It is difficult to believe that a mix of Big Bird and Black Sabbath could bring a person to their knees. But, as officers in the US army's psychological operations (PSYOP) know, few combinations are more effective at breaking a prisoner's will. In Iraq, Sesame Street and heavy metal were combined to keep prisoners awake for long periods. "They can't take it," an officer was quoted as saying in 2003. "If you play it for 24 hours, your brain and body functions start to slide…your will is broken."
Sleep is one of the enduring mysteries of human consciousness. We need it for our cognitive and physical wellbeing, and, indeed, for our very survival. Yet, despite decades of research, the scientific explanation for why we need sleep, what it does, and how it evolved, remains elusive. It is not for want of theories; in fact, the problem is that there are too many seemingly contradictory ones.
Many non-scientific theories have filled the gap —most of them worrying that we aren't getting our full eight hours. It is often claimed that the harried 21st-century man sleeps, on average, at least an hour a night less than in the 19th century. Popular sleep-depriving villains range from too much television and internet to overwork. The think tank Demos even ran a project in 2004 calling for a "well-slept society," an idea so revolutionary that the Observer heralded it as one of the "ideas of the year."
Science is now catching up. Over the last decade the increased use of magnetic resonance imaging (MRI) has helped to identify the patterns of neuronal electrical activity associated with cognitive processes. This has led to big advances in our understanding of what sleep does, and in particular what it means for creativity, memory and even our weight.
We also now know more about what happens when we don't get enough. The celebrated Whitehall II study, which tracked more than 10,000 British civil servants over a number of years, has recently proved some of the worriers right. Making do with five hours a night—the "Thatcher syndrome," named after the famously hard-working prime minister—is damaging for most people. The civil servants gave details of their sleep patterns in the late 1980s, and then again in the early 1990s. Those whose sleep had fallen from seven to five hours a night or less doubled their risk of dying from a cardiovascular problem, such as a heart attack, compared to those who did not change their sleeping habits. (The researchers took into account factors such as smoking and blood pressure). These results are corroborated by other studies, which have linked poor sleep to everything from headaches to type 2 diabetes and heart problems.
Sleep: It's in the genes
The fact that advanced animals have two types of sleep is evidence that sleep has evolved in tandem with the brain. Yet its evolutionary roots go back to our single-cell bacterial ancestors, many of which modify their metabolic activity according to circadian rhythms set by movement of the Earth and stars. More recognisable sleep-like behaviour first occurred in primitive animals, like worms, which experience periods called "lethargus," when they respond less but form more new neuronal connections in their nervous systems. These creatures were also the first in evolutionary history to link their nervous systems to their equally primitive immune systems. Such crosstalk between the nervous and immune systems evolved greatly in higher animals and human beings.
The worm shows a clear evolutionary link between sleep and health. But worms would still make sturdy prisoners. Undeterred by death metal, they suffer no ill-effect from lethargus deprivation. One step up the evolutionary tree, though, scientists know that fruit flies typically enjoy lethargus for eight to 14 hours a day. And they have a gene—codenamed "shaker" because its absence causes wobbly legs linked to sleep deprivation. The shaker helps conduct electric charge across the nervous system; its absence leads to a drastic reduction in sleep, and early death. Take another step up the chain, to mice, and knocking out a similar gene leads to a much more dramatic drop in lifespan. And for higher animals, sleep becomes as necessary for survival as water, food and oxygen.
Yet, in contrast to our other elementary survival needs, sleep does not seem to have been an inevitable product of evolution. Instead, science suggests that sleep was written into our genes at an early stage by mutation. Only from then did it become a central pivot for the operation of natural selection. Any mutation that reduces our need for sleep tends to harm us in other ways, preventing its selection. But the ability to go without sleep for a few days is also essential: migrating birds cannot sleep properly during flights, while young whales have to be able to avoid hungry sharks. Most animals have evolved to cope with the occasional all-nighter. And some have evolved an even cleverer strategy. Lizards, for instance, are particularly exposed to predators. In response, they have evolved "uni-hemisphere" sleep, staying awake in one hemisphere while sleeping in the other. Some mammals take it even further: long distance birds stay awake for days by taking their non-REM sleep in one hemisphere at a time. REM sleep, meanwhile, is taken in short snatches. You can't see it from the ground, but birds on the wing are often napping in the sky.
These findings conflict with what we might call the "conking out" theory of sleep. Our common association of sleep with loss of consciousness is too simplistic. In humans as well as lizards, parts of the brain can be asleep while others are alert (something known as "local" sleep). Whispering "cat" in the ear of someone sleeping, for instance, can introduce felines into dreams. Sleep is regulated by both neuronal networks that span the whole brain, and local "napping" regions, particularly the cortex: the crux of human consciousness, memory and language. Only higher animals have an advanced cortex. And only higher animals enjoy local sleep.
The mysteries of REM
Conking out, then, cannot be a good definition of sleep. Instead, science now tells us that sleep is really more about the suspension of normal waking activities. It is this that allows the brain to recover. While we are awake our brain has to deal with vast quantities of external stimuli, all fostering small connections between different nerve networks. We need sleep to sift the important connections from the unimportant ones so we can start afresh the next day. And this brings us back to the difference between the two types of sleep.
