PURPOSE: South Korean adolescents have been identified as a chronic partial sleep-deprived population in several previous studies. This study was to identify the current nocturnal sleep duration by age in South Korean adolescents and to analyze the association of emotional status factors such as subjective happiness, depression and suicidal attempts, and health-risk behaviors with the nocturnal sleep duration. METHODS: The findings in this study are based on the data obtained from the 7th Korea Youth Risk Behavior Web-Based Survey (KYRBWS-VII), a cross-sectional, annual survey of the health-risk behaviors in a representative sample of South Korean middle- and high-school students aged 13-18 years, which was conducted in 2011. Out of 75,643 students from 800 schools across the nation, 75,205 students were selected by using the complex sampling design of the survey. We analyzed the relationships between the duration of nocturnal sleep and emotional status (subjective happiness, stress level, depression, and suicidal thoughts), health-risk behaviors (smoking, drinking), and sleep satisfaction. RESULTS: There was a significant relationship between the duration of sleep and the emotional status. In general, happy students had the longest nocturnal sleep duration, regardless of their grades. Self-rated stress levels and depression in adolescents were inversely proportional to nocturnal sleep duration. Health-risk behaviors such as smoking and alcohol consumptions were more common in sleep-deprived students (P<0.001). CONCLUSION: Appropriate nocturnal sleep duration is critical for emotional health as well as prevention of suicide in the adolescent population.