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.
Till o’er their brows death-counterfeiting sleep
With leaden legs and batty wings doth creep.
William Shakespeare, A Midsummer Night’s Dream (1595–6)
Accidents are one of the leading causes of death in developed nations, and sleepy people are responsible for many of them. A remarkably large proportion of vehicle accidents are the direct or indirect result of tired drivers losing concentration or falling asleep at the wheel. A few examples may give a flavour of the carnage they cause. In March 1994 near Barstow in California, a pickup truck carrying 20 people veered off the road and crashed into a culvert after the driver apparently fell asleep at the wheel. The driver survived but 12 passengers died. In July 1995 near Roquemaure in France the driver of a bus carrying Spanish students from Amsterdam to Barcelona seemed to nod off then wake abruptly as his bus scraped a passing truck. He lost control and the bus swerved wildly before rolling over several times. The accident caused 22 deaths and 32 injuries. In February 2001 a sleep-deprived driver caused the Selby rail disaster in the UK, after he fell asleep at the wheel and his vehicle crashed onto a railway line. Ten train passengers died. The driver, Gary Hart, admitted getting no sleep the night before the crash, but claimed he could still drive safely. He was sent to prison.
Scientists have judged that sleepiness is a factor in at least 10 per cent of fatal car crashes in the USA and more than 50 per cent of fatal crashes in which a truck driver is killed. A 1994 report by the US National Commission on Sleep Disorders concluded that driver fatigue contributed to 54 per cent of all vehicle accidents in the USA. A comparable situation applies in the UK and elsewhere. Research concluded that at least 10 per cent of vehicle accidents in the UK are related to sleepiness, though some experts have put the figure much higher. Two large surveys in England found that sleepiness was a causal factor in 16 per cent of accidents to which the police were summoned and at least 20 per cent of accidents on motorways. Driving on a motorway is generally more monotonous than driving on a minor road, and monotony heightens the risk that a tired driver will fall asleep at the wheel. Half the drivers involved in these sleep-related accidents were men under the age of 30 and many of the accidents involved truck drivers, company cars or workers returning home from night shifts.
For every sleepy driver who actually crashes there are uncounted numbers who have had near misses. A large survey by the British Transport Research Laboratory found that 29 per cent of drivers had come close to falling asleep at the wheel within the previous year, while other research established that at least 5 per cent of middle-aged male drivers had actually fallen asleep while driving on several occasions. Not surprisingly, drivers suffering from moderate or severe daytime sleepiness are at least twice as likely to have a vehicle accident.
The official statistics tend to underestimate the true extent of the sleepiness problem, and it is easy to see why. Drivers who survive crashes are naturally reluctant to admit that they dozed off at the wheel, even if they recollect doing it. And it is difficult to prove legally that sleepiness caused a crash (especially if the driver is dead). Unlike alcohol, drugs or mechanical defects in a vehicle, sleepiness leaves few evidential traces. You can easily measure how drunk someone is at the roadside immediately after an accident. But measuring sleepiness is neither quick nor easy, and in practice it is simply not done.
The systematic
