Background: Limited research has examined the association between sedentary behavior, sleep duration, and daytime sleepiness. Therefore, the purpose of this study was to examine these associations, and see if these associations were independent of physical activity (which is known to influence nighttime sleep and daytime tiredness). Methods: Data from 2005 to 2006 National Health and Nutrition Examination Survey were used. Sedentary behavior was measured through accelerometry with sleep-parameters measured through questionnaire. Results: After adjustments, and for every 60 min increase in sedentary behavior, participants were 16% more likely to almost always feel unrested during the day and 22% more likely to almost always feel overly sleepy during the day. Sedentary behavior was not associated with sleep duration at night (β = −0.0002; 95% confidence interval: −0.0008 to 0.0003; P = 0.40). Conclusion: To reduce perceptions of tiredness during the day, a sensible strategy may be to increase the frequency of sedentary breaks.