The relationship between good health and sleep duration has been clearly established in epidemiological studies, although exactly which factors are involved in altering sleep patterns remains to be fully determined. This, at least in part, reflects the complex and multifactorial mechanisms involved in sleep. Subjective sleep duration has slowly declined over the last 50 years, and suboptimal sleep duration and poor sleep quality are becoming widespread in modern society. Suboptimal sleep duration has a strong association with mortality and morbidity; hence there is a need to more clearly understand the mechanisms involved in regulating sleep duration and patterns and to identify high-risk individuals who are in greatest need of preventive strategies. This review focuses on epidemiological evidence of the relationship between sleep duration and various risk factors, including socioeconomic, behavioural and metabolic aspects.