Lack of sleep is the most important factor in determining whether someone will catch a cold, according to an American study.

People who sleep for under six hours a night are four times more likely to succumb to the virus than those who manage at least seven, it was found.

Sleep was found to be a more important factor than stress, smoking, education, or income, all of which are known to impact on overall health.

"It didn't matter how old people were, their stress levels, their race, education or income. It didn't matter if they were a smoker. With all those things taken into account, statistically sleep still carried the day," said Dr Aric Prather, lead author of the study and assistant professor of psychiatry at the University of California.

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"It goes beyond feeling groggy or irritable. Not getting sleep fundamentally affects your physical health."

More than a third of people in Britain sleep for less than six hours a night, according to the Sleep Council.

Public Health England is due to start a health campaign to encourage the middle-aged to get more sleep.

Dr Prather's previous studies have shown that those who sleep fewer hours gain less protection from a vaccine against illness.

In the latest study, published in the journal Sleep, researchers monitored 164 volunteers for two months, noting their sleeping patterns, alcohol and cigarette use and stress levels.

They were then administered the cold virus and tested daily over the course of a week. Participants who had slept less than six hours a night the week before were 4.2 times more likely to catch the cold compared with those who got more than seven hours of sleep.

Those who slept less than five hours were 4.5 times more likely to become ill.

A recent study by scientists at Surrey University found that people who get less than six hours of sleep have changes to 700 different genes. Prof Derk-Jan Dijk, who leads a sleep and physiology unit at Surrey, said sleep was a fundamental pillar of health.

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