighting – just much less.

There have only ever been two ways for humans to deal with the night: to sleep and doze through it or to light it artificially. Until the nineteenth century the only practical source of artificial light was fire in one form or another. When humans depended on expensive candles or oil for artificial light they went to bed earlier and stayed there longer, unless they were in the wealthy minority. Few people did much work after dark. And when they did use artificial lighting, the fires and candles (and later, the gas mantles) generated light of insufficient intensity to reset their internal biological clocks in the way that much brighter electric lighting can. One electric light bulb produces as much light as a hundred candles and for only a tiny fraction of the cost. Unlike our ancestors, we no longer have to sleep, doze or stay in bed just because it is dark.

To appreciate how different life was for the majority of people living in temperate or northern climates, we need only wind the clock back to the eighteenth century. For most working folk, especially in winter, the sun provided the only serious illumination. The Natural History of Selborne, which was written by an English country clergyman called Gilbert White and published in 1788, describes life in a small village in rural England. White’s parochial history is said to be the fourth most published book in the English language. In one of his glimpses into the lives of Selborne’s human inhabitants, White reminds us that in the days before electric lighting, few people could afford the luxury of routinely staying awake for long during the hours of darkness. The villagers burned rushes to produce light, and even rushes cost money:

Working people burn no candle in the long days, because they rise and go to bed by daylight. Little farmers use rushes much in the short days, both morning and evening in the dairy and kitchen; but the very poor, who are always the worst economists, and therefore must continue very poor, buy an halfpenny candle every evening, which, in their blowing open rooms, does not burn much more than two hours. Thus have they only two hours’ light for their money instead of eleven.

In rural northern Europe of the Middle Ages it was pointless or impossible to work the fields during the dark days of winter, and too costly to heat and light the home all day. Whole families would therefore take to their beds for days at a time. You might not relish the prospect of spending days in bed with nothing to do. (Or perhaps you would?) Boredom would be the big enemy. Boredom, however, is a modern concept. Being alone with our thoughts and dreams is no longer enough for us.

Cold weather was another good reason for staying in bed, as Samuel Pepys recorded in this entry from his diary, written in December 1661:

All the morning at home, lying abed with my wife till 11 a-clock – such a habit we have got this winter, of lying long abed.

But even in warm, sunny climates, our ancestors probably spent more of their time in bed, especially in civilisations that practised the siesta.

In modern industrialised societies we are exposed to an artificial day that is extended by electric lighting and typically lasts for at least 16 hours, regardless of season. The marked seasonal fluctuations in the conception rate, which were once associated with the long winter nights, have almost disappeared now. Moreover, we now pack all of our sleep into a single block of time during the remaining seven or eight hours of darkness. This pattern of sleeping is biologically unusual: in most other species, sleep is split into two or more separate episodes in each 24-hour period. As we shall see, there are reasons for supposing that humans have not always slept in a single, compressed block.

Our daily cycle of sleep and wakefulness is largely determined for us by clocks rather than tiredness. Many of us go to bed when it is time to go to bed, not when we are tired, and wake when we have to wake, not when we choose to. Clocks with minute hands did not become available until the seventeenth century. Until quite recently in history, the majority of people relied on the sun for their timekeeping and lived in a world where the light intensity changed gradually at dawn and dusk, not instantaneously with the flick of a light switch. Moreover, they did not work in offices, factories or shops where they were required to be present at a certain time early every day.

How do humans sleep when left to their own devices in a world where it is dark for more than half the day – as would have been the case in pre-industrial northern countries during winter? To find out, Thomas Wehr at the National Institute of Mental Health in Maryland exposed volunteers to an experimental environment where it was dark for 14 hours a day and they could sleep freely. To begin with, the volunteers slept a lot (some more than 12 hours a day) as they caught up on their backlog. On average, they clocked up an additional 17 hours of sleep during the initial adjustment period. After their sleep deficits had been paid off, they settled down to an average sleep duration of eight and a quarter hours a day. Their mood and energy levels during the day improved consistently over the course of the experiment. When they were awake, they felt more awake and were more awake.

As well as sleeping longer, Wehr’s subjects also slept differently. Under these conditions of long, dark days and with nothing much to do, their sleep spontaneously divided into two distinct blocks. Typically, they would lie in a state of quiet rest each evening for about two hours before falling asleep. Then they would sleep for about four hours, usually waking at the end of an episode of dreaming. After another couple of hours of quiet rest they slept for a further four hours. On waking in the early morning they would lie in quiet rest for another couple of hours before rising.

Under these pseudoprimitive conditions, then, sleep was preceded, punctuated and terminated by long periods of quiet restfulness. This pattern of sleeping in two distinct blocks of time is known as biphasic sleep. It is typical of many mammals living in the wild and was probably the natural sleep pattern of our ancestors. We all retain the biological capacity for biphasic sleep, despite the profound changes in humanity’s environment since the advent of artificial lighting and the 24-hour society. A group of thoroughly modern Americans reverted to biphasic sleep within days of being given the opportunity. The nearest contemporary equivalent is the afternoon sleep of the siesta, a custom that still survives in some countries.

The predominant lifestyles of artificially-lit industrialised societies have led us to compress our sleep into a single block of seven or eight hours, as though we were living permanently in midsummer (but usually without the siesta). We have jettisoned the additional hours of quiet rest and the seasonal variations that once accompanied human sleep. We have also lost the main channel that once existed to our dreams. During the 14-hour nights, as they alternated between sleep and quiet rest, Wehr’s volunteers usually awoke from dreaming, giving them ample opportunity to lie quietly in the dark and contemplate their dreams. In later chapters we shall consider why dreaming evolved and what it does for us.

A quite different reason for believing that many people nowadays are chronically sleep-deprived is the mass of evidence that sleepiness is a major cause of accidental injuries and deaths. We shall now look at how sleepiness jeopardises safety-critical activities such as driving a car, flying an aeroplane, being a doctor, running a country and operating a nuclear power plant.