Morningness-Eveningness (ME) can be defined by the difference in individual diurnal preference observed from general behavioral patterns including sleep habits. The Horne & Östberg Morningness-Eveningness Questionnaire (MEQ) has been used for classifying ME types. We examined the reliability of a Korean version of the MEQ (Korean MEQ) and verified its validity by comparing responses on the Korean MEQ to objectively-recorded sleep-wake rhythms. After translating and back translating the MEQ from English into Korean, we examined the internal consistency of 19 items of the Korean MEQ in 91 subjects, and the test-retest reliability in 21 subjects who took the Korean MEQ twice, 4 weeks apart. The Korean MEQ was then administered to 1022 young adult subjects. A subset of 46 morning, neither, and evening type subjects took part in a validation study in which their rest-activity timing was collected by actigraphy for 7 days. Cosinor analyses on these data were done to obtain the acrophase and amplitude of the sleep-wake rhythm. Cronbach's alpha of the total scores from the Korean MEQ was 0.77, and the test-retest reliability intra-class correlation coefficient was 0.90 (p < 0.0001). There was a significant negative correlation between Korean MEQ score and reported sleep-wake timing among the entire cohort (p < 0.0001). There was a significant difference in bedtime and wake time (on both work and free days), and in the mean sleep-wake rhythm acrophase, between ME types (p < 0.01). In this study, the validity of the Korean MEQ was verified by illustrating the difference in acrophases of the sleep-wake rhythm between the ME types in young adults.