Insomnia, a prevalent sleep disorder, affects 6-10% of the global population and causes significant functional impairments. Chronotype, individual diurnal preferences, influences health outcomes and varies by sex. However, evidence on sex-specific chronotype distribution and its relationship with insomnia in Asian populations remains limited. This study aimed to examine how chronotype is distributed differently between men and women, and to explore its relationship with insomnia symptoms using nationwide population-based data from Korea. We analyzed data from 2,838 participants aged 20-59 years from the baseline assessment phase of the Circannual Change in Headache and Sleep Study. The Morningness-Eveningness Questionnaire was administered to assess chronotype, while insomnia symptoms were estimated using the Insomnia Severity Index. Covariates included sociodemographic factors, depression, employment, smoking and alcohol consumption, body mass index, and average sleep duration. Women reported a higher prevalence of insomnia symptoms than men (14.5% vs. 11.7%, = 0.027) and were more likely to be evening types (26.4% vs. 20.6%, < 0.001), whereas men were more likely to be morning types (11.2% vs. 7.0%, < 0.001). These sex-specific differences in chronotype distribution were consistently observed across individuals in their 30s to 60s. Across chronotype groups, the prevalence of insomnia symptoms was lowest in morning types (4.7%) and highest in evening types (18.4%, < 0.001). In adjusted analyses, morning type was significantly associated with reduced odds of insomnia (OR = 0.47, 95% CI = 0.22-0.98), whereas evening type was not significantly associated with insomnia risk. Stratified analyses suggested a stronger protective effect of morning type among women, although the chronotype-by-sex interaction term did not reach statistical significance. This nationwide Korean study found that women tended to be evening type, whereas men were more frequently morning type. Morning type was also linked to a lower risk of insomnia symptoms in the overall population, with the association being more pronounced in women. Together, these findings confirm sex-specific differences in chronotype distribution in Korea and demonstrate the independent protective effect of morning type against insomnia. p p p p
