Researchers have known that too little sleep affects mental performance. Now they are finding that sleep loss affects a whole lot more, including the immune system, cardiovascular health, and even hunger regulation.

Epidemiologic studies around the world show that time spent in sleep is linked to all causes of death and to conditions that include cardiovascular risk, hypertension, diabetes, and stroke, says David Dinges, a sleep researcher at the University of Pennsylvania School of Medicine in Philadelphia. “The risk goes up when you sleep below seven hours or above eight, especially below seven,” he says.

Several large studies have been designed to look at the impact of sleep disturbance on physical health, including the Sleep Heart Health Study, a multicenter study implemented by the National Heart, Lung, and Blood Institute at the National Institutes of Health. After compiling the complete health history of more than 1,000 volunteers and obtaining basic information such as weight and blood pressure, the researchers fitted each participant with equipment that enabled the researchers to measure electrical currents in the brain, oxygen saturation in their breath, and chest and body movement.

Because different stages of sleep are associated with different electrical patterns as measured by electroencephalogram (EEG), the researchers could see who slept soundly and who had broken sleep. Eyal Shahar of the University of Minnesota, one of the Sleep Heart Health Study leaders, found that people who had severe sleep apnea, in which they stop breathing for a short period of time repeatedly during the night, were 42 percent more likely to have heart problems and 58 percent more likely to suffer a stroke than participants who did not have the disorder.

In a separate study, scientists found that patients treated for sleep apnea had a lower risk of heart attack and stroke, and were less likely to require bypass surgery than those who had untreated sleep apnea of a similar severity. Although neither of these studies shows definitively that cardiovascular problems are caused by sleep apnea or disrupted sleep, the data suggest that there is an important interaction between the two processes.