Counting Sheep: The Science and Pleasures of Sleep and Dreams. Paul Martin

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СКАЧАТЬ temperature gradient across their body. The hands and feet are normally a degree or two cooler than core body temperature, but the temperature difference dwindles to nothing as sleep approaches.

      A further demonstration of the linkage between warm appendages and the onset of sleep came from a study of people suffering from a disorder known as vasospastic syndrome. This condition is caused by faults in the physiological mechanisms controlling the peripheral blood vessels, which become less able to dilate. One of the main symptoms is cold hands and feet. As predicted, the cold-toed victims of vasospastic syndrome took longer than normal to fall asleep at night.

      The importance of a declining body temperature means that artificial heat sources like electric blankets can disturb sleep. An electric blanket operating between the early hours of the morning and waking will typically increase your core body temperature by about 0.2 degrees Celsius. Even this small increase in body temperature is enough to disrupt sleep.

      The sleep-inducing effect of a falling body temperature helps to explain why vigorous physical exercise, which raises body temperature, is not a good idea just before going to bed. It also reminds us why it is inadvisable to eat a large meal shortly before bedtime. The digestive processes that follow a large meal evoke a rise in metabolic rate, which in turn raises body temperature. In an ideal world, a large evening meal would be eaten at least three hours before bedtime. However, this helpful advice is of little use to the many people who work long hours and face long journeys to get home afterwards. They may have barely enough time to prepare and eat an evening meal before going to bed – another example of how lifestyles can conflict with good sleep.

      One popular notion that reportedly fails to stand up to scientific scrutiny is that we fall asleep faster after orgasm. A group of enterprising researchers conducted an experiment in which they monitored the sleep of men and women under three different conditions: after the subjects had masturbated to orgasm, after they had masturbated without orgasm, and after they had simply read some nonsexual material. Recordings of their subsequent sleep yielded no evidence that masturbation, with or without orgasm, affected any aspects of sleep, implying that post-coital sleepiness has nothing to do with the attainment of orgasm. (You may find this hard to believe.) This is clearly an area crying out for more research.

The sleep cycle

      Sleep flooded over him like a dark water.

      Jorge Luis Borges, Labyrinths (1964)

      Two broadly different states are conventionally bracketed together under the general heading of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further subdivided into four different stages, based on their characteristic EEG patterns. Each sleep stage has its own distinctive pattern of brain activity. The various stages and types of sleep alternate cyclically throughout a night’s sleep.

      As you become sleepier, your EEG pattern changes. If you are tired enough, this can happen even when you are walking around and supposedly wide awake. The pre-sleep state of quiet restfulness is heralded by the appearance of brain waves of lower frequency and higher voltage, called alpha waves. If you are very sleepy, but still awake, your alpha waves will be accompanied by slow, rolling movements of your eyes. Nearly there. Then you are asleep.

      The initial phase of sleep, which has the prosaic name of stage 1, typically lasts only a few minutes. Your muscles start to relax. If you are trying to sleep in a sitting position, the relaxation of your neck muscles will allow your head to slump forward, briefly waking you; your head straightens, you nod off again, and so on. hat is why you ‘nod off’. You can easily be roused into wakefulness from stage 1 sleep. If someone does wake you during stage 1 sleep you may be aware that you have been asleep, or you may be equally convinced that you have been awake the whole time. Stage 1 sleep is accompanied by a further slowing of the brain-wave patterns.

      The next phase is known, predictably, as stage 2 sleep. This is signalled by the appearance on the EEG of two specific brain-wave patterns called K complexes and sleep spindles. The K complex is a single, strong wave that lasts less than a second. The sleep spindle is a brief burst of waves lasting less than a second. A sleep spindle on an EEG trace looks like a spindle moving along a loom, hence its name. During stage 2 sleep your eyes are still and your muscles are relaxed. You are less easily awoken by stimuli and you appear to an observer to be sound asleep. Altogether, stage 2 occupies about 45–50 per cent of a night’s sleep.

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