Purpose: Korean adolescents have severe nighttime sleep deprivation and daytime sleepiness because of their competitive educational environment. However, daytime sleep patterns and sleepiness have never been studied using age-specific methods, such as the pediatric daytime sleepiness scale (PDSS). We surveyed the daytime sleepiness of Korean adolescents using a Korean translation of the PDSS. Methods: We distributed the 27-item questionnaire, including the PDSS and questions related to sleep pattern, sleep satisfaction, and emotional state, to 3,370 students in grades 5-12. Results: The amount of nighttime sleep decreased significantly with increasing age. During weekday nights, $5-6^{th}$ graders slept for $7.95{pm}1.05h$, $7-9^{th}$ graders for $7.57{pm}1.05h$, and $10-12^{th}$ graders for $5.78{pm}1.13h$. However, the total amounts of combined daytime and nighttime sleep during weekdays were somewhat greater, $8.15{pm}1.12h$ for $5-6^{th}$ graders, $8.17{pm}1.20h$ for $7-9^{th}$ graders, and $6.87{pm}1.40h$ for $10-12^{th}$ graders. PDSS scores increased with age, $11.89{pm}5.56$ for $5-6^{th}$ graders, $16.57{pm}5.57$ for $7-9^{th}$ graders, and $17.71{pm}5.24$ for $10-12^{th}$ graders. Higher PDSS scores were positively correlated with poor school performance and emotional instability. Conclusion: Korean teenagers sleep to an unusual extent during the day because of nighttime sleep deprivation. This negatively affects school performance and emotional stability. A Korean translation of the PDSS was effective in evaluating the severity of daytime sleepiness and assessing the emotional state and school performance of Korean teenagers.