An electroencephalograph (EEG) uses electrodes on the scalp to measure brain activity. When we wake up, whether locally or fully, these electrical rhythms become more irregular. But the EEG is also irregular during REM sleep, meaning, in electrical terms at least, that this is more of a hybrid between being asleep and awake. People are much more likely to wake up during REM sleep. And they are also more prone to sexual arousal—one reason for the phenomenon known as "morning glory" (nocturnal penile tumescence). Dreaming, too, occurs in REM sleep and can in effect be a form of simulated consciousness. Some people even dream that they are conscious, observing themselves asleep, like a dream within a dream.
The evolution of REM sleep is still hotly debated. It is associated with warm bloodedness. However EEG patterns in lizards suggest that its origins lie in reptiles, the common immediate ancestor of both mammals and birds. If true, this means it probably evolved for different reasons, as reptiles lack a sophisticated cortex. Only much later, according to this theory, did REM sleep begin to play a vital role in creativity, memory and learning.
A variety of recent studies have shown that people perform better at tasks—ranging from the regurgitation of facts to juggling—after sleep. Intriguingly we tend also to do better at recently learned tasks two or three days later, rather than the next day, even if we haven't practised them in the meantime. Those couple of extra nights, on their own, make us smarter. We humans, it seems, "practise" during sleep. Anecdotal evidence certainly backs this up: the golfer Jack Nicklaus claimed to correct his swing while dreaming; Paul McCartney says he literally dreamt up the tune for "Yesterday." Such tales are now backed up by studies showing that people woken during REM sleep are better at a variety of mental tasks, including solving anagrams. Natural selection seems to have honed REM sleep, in humans at least, partly to assist with solving difficult problems, presumably associated with survival. It also seems to help us to think creatively and keep our emotions on an even keel. Short-term deprivation of sleep, REM sleep in particular, impairs rational decision making, and can even induce temporary psychosis. Psychotic illnesses, like schizophrenia, are also associated with disordered sleep patterns.
If REM sleep is our brain's spark, non-REM sleep is its filter. It is in deep (slow wave) non-REM sleep that we discard transient memories and select a few for longer storage. For mammals this occurs in the neocortex, where it seems that representations of recent experiences are consolidated. During non-REM sleep they are transferred from the hippocampus, the landing pad where they are initially processed, to the neocortex. This process involves deleting unwanted memories, rather than positive selection, although details of the process are still vague. In effect, memory is really not about remembering at all; it's about what we choose to forget. And it is this process of active forgetting that requires disruptive waking activities to be switched off. This could explain why, even though we may be "locally" awake, we lose consciousness during sleep, in order to reboot and refresh our brains.
The brain isn't the body's only sleep centre, however. The central nervous system, and in particular the vagus nerve, may play a role in linking sleep to the immune system. The nerve's fibres branch out through the neck and chest and down into the abdomen, controlling automatic functions (like breathing), while conducting information to and from the immune system. In this way it provides the communication lines for the "inflammatory reflex," sometimes described as our sixth sense, giving early warning of infection. But the vagus nerve also connects with neuronal pathways regulating sleep, and experimental research suggests that stimulation of the nerve could help to treat sleep disorders. Such disorders also seem to disrupt the immune system and have been linked to Crohn's disease, high blood pressure and heart disease.
Sleep, depression and torture
Sleep plays a part in two other modern maladies. Depression is linked to the neuroendocrine system and especially low serotonin, the "happiness molecule" that regulates both mood and sleep. Sleep deprivation can lower serotonin, triggering depression, along with other effects like impaired immunity. In this way, sleep, depression and physical health go hand in hand. Perhaps more surprisingly sleep is also linked to obesity. A Warwick University study of 28,000 children and 15,000 adults in 2006 found that sleeping less than five hours a night doubled obesity rates. Cause and effect are hard to disentangle: depression, for example, can cause weight gain. But the study suggests that the hormones controlling appetite react to sleep, suggesting a three-way link—between sleep, health and appetite.
The importance of sleep to our health is attracting more attention, especially in richer countries where average sleep times are falling and sleep disorders are increasingly common. Such disorders occur everywhere in the world, but are accentuated among the affluent and over-stimulated—who sleep less and perhaps don't recognise that an extra hour could make some of their pill-popping redundant.
We also now know that sustained sleep deprivation causes psychological damage—from attention deficits and poor short-term memory to speech impairment and chronic depression—that is not restored by resumption of normal sleep patterns. Science has scotched the notion that sleep deprivation effects are "all in the mind." Rats, if totally deprived of sleep, die within two to three weeks. Human beings are more resilient. But without sleep, we too eventually die. It is this growing knowledge about the consequences of sleep deprivation that is changing attitudes. Inmates of Guantánamo Bay may still be kept awake with blaring Metallica or ACDC. But other countries, including Israel, have now banned the use of sleep deprivation on prisoners, recognising that it is in some respects even more harmful than physical torture. We would all sleep more soundly if the rest of the world followed suit.
