A U-shape Relationship Was Found with Re- Gard to Sleep Duration and Outcome Studies Which Showed That Both Short and Long Sleep

177 A U-SHAPE RELATIONSHIP WAS FOUND WITH REGARD TO SLEEP DURATION AND OUTCOME STUDIES WHICH SHOWED THAT BOTH SHORT AND LONG SLEEP durations were associated with the risk of all-cause death among Caucasians,1-3 Japanese,4,5 and Taiwanese.6 However, the association between sleep duration and cardiovascular disease (CVD) risk was inconsistent: a U-shape relationship was found among Chinese in Singapore7 and white women,8 but other studies only showed an inverse relationship.9 Short sleep duration has been linked to increased obesity and weight gain,10-12 diabetes,13 and hypertension14,15; and this association has been attributed to the relationship between sleep duration and all-cause death and CVD risk. However, the association between long sleep duration and CVD risk remains unexplained. Ethnic difference for the risk of sleep duration and sleep quality, such as insomnia severity, has been reported. Ethnic and geographic variation in self-reported sleep duration has been noted.16,17 A nationwide survey of 32,749 adults aged 18 years or older in the United States showed that compared with Caucasians, African Americans and other non-Hispanics, including Asian Americans, were more likely to be short or long sleepers, and the relative risks ranged from 1.26 to 1.62.16 In a population-based survey of 3,158 adults, the prevalence of reduced habitual sleep time was higher in African Americans than Caucasians (18.7% vs. 7.4%).17 However, African Americans were less likely to have sleep complaints among a crosssectional study based on 13,563 adults aged 47 to 69 years.18 Evidence regarding the prevalence and the risk of sleep duration among ethnic Chinese is relatively rare.6,19 Furthermore, the issues on association between sleep duration and the risk of all-cause death and CVD events need to