We examined the dose–response relationship between long sleep duration and health outcomes including mortality and the incidence of diabetes mellitus, hypertension, cardiovascular diseases, stroke, coronary heart diseases, obesity, depression and dyslipidemia. We collected data from 5,134,036 participants from 137 prospective cohort studies. For the independent variable, we categorized participants at baseline as having long sleep duration or normal sleep duration. Risk ratios (RRs) for mortality and incident health conditions during follow-up were calculated through meta-analyses of adjusted data from individual studies. Meta-regression analyses were performed to investigate the association between each outcome and specific thresholds of long sleep. Long sleep was significantly associated with mortality (RR, 1.39; 95% CI, 1.31–1.47), incident diabetes mellitus (1.26, 1.11–1.43), cardiovascular disease (1.25, 1.14–1.37), stroke (1.46, 1.26–1.69), coronary heart disease (1.24, 1.13–1.37), and obesity (1.08, 1.02–1.15). Long sleep was not significantly related to incident hypertension (1.01, 0.95–1.07). Insufficient data were available for depression and dyslipidemia. Meta-regression analyses found statistically significant linear associations between longer sleep duration and increased mortality and incident cardiovascular disease. Future studies should address whether the relationship between long sleep and health outcomes is causal and modifiable.