Once again, science supports what your grandmother told you: A good night's sleep helps your body fight a cold.

　　People who averaged fewer than seven hours of sleep per night in the weeks before being exposed to the cold virus were nearly three times as likely to get sick as those who averaged eight hours or more, a new study found.

　　Researchers used frequent telephone interviews to track the sleep habits of more than 150 men and women aged 21 to 55 over the course of a few weeks. Then they exposed the subjects to the virus, quarantined them for five days and kept track of who got sick.

　　Besides sleeping more, sleeping better also seemed to help the body fight illness: Patients who fared better on a measure known as 'sleep efficiency' - the percentage of time in bed that you're actually sleeping - were also less likely to get sick.

　　The results held true even after researchers adjusted for variables such as body-mass index, age, sex, smoking and pre-existing antibodies to the virus.

　　Like your grandmother, the researchers aren't exactly sure why sleeping better makes you less likely to develop a cold. But they do take a stab at the answer: 'Sleep disturbance influences the regulation of proinflammatory cytokines, histamines, and other symptom mediators that are released in response to infection.' In plain English, maybe tossing and turning when you're infected with the cold virus contributes to the symptoms that define a cold.

　　The researchers were based at Carnegie Mellon, the University of Pittsburgh and the University of Virginia, and the study was funded by the National Institutes of Health.

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