In the field of sleep medicine there has been a great deal of controversy as to whether or not it is unhealthy to sleep for too long. Numerous studies have been published showing short sleep duration to be associated with hypertension, diabetes, and premature death. However, in several studies, long sleep as defined by sleeping greater than eight hours has also been associated with premature death. This has become a source of debate, as its basis has been hard to explain. In a study published this month in the American Journal of Epidemiology titled Sleep Duration and Survival Percentiles Across Categories of Physical Activity, I believe we finally have an answer.
This was an extensive study performed in Sweden during a 15-year period. Over 75,000 men and women were followed regarding death. They were divided based on their sleep duration and level of activity. Those who slept less than six hours a night were defined as short sleepers and those who slept greater than eight hours were defined as long sleepers. Individuals who slept around seven hours were in the majority and defined as normal. A low level of activity was defined as those who walked or bicycled less than 20 minutes per day, had sedentary work/occupations, and spent most of their leisure time doing such things as watching television. Higher levels of activity were defined by increased walking, biking, non-sedentary occupations, and exercising regularly.
The researchers found that short sleepers, regardless of physical activity, had a much higher incidence of death when followed over 15 years, than did normal sleepers. However, in the long sleeper group, those that slept greater than eight hours, physical activity was the most important factor in predicting early mortality. In those that slept greater than eight hours, shorter survival was seen only in those with the lowest levels of physical activity.
The major causes of death in all groups were cardiovascular disease and cancer. In long sleepers, a lack of physical activity predicted cardiovascular causes of death such as heart attack and stroke, but not cancer.
What is the take home message of this important study? First, it once again reveals that those of us who get less than six hours of sleep are at increased risk for early death even if we are active and exercise regularly. Second, it finally seems to answer the age-old question of “can you sleep too much?” It would appear that the answer is no, not if you maintain a reasonable level of physical activity.