of sleep and the resulting daytime tiredness are associated with greater sickness absence from work. An investigation of absenteeism among French employees found that those who reported feeling very sleepy at least three days a week were more than twice as likely to take sick leave as their less sleepy colleagues. Sleep problems are also predictive of long-term work disability. Norwegian research found that adults who were experiencing mediocre or poor sleep were more than twice as likely to face long-term work disability a few years later. People who sleep badly also tend to eat badly, which may contribute to their health problems.
Good sleep, on the other hand, fosters mental and physical health. Psychological wellbeing, physical health and longevity are all statistically associated with healthy lifestyle practices, one of which is good sleep. The lifestyle factors associated with a lower risk of dying prematurely include taking physical exercise, not smoking and getting seven or eight hours of sleep a night. For example, recent research that investigated longevity in Japanese people uncovered three important factors, each of which was independently linked with a reduced risk of dying. These factors were walking for at least one hour a day, ikigai (a sense that your life is meaningful), and sleeping for at least seven hours a night. There is even some tentative evidence that people who habitually go to bed early live longer. A study of people aged over 80 found that these long-lived individuals all reported having gone to bed early throughout their lives. However, retrospective evidence of this sort must always be taken with a large pinch of salt. (But not too much salt, because that would be unhealthy.)
At the other extreme, excessive sleep is also linked statistically with poor health – probably because sleeping for unusually long periods is often a sign of illness. Scientists discovered in the 1970s that people whose normal nightly sleep duration was either unusually short (less than four hours) or unusually long (more than nine or ten hours) had a higher than average risk of dying prematurely. Similarly, a study of elderly British people found that those who spent 12 or more hours a day in bed had a significantly higher mortality rate, while those who spent the proverbial eight hours a day in bed had the lowest mortality rate.
Excessively long sleep is often a consequence of heart disease or other medical conditions, so it would be a mistake to generalise this finding very far. There is no reason to suppose, for example, that sleep-deprived teenagers or exhausted adults who lie in bed at the weekends will die younger as a consequence of snatching a few extra hours of rest. Too much sleep can make you feel temporarily below par, however. Experiments have confirmed that healthy people who normally feel refreshed after eight hours of sleep tend to feel groggy and perform badly after they have slept (on request) for 10 or 11 hours. On the other hand, how many healthy adults routinely sleep for 10 or 11 hours at a time? Few of us have any reason to fret about the dangers of sleeping too much.
We term sleep a death, and yet it is waking that kills us, and destroys those spirits that are the house of life.
Sir Thomas Browne, Religio Medici (1642)
What happens when sleep deprivation is taken to the extreme? If a slight insufficiency of sleep makes us feel unwell, would a prolonged absence kill us? Setting aside purely anecdotal accounts, science has unsurprisingly not investigated whether forcibly depriving humans of sleep is fatal. Ethical committees would tend to frown upon applications from scientists proposing to test this experimentally. But the evidence from other species is clear. Animals that have been experimentally deprived of sleep for long enough invariably die. There is no reason to suppose that humans are fundamentally different.
Some of the earliest experiments on extreme sleep deprivation were performed in the late nineteenth century by a Russian scientist called Marie de Manacéïne. She deprived puppies of sleep by keeping them constantly active. They all died within four or five days, despite every effort to keep them alive. The younger the puppy, the more rapidly it succumbed. Marie de Manacéïne also noticed a progressive decline in the body temperature of the sleep-deprived animals, a phenomenon that is now known to be a standard symptom of prolonged sleep deprivation in humans and other species. She concluded that sleep was even more crucial for survival than food:
As a rule, the puppy deprived of sleep for three or four days presents a more pitiful appearance than one which has passed ten or fifteen days without food. I can speak from observation, as I was obliged to make experiments on the results of want of food as well as of sleep, and I became firmly convinced that sleep is more necessary to animals endowed with consciousness than even food.
Italian scientists working at the end of the nineteenth century kept adult dogs awake by making them walk. The sleep-deprived dogs all died after 9–17 days, regardless of how much food they ate.
One objection to experiments such as these (apart from the obvious ethical one) is that the scientists had to use increasingly stressful methods to keep the animals awake, so perhaps it was the stress that killed them rather than the sleep deprivation itself. Prolonged stress can impair the immune system and make an animal more vulnerable to infection. However, more recent research has managed to sidestep this methodological problem.
In a long series of experiments, Alan Rechtschaffen and colleagues at the University of Chicago systematically investigated how prolonged sleep deprivation affects rats. They used an experimental procedure known as the disc-over-water method, which works like this. Two rats – the experimental subject and the ‘yoked control’ – are placed on a turntable mounted over a shallow bath of water. The brain-wave patterns of both animals are continuously monitored to detect the onset of sleep. When the experimental rat’s brain waves indicate that it is falling asleep, the turntable automatically revolves slowly, waking the unfortunate rat and forcing it to walk in the opposite direction to avoid being pitched into the water. The control animal, which is on the other side of the turntable and separated by a partition, receives precisely the same treatment at precisely the same times. The crucial difference is that the turntable movements are unaffected by its sleep. The control animal is therefore able to get some sleep when the experimental animal is awake. This cunning technique has the advantage – from the human experimenter’s point of view – of preventing the experimental subject from sleeping without having to subject it to other noxious stimuli.
Rats that are prevented in this way from sleeping invariably die after two or three weeks. The control animals, which experience the same stimuli but not the complete loss of sleep, survive and display relatively few symptoms. Before they die, the sleep-deprived rats all exhibit the same horrible syndrome. This is characterised by a debilitated appearance, skin lesions, increased food intake, weight loss, increased metabolic rate, increased levels of the hormone noradrenaline, and declining body temperature. Some of these changes are symptomatic of excessive heat loss from the body, which has led some scientists to suggest that sleep is crucial, among other things, for the regulation of body temperature.
A progressive rise in metabolic rate (the rate at which the body consumes energy) is an early symptom of sleep deprivation. Sleep-deprived rats eat more to compensate for their rising energy expenditure, but their weight and body temperature nonetheless continue to fall. Feeding them an easily digestible diet helps to slow this process somewhat, but it does not prevent them from dying. An increase in appetite is one of the less obvious effects of sleep deprivation in humans as well. Might it be that chronic sleep deprivation is one of the factors helping to fuel the epidemic of obesity that is currently sweeping the USA, UK and other industrialised nations?
Health may be as much injured by interrupted and insufficient sleep as by luxurious indulgence.
William Kitchiner,
