Study sample

To evaluate the relationship between sleep duration and RA, the present study analysed data from the fourth (2007–2009), fifth (2010–2012) and sixth (2013) KNHANES assessments performed by the Korean Ministry of Health and Welfare. The KNHANES is a cross-sectional survey based on stratified multistage probability sampling units of Korean households that targets members of the civilian non-institutionalised South Korean population who are 1-year of age or older. The samples were determined by the household registries of the 2005 National Census Registry.

The total target population initially consisted of 24 871, 25 534 and 8018 participants who completed the 2007–2009, 2010–2012 and 2013 KNHANES assessments, which had average response rates of 78.4%, 80.8% and 79.3%, respectively. The information from 14 305 individuals aged 1–18 years old were excluded from the present analyses while the information of 44 118 individuals aged 19 years and older were included. Additionally, the present study excluded 6036 individuals with missing data regarding age, occupation, income and/or marriage status and 103 individuals with missing data regarding smoking, drinking, perceived stress, exercise, sleep duration, RA, hypertension, and/or dyslipidaemia. Thus, a total of 37 979 individuals were selected for the final analyses in the present study. Since all KNHANES data are available publicly, this study did not require approval from an institutional review board.