Instability in daily life and depression: The impact of sleep variance between weekday and weekend in South Korean workers.

South Korean workers have problems getting optimal sleep because they work excessively, and this affects their sleep duration and quality. To compensate for their lack of sleep on weekdays, South Korean workers have a different lifestyle on weekends, which is reflected by the terms 'Hot Friday' and 'Monday Sickness'. However, an irregular lifestyle may be a risk factor for poor health even if insufficient sleep during weekdays is supplemented by sleep on the weekend. The objective of this study was to analyse the association between depressive mood and sleep duration variance on weekdays and weekends among South Korean workers. Data from the 2016 Korea National Health and Nutrition Examination Surveys were used. Linear regression analysis was performed to determine the association between variance in sleep duration and the Patient Health Questionnaire-9 (PHQ-9) score. Additional sub-group analyses were performed to evaluate associations between variance in sleep duration and sex, marital status and perceived health status. The average sleep duration was 419.44 min on weekdays and 463.29 min on weekends. The average sleep duration on weekends was 11.73% longer than on weekdays. There was a positive association between variance in sleep and PHQ-9 score (per 1% increased, β = 0.002, p = .0074), and variance in sleep duration affected health regardless of the direction of the change (positive: increased sleep or negative: decreased sleep). Further, sub-group analyses showed that variances in sleep duration are associated with increased PHQ-9 scores in females and in individuals who were single or who had problems in their marriage. Our findings suggest that there is a relationship between variance in sleep duration and depressive symptoms among South Korean workers. Thus, healthcare professionals should suggest alternatives for optimal sleep duration and sleep methods to ensure the psychiatric health of workers.