Not getting enough sleep each night – anything less than six hours – could increase your chances of catching a cold, U.S. researchers have found.

Their study indicates that people who sleep six hours a night or less are four times more likely to catch a cold if exposed to the virus than people who get at least seven hours of sleep nightly. 

It has long been suspected that a good night's sleep is crucial for the body's maintenance of a strong immune system.

The findings, published in the journal Sleep, firmly support previous studies that indicated sleep-deprived people are more likely to fall victim to infectious diseases, and that they take longer to recover.

"Americans are not getting enough sleep. They know that and we know that," said lead study author Aric Prather of the University of California, San Francisco.

The study, a collaboration between UCSF, Carnegie Mellon University and the University of Pittsburgh Medical Center, is the first to utilize objective sleep measures to correlate people's usual sleep habits and their possible risk of becoming ill.

For the study, 164 volunteers between the ages of 18 and 55, both men and women, had their sleeping habits tracked and were then infected with a live cold virus, rhinovirus — the most common cause of the common cold.

They were monitored for a week to see if, and when, they would fall victim to the cold virus.

The amount of sleep the volunteers were getting proved more important than any other factor — including stress levels, age, income, education, income or whether they were smokers, the researchers report.

"The role that sleep has on the immune system is well-established, though not completely understood," noted Prather.

When otherwise healthy people are deprived of sleep, he explained, there are changes in factors such as the kinds of immune cells circulating in the bloodstream, or the kinds of chemical messages seen in cells involved in immune system communication.

Around 20 percent of American sleep less than six hours a night, according to a survey by the National Sleep Foundation.

That has led the U.S. Centers for Disease Control and Prevention to express concern about an "epidemic" of insufficient sleep, the study authors note.

Other experts said the study findings were no surprise.

"This is a confirmatory study of other studies that have been moving our thinking in this direction," said Brown University's Mary Carskadon, a professor of psychiatry and human behavior. "Short sleep has been linked to a greater risk for cancer and other more chronic conditions, and consistently shorter sleep has clearly been associated with a higher likelihood of illness."