Study objective: Sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality. Whether this association is causal and what pathways explain this association are unknown. We sought to identify factors that could potentially explain the association between long sleep and mortality.

Design: Cross-sectional epidemiologic survey.

Participants: Middle-aged women (n = 60,028) participating in the Nurses Health Study II who reported a habitual sleep duration of 7 hours or more.

Results: Multiple sclerosis (odds ratio [OR] = 3.7, 95% confidence interval [3.0-4.5]), antidepressant use (OR = 3.1, [2.9-3.3]), benzodiazepine use (OR = 3.0 [2.6-3.3]), and systemic lupus erythematosus (OR = 2.9, [2.3-3.6]) were the factors most strongly associated with prolonged sleep. Combining these data with prevalence information and a range of plausible associations with mortality, the confounding rate ratio was estimated. This parameter is the ratio of the unadjusted long sleep-mortality rate ratio to the rate ratio adjusted for the factor and measures the extent that the factor can alter the long sleep-mortality association, either through confounding or as a causal intermediate. Based on this parameter, psychiatric and socioeconomic factors have the greatest potential to influence the long sleep-mortality relationship. Assuming each factor doubles mortality risk, the confounding rate ratios for depression, antidepressant use, and unemployment were 1.10, 1.18, and 1.12. Lesser influential factors were benzodiazepine use, poverty, low societal status, sedentary lifestyle, and obesity.

Conclusion: Depression and low socioeconomic status are strong candidates for producing the statistical association between long sleep and mortality, either as confounders or as causal intermediates. Future causal research on the effects of long sleep should include a detailed assessment of psychiatric disease and socioeconomic status.