ObjectiveZZThis study examined the association between the sleep patterns and depression of middle school students in Seoul.MethodsZZ450 adolescents attending middle school (mean age 14.07 years, 40% male) responded to the survey. The participants answered the self-administered questionnaires about sleep patterns (Morningness-Eveningness Questionnaire, MEQ), sleep problems (Insomnia Severity Index, ISI ; Epworth Sleepiness Scale, ESS), and depressive symptoms (Children's Depression Inventory, CDI).ResultsZZThe depressive group represented 25.9% of the total study sample with females exhibiting higher depressive index scores than males. Although the difference in the total sleeping time between the depressive group and control group was unspecified, the ISI and ESS scores were significantly higher in the depressive group than in the control group. In addition, there was a significant positive correlation between the total ISI and CDI scores. Those who scored higher than 8 on the ISI were 2.24 times more likely to belong to the depressive group, and those who scored higher than 7 on the ESS were 2.23 times more likely to belong to the depressive group.ConclusionZZThe depressive group tended to suffer from insomnia and experience more severe daytime sleepiness than students in the control group. In addition, students suffering from more severe insomnia tended to have higher CDI scores than their counterparts. Furthermore, students suffering from insomnia and/or experiencing daytime sleepiness were at least twice as likely to develop depression than other students.J Korean Neuropsychiatr Assoc 2017;56(2):78-83 KEY WORDSZZ Depression · Sleep pattern · Adolescents · Middle school student.