Do you feel exhausted after lunch? Then, like nine out of ten of your colleagues, you are sleep deprived - and the consequences could be catastrophic. Ayala Ochert talks to sleep researcher William Dement

For most people the name Exxon Valdez instantly conjures up images of the ill-fated oil tanker that so famously spilled its load into Prince William Sound in Alaska, causing appalling damage to the environment and killing thousands of birds and seals. Those who followed the incident closely might recall that it was the captain's drinking that caused the disaster.

But that recollection would be wrong. Despite the deluge of press attention on the spill and its aftermath, the most important message was somehow lost, says William Dement, director of Stanford University's centre for sleep disorders. "Because of the media emphasis on the captain's drinking, I was never able to exploit the impact of the catastrophe for getting knowledge about sleep deprivation into the mainstream," he laments.

True, the captain of the Exxon Valdez had been drinking. But when the accident happened he was below deck and had left the vessel in the charge of the third mate. The third mate had not been drinking - but he might as well have been. Having slept just six hours in the previous 48, he was seriously sleep deprived. The result was one of the worst environmental disasters of recent years.

ÖйúAƬ

For the rest of us, the effects of sleep deprivation may be no less devastating. "No matter how bad we think it is, the truth is worse," claims Dement. Just a few nights of poor sleep can make us nod off at the wheel for long enough to cause an accident - in fact, a quarter of all fatal car accidents have been blamed on lack of sleep. "I think the big change will come when people start to equate sleep deprivation with alcohol consumption," Dement says. "If you don't think people should drive drunk, then they shouldn't drive when they're sleep deprived."

He is hoping that new research will help bring about that shift in attitude. In the study, people who had slept four hours a night for six nights were compared with people who had been drinking. The tests showed that their judgement was as impaired as that of someone who was well over the legal limit.

ÖйúAƬ

Dement was not always so aware of the dangers of sleep deprivation. In fact he put himself through years of it when, as a young researcher, he spent hundreds of nights trying to stay awake to watch the brain waves of his sleeping subjects.

His interest in sleep began in 1950 while he was a medical student at the University of Chicago. After attending a lecture by the late Nathaniel Kleitman, one of a handful of sleep experts at that time, Dement was hooked and soon after began working for him. Dement was fascinated by Freud and psychoanalysis and hoped that researching sleep might lead to an understanding of the physical workings of what Freud called the "unconscious mind".

While working for Kleitman, Dement witnessed the discovery of rapid-eye movement, so-called REM, sleep. It became clear that dreaming occurred during these episodes of vigorous brain activity. The breakthrough hit the headlines and the young researcher became something of a celebrity. "For a kid from Walla Walla and a sophomore medical student, it was heady stuff," he recalls. Now Dement is considered the world's leading authority on sleep and sleep disorders.

Modern dream research tends towards the view that dreams are nothing more than our mind's interpretation of random nerve signals caused by the state of being in REM sleep, but Dement disagrees. "There's just too much of a relationship between dream content and important events in life (to call it random)."He frequently recalls a dream he had in 1964 that changed the course of his life. A heavy smoker at the time, he dreamt that he was in his doctor's office, looking at an X-ray film of his own chest, as he was diagnosed with lung cancer. The experience was so real and had such an impact that the next morning he resolved never to smoke again.

Although he found dream research fascinating, Dement also knew that very little was known about sleep disorders or even about normal sleep, so he switched focus. The news from his own research is not good. As many as 90 per cent of people are sleep deprived and about 60 per cent suffer from some sort of sleep disorder. Viewing those statistics, you can begin to understand what Dement means when he refers to sleep disorders and sleep deprivation as "collectively the biggest public health problem in America".

Most people who think they are not sleep deprived actually are, says Dement. If you are getting the right amount of sleep every night, you should not be tired during the day. If you tried to take a nap, you would find it impossible. "When people use the words tired, exhausted, fatigued, not motivated, low energy - that is all sleep deprivation," he says.

Daytime sleepiness is a direct consequence of what researchers call "sleep debt". Every hour of lost sleep adds to this debt. Because few people know how to manage their sleep, most people are walking around - or, more worryingly, driving - with 20 to 30 hours of sleep debt.

ÖйúAƬ

We may also underestimate how much sleep we need. In a recent experiment, college students were allowed eight hours in bed every night for 25 nights. Since most took a while to fall asleep but had a daily sleep requirement of eight hours or more, by the end of the experiment they had added about ten hours to their sleep debt. Tests of their mental and physical performance showed that they were significantly impaired.

ÖйúAƬ

The reason we are unaware of our burden of sleep debt lies in the nature of sleep cycles, explains Dement. There are two opposing forces - one that tries to put us to sleep and another that strives to keep us awake. While our sleep drive grows incrementally hour by hour, making us progressively sleepier, our biological clocks periodically try to wake us. There are two peaks of "clock-dependent alerting", one in the morning and another stronger peak in the evening. "Your experience of sleepiness is that it is like smoke - you can just blow it away. You don't have the sense that it is building up," says Dement. But that is exactly what happens.

While most people would benefit from getting extra sleep, for some people it is the quality rather than the quantity that counts. About 24 per cent of men and 9 per cent of women suffer from obstructive sleep apnoea, a disorder in which their breathing stops and they wake up, perhaps hundreds of times each night. The awakenings are so brief, sufferers are usually oblivious to them, except that they struggle against monumental tiredness throughout the day. The disorder also contributes to heart disease. "I expect it to account for at least one quarter of the problem, maybe 50 per cent," says Dement, pointing out that apnoea is eminently treatable.

Dement sees his most important mission as educating people about sleep debt. "I put sleep first," he says. "If sleep professors don't put sleep first, who will?" And he seems to be having an impact on at least one group of people - his students. During his undergraduate course "Sleep and Dreams" he encourages his students to cut their sleep debt. Most report remarkable results after a few weeks. They stop falling asleep during lectures, concentrate better and feel more motivated to study.

The father of one student, a minister, was so impressed he recently delivered a sermon about Dement. "He likened me to Jesus Christ. He told his congregation that the students are now 'living according to the higher wisdom of Professor Dement'," he says, clearly delighted with the comparison.

The Promise of Sleep (William Dement and Christopher Vaughan) is published by Macmillan, Pounds 13.59.

PAY OFF YOUR SLEEP DEBT

Do you feel tired in the afternoons? If so, monitor your sleepiness at the same time each day for a week. If you get progressively more tired over the week, you need more sleep.

Most people need about eight hours every night just to maintain their current sleep debt. Start "paying off" your debt by adding extra hours of sleep to your daily requirement. Most people have about 30 hours of sleep debt to pay it off.

You will know when you have made your final payment because you will wake up in the morning without an alarm and feel wide awake all day long. "When people significantly lower their sleep debt, they experience a miracle," Stanford University's William Dement claims.

ÖйúAƬ

Stanford University's Centre of Excellence for the Diagnosis and Treatment of Sleep Disorders: National Sleep Foundation: