BACKGROUND: The prevalence of metabolic syndrome is increasing worldwide, and previous studies have shown that inadequate sleep duration and skipping breakfast may be related to metabolic syndrome. Therefore, we investigated the effects of inadequate sleep and skipping breakfast on metabolic syndrome using data from the Korea National Health and Nutrition Examination Survey (KNHANES) IV & V reports (2007-2009 and 2010-2012, respectively). METHODS: The sample included 12,999 subjects who participated in the KNHANES IV & V. Sleep duration and breakfast eating were self-reported, and metabolic syndrome was defined according to the modified National Cholesterol Education Program Adult Treatment Panel III guidelines. Subjects were divided into 12 groups according to breakfast eating and sleep duration patterns, and multiple logistic regression analyses adjusted for age, sex, household income, education level, smoking status, alcohol drinking, physical activity, and total daily energy intake were conducted. RESULTS: In subjects under 50 years of age, sleeping less than 6 hours was significantly associated with increased metabolic syndrome except among those who ate breakfast on only 1 of the past 2 days. In subjects over 50 years of age, sleeping less than 6 hours was significantly associated with a decreased risk of metabolic syndrome among those who ate breakfast on both days. CONCLUSION: In conclusion, significant associations between metabolic syndrome and sleep duration were identified, and these associations differed according to age group.