OBJECTIVES: To compare daily and hourly activity patterns according to sex and age. DESIGN: Cross-sectional, observational. SETTING: Nationally representative community sample: National Health and Nutrition Examination Survey (NHANES) 2003-04 and 2005-06. PARTICIPANTS: Individuals (n = 5,788) aged 20 and older with 4 or more valid days of monitor wear-time, no missing data on valid wear-time minutes, and covariates. MEASUREMENTS: Activity was examined as average counts per minute (CPM) during wear-time; percentage of time spent in nonsedentary activity; and time (minutes) spent in sedentary (<100 counts), light (100-759), and moderate to vigorous physical activity (MVPA (>= 760)). Analyses accounted for survey design, adjusted for covariates, and were sex specific. RESULTS: In adjusted models, men spent slightly more time (similar to 1-2%) in nonsedentary activity than women aged 20 to 34, with levels converging at age 35 to 59, although the difference was not significant. Women aged 60 and older spent significantly more time (similar to 3-4%) in nonsedentary activity than men, despite similarly achieved average CPM. With increasing age, all nonsedentary activity decreased in men; light activity remained constant in women (similar to 30%). Older men had fewer CPM at night (similar to 20), more daytime sedentary minutes (similar to 3), fewer daytime light physical activity minutes (similar to 4), and more MVPA minutes (similar to 1) until early evening than older women. CONCLUSION: Although sex differences in average CPM declined with age, differences in nonsedentary activity time emerged as men increased sedentary behavior and reduced MVPA time. Maintained levels of light-intensity activity suggest that women continue engaging in common daily activities into older age more than men. Findings may help inform the development of behavioral interventions to increase intensity and overall activity levels, particularly in older adults.