A study suggests that people who sleep less than seven hours a night are three times more likely to get sick after being exposed to a cold virus than those who are more well-rested.

Volunteers who spent less time in bed, or who spent their time in bed tossing and turning instead of sleeping, were much more likely to catch a cold when viruses were dripped into their noses. People who slept longer and more soundly resisted infection better.

Psychology professor Sheldon Cohen and his colleagues at Carnegie Mellon University in Pittsburgh interviewed 153 healthy men and women ages 21 to 55 every day for two weeks. They asked how long they slept, how much they tossed and turned before dropping off, and whether they felt rested in the morning, in addition to other factors.

After that, the volunteers were quarantined for six days in a hotel and given cold-virus-containing nose drops at a dose about 125 times the amount that it takes to infect cells in a laboratory.

Researchers found that 88 percent of people became infected with the virus (as measured by cold virus in their nose or antibodies in their blood), but not all of those people actually got sick. About 43 percent of the volunteers had signs of infection plus cold symptoms, such as a stuffy nose, cough, and sore throat.

Those who reported fewer than seven hours of sleep on average were 2.94 times more likely to develop sneezing, sore throat and other cold symptoms than those who reported getting eight or more hours of sleep each night.

The researchers don't know why less sleep is associated with a greater susceptibility to colds. They believe that sleep deprivation or sleep disturbances may affect immune system signalling chemicals called cytokines or histamines.