Astudy from the University of Washington, in Seattle, tested the relationship of immune system functioning to lack of adequate sleep. To rule out genetic factors, which experts say account for 31 to 55 percent of individual sleep patterns, researchers tested blood samples from 11 pairs of adult identical twins (genetic matches) with differing sleep habits. They found that the immune system was depressed in the twin that slept less.

According to the Centers for Disease Control and Prevention, Americans are sleeping 1.5 to two hours less than they did 100 years ago, and more than 30 percent of working people average fewer than six hours a night.

Dr. Nathanial Watson, lead author and co-director of the university’s Sleep Medicine Center at Harborview Medical Center, observes, “Seven or more hours of sleep is recommended for optimal health.”