Lack of Sleep = Cold?

Studies show that poor sleep and susceptibility to colds go hand in hand. In a recent study, scientists followed 153 men and woman for two weeks, keeping track of their duration and quality of sleep. Then, during a five-day period, they exposed the subjects to cold viruses. It turned out that those who slept an average of fewer than seven hours a night were three times as likely to get sick than those who averaged at least eight hours.
It seems that sleep and immunity are tightly linked. Studies have shown that mammals that produce greater levels of white cells also require the most sleep. And researchers have shown that species that sleep more have more resistance against pathogens.