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to sleep after learning new information were found to have a better recall of the data they had learned than those who had not slept.

      To most of us, the benefits of sleep are evident from the way we feel after a good night’s rest. But perhaps a better way to understand the role of sleep is to look at what happens when we don’t sleep.

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       Rest and sleep

       Scientists are baffled about the role of rest in sleep, as the amount of energy saved during sleep is only 100 kcal – the same number of calories as in a large piece of toast.

      Effects of sleep deprivation

      Classic sleep deprivation experiments consist of depriving subjects of one night’s sleep, then asking them to listen to about 1800 bleeps for an hour or so. About 40 of the bleeps are a second shorter than the others, and these are the ones the subjects have to react to. (Most errors of detection generally occur in the last 15 minutes of the task.) Experiments such as this have proved useful to scientists’ understanding of the consequences of lack of sleep. Findings have shown the main short-term effects to be as follows:

      • General lack of wellbeing. Lack of sleep can cause fatigue and grogginess.

      • Concentration and vigilance. Experiments have invariably shown damaging effects in these areas. People who have been sleep-deprived are more likely to have difficulty taking in information and to make mistakes at work. In real-life situations requiring constant vigilance, such as driving, the dangers are obvious. Statistics show that 20 per cent of all road accidents are caused by fatigue and that many of these accidents will lead to fatalities.

      • Memory. Many people complain that they are more forgetful when they do not get enough sleep. This could be down to a concentration problem but it may also be that sleep deprivation makes it more difficult to retrieve information from the brain’s memory store.

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       Sleep deprivation

      • 20-25 hours of sleep deprivation reduces mental performance to the same level as someone with a blood/alcohol concentration of 0.1 per cent, which is greater than the current maximum for legal driving in the UK – 0.08 per cent.

      • The US Department of Transportation estimates that 100,000 accidents every year are caused by people feeling fatigued and/or drowsy, and that it leads to 4 per cent of all traffic-related deaths.

      • Mood. Lack of sleep can lead to irritability and over-anxiety, which can have damaging effects on your social life, family and other relationships.

      • Immunity. Evidence suggests that lack of sleep may affect the immune system. After vaccination, subjects who may have been sleep-deprived have 50 per cent fewer antibodies than those who have slept adequately. Sleep and the immune system are strongly linked; bacterial cell walls can stimulate the sleep centres directly.

      • Rational decision-making. Studies show that sleep deprivation can affect general judgement and decision-making abilities, and that people who are sleep-deprived have difficulty in responding to rapidly changing situations. The real-life consequences can be grave. Fatigue is now known to have been a contributory factor in many international disasters such as the nuclear explosion at Chernobyl, the Exxon Valdez oil spill and the Challenger shuttle explosion.

      Apart from these common short-term effects of sleep deprivation, there are also long-term consequences. American research suggests that long-term sleep deprivation (defined as interrupted sleep over a period of about a year) may be linked with obesity. Studies carried out at Colombia University have shown that 73 per cent of people who sleep only 2-4 hours a night are more likely to be obese than those who sleep for seven hours. The reason is unclear but it may be because chemicals that play a key role in appetite and weight gain are released during sleep. Other long-term consequences include extreme anxiety, depression, specific sleep-related disorders and even psychosis.

      Key turning points in sleep research

      Progress in sleep studies changed significantly when it was found that the brain’s activity could be measured objectively. Here is a summary of the main findings that led to this discovery.

      • In the 19th century, British researcher Richard Caton measured the brain’s electrical activity by placing sensors on to the scalp’s surface. He noted that the activity was not constant but increases and decreases over time.

      • In the late 1920s, German psychiatrist Hans Berger measured brain activity in the belief that it would help him to calculate psychical energy. Largely discredited, he tragically committed suicide. However, his work on measuring the electrical activity of the brain was pivotal in the development of sleep research.

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       REM and dreams

       The discovery in the late 1950s/early 1960s of the connection between REM sleep (see page 20) and dreaming was one of the most exciting in sleep science because it proved without doubt that the brain was active during sleep. The findings marked the beginning of a new impetus in sleep research which lasted through the 1960s, when psychedelia was much in vogue. By the 1970s, interest had declined.

      • In 1939, while working at Chicago University, Nathaniel Kleitman – often called ‘the father of sleep’ – published the first major book on sleep, Sleep and Wakefulness (1939). The generally held view of the scientific and medical establishment was that sleep is a passive condition. Kleitman was one of the few people in the world working on sleep at the time.

      • In 1953, PhD student Eugene Aserinsky, while working with Kleitman, noted that the eyes move rapidly during sleep, eventually leading to the name of this state as Rapid Eye Movement (REM) sleep. Around this time, William (Bill) Dement joined them and all three were involved in the discovery that subjects awoken out of REM sleep often report dreaming – a turning point in knowing, as opposed to inferring, what goes on in the mind.

      How sleep works

       Sleep is a highly complicated but ordered process that is controlled by special wakefulness and sleep centres in the brain that work in tandem with hormones and our own internal body clock. The main players in this fascinating process are described below.

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       24-hour cycle

       Most living organisms, plants and animals, live according to a 24-hour cycle that is dominated by light and darkness. Even death can be part of this cycle, with cardiac arrests and strokes occurring mainly between 6 a.m. and 12 noon – perhaps because this is when blood tends to clot most.

      Clocks, cycles and rhythms

      We are all governed by a 24-hour cycle called a ‘circadian rhythm’, taken from the Latin words circa, meaning ‘around’, and die, meaning ‘day’. Circadian rhythms underpin everything, from hormone production to when we feel like getting up or going to bed. Our body temperature has a 24-hour rhythm too; minimum body temperature usually occurs around 4 a.m., maximum body temperature around 10-11 p.m. Sleep also roughly follows a 24-hour rhythm.

      For most of us, a typical cycle means falling asleep between around 11 p.m. and midnight, and waking up between 6 a.m. and 8 a.m., indicating that we are biologically programmed to be able to fall asleep and wake up at around those times. However, not

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