sleep durations reported in surveys may be difficult to differentiate from simply spending a long time in bed each day unrelated to actual sleep time. These findings question the generality of the association between long sleep times and mortality because the reports may not have reflected actual sleep. In addition, other lifestyle, health, or socioeconomic factors could be responsible. Finally, it also suggests that interventions to decrease sleep length as a means to increase life span in younger long sleepers might be misdirected. Further research is required to provide clarification regarding the possibility of an independent association between lifetime long sleep patterns and mortality.
In summary, there is a clear association of short sleep times with numerous health problems and increased risk of death. Long sleep durations are also associated with an increased risk of death but underlying causes are less apparent. However, all of these conclusions are limited by the reliance of studies on one or few brief questions related to sleep and later statistical association. More extensive sleep data from objective measures or even a study that follows well-defined short and long sleepers for several years would provide much better insight into sleep duration and health-related outcomes.
Individual Differences in Sleep Duration
The majority of research has assumed that normal adults sleep for 7-8 hours each night. Although this statement is generally true, it is also the case that each individual has a unique amount of sleep that is required to be awake and alert during the day and this amount changes across the lifespan. For example, newborns typically sleep 16 or more hours per day. Sleep time for children and adolescents declines then becomesmore stable through adulthood. In a recent study of age-related changes (31), groups of young (mean 22 years) and older (mean 68 years) healthy individuals were put in an isolated environment where they were scheduled to sleep for 12 hours at night followed by 4 hours in an afternoon nap for several days. Both groups slept more during the first 24 hours (about 12 hours for the young adults and 9.5 hours for the