While body functions slow down to repair the wear and tear of a busy day, a person is likely to toss and turn up to 70 times a night, sweat almost a quarter of a litre of water and "grow" about 2cm as the spine elongates.
The brain embarks on a series of well-ordered cycles, each lasting 90 to 120 minutes and made up of five stages. Brainwaves remain muted for four of these, but come the fifth, and the peace is rudely interrupted. The brain partially emerges from its deep sleep, although consciousness does not return. Behind their lids the eyes swivel furiously. This is known as rapid eye movement and betrays the start of vivid dreaming.
Some rather drab dreams may surface, sometimes accompanied by twitching, and even walking and talking. But during REM sleep, much more dramatic, colourful and unpredictable dreaming grips the sleeper for perhaps 20 minutes at a time. Debate still rages among experts as to the purpose of dreaming. Some argue the evidence indicates only that dreams are an unusual by-product of the brain "stretching its muscles" without waking the sleeper. Others are convinced they are an integral part of the process when memories of the day are sorted and assessed.
It seems modern culture is starting to rob many people of the chance to dream. Some scientists worry that we are being overtaken by an epidemic of sleeplessness. Children, they warn, are especially at risk. A recent poll showed a third of people questioned in the UK admitted having trouble falling asleep. As many as one in four adults in the UK suffers from chronic lack of sleep.
Initially, the sleep-deprived individuals think fuzzily, have difficulty concentrating, grow irritable and become increasingly unco-ordinated. Motoring becomes potentially lethal - research at Loughborough University has implicated drowsiness in up to 20 per cent of road accidents. If a person loses an hour or more sleep each night, his or her health starts to deteriorate. The immune system becomes compromised and the person becomes increasingly prone to infection. There are suggested links to depression and other mental disorders, diabetes, obesity, strokes, cancer and heart disease.
The light bulb has led the assault on human sleep patterns in recent years, usurping the role that sunlight once had on determining when a person should turn in for the night.
The polar bear can rest easy. Sleep problems appear to be a uniquely human peril.
STEVE FARRAR Weblinks Loughborough Sleep Research Centre: http:www.lboro.ac.ukdepartmentshugroupssleepSleeping disorders: http:www.rcpsych.ac.ukinfohelpsleepindex.htm The Selby rail crash: http:news.bbc.co.ukhienglishukenglandnewsid_17030001703959.stm