People who get less than seven hours of shut-eye nightly are three times more likely to catch a cold than those who get eight or more hours, according to a new study. Researchers speculate that a lack of sleep may compromise immune function, making people more vulnerable to the common cold. 

"The really striking thing about this study for us is how little differences in sleep can have a big impact on your susceptibility," says Sheldon Cohen, a psychoneuroimmunologist at Carnegie Mellon University in Pittsburgh, Pa., and lead author of the study published today in the Archives of Internal Medicine

Previous research has shown that sleep deprivation may trigger changes in the immune system that could make a person more vulnerable to infection. For example, poor sleep may lead to a dip in the number of killer T-cells, which destroy viruses and bacteria, as well as to lower levels of interleukin-2, a protein that stimulates production and growth of many infection-fighting cells, including T-cells. But this is one of the first studies that links sleep deficits to increased susceptibility to the rhinovirus, the most common culprit behind, well, the common cold.

The scientists studied 153 healthy men and women between the ages of 21 and 55 brave enough (or masochistic enough) to deliberately subject themselves to the rhinovirus. (The $800 participation fee might also have helped.) Every day for two weeks, the researchers asked subjects a battery of questions on the phone about their previous night's slumber, such as the time they laid down and the quality of their rest.

Next, Cohen and his colleagues quarantined the subjects (put each one up in his or her own hotel room with instructions to stay at least several feet away from other people to limit spread of the infection) and gave them nasal drops packed with rhinovirus. The team kept track of cold symptoms such as congestion, sneezing, and runny nose, and measured the amount of mucus the participants produced (don't ask) over the next five days. To determine if the subjects had indeed been infected with the rhinovirus, the researchers screened their blood for antibodies, or immune system proteins, produced in response to the rhinovirus.

The results: Subjects who snoozed for less than seven hours were 2.94 times more likely to develop a cold than those who slept for eight or more hours.  Participants who had the best sleep efficiency (defined as the percentage of pillow time actually spent sleeping as opposed to tossing and turning) were the least likely to get sick.

"People who have sleep efficiencies between 92 and 98 percent," Cohen says, "are four times more likely to get colds than people who have 99 to 100 percent efficiency." So what's more important: sleep duration or sleep efficiency? Cohen says both are key; someone who sleeps more efficiently is also likely to sleep for longer periods.

Cohen, who has studied susceptibility to viruses for three decades, says sleep is just one of many lifestyle factors that may influence susceptibility to the common cold. Another biggie is stress, which is known to take a toll on immune systems and heighten risk to various ailments.
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