Examine the association between sleep duration and suicidal ideation in Korean adults.Cross-sectional survey.Data obtained by the Korea National Health and Nutrition Examination Survey IV (2007-2009) using a rolling sampling design involving a complex, stratified, multistage, and probability-cluster survey of civilian non-institutionalized Korean residents.A total of 15,236 subjects (6,638 males and 8,598 females) ≥ 19 years old.The weighted prevalence of self-reported short sleep duration (≤ 5 h/day) was 11.7% in males and 15% in females, and of long sleep duration (≥ 9 h/day) was 6.7% in males and 8.9% in females. A U-shaped relationship existed, with both short and long sleep durations associated with a higher suicidal ideation risk. Multiple logistic regression analysis was used to analyze the relationship between sleep duration and suicidal ideation, adjusting for sociodemographic factors, health behavior, and health status. After controlling for covariates, people with short sleep were 38.1% more likely to have suicidal ideation (OR = 1.381, 95% CI 1.156-1.650) than people with sleep duration of 7 h/day. Suicidal ideation was 1.196 times higher (95% CI: 0.950-1.507) in long-sleeping people than people sleeping 7 h/day, although statistically not significant. Inclusion of depressive mood (a potential confounder) in multiple logistic regression models attenuated but did not eliminate the sleep duration/suicidal ideation association.Sleep duration and suicidal ideation were assessed only by self-report.The sleep duration/suicidal ideation relationship is U-shaped in the Korean adult population. Self-reported habitual sleep duration may be a useful behavioral indicator for both individual and societal suicidal ideation risk.