The absence of daily robust light-dark exposure patterns may contribute to sleep disturbances in persons with Alzheimer’s disease and related dementias (ADRD). Personal light-dark and activity-rest patterns were measured for healthy older adults and for persons with ADRD. Persons with ADRD experienced lower light levels, exhibited lower activity levels, and had greater levels of circadian disruption than healthy older adults during winter. Seasonal differences were observed for persons with ADRD; lower levels of light exposure and greater levels of circadian disruption were seen during the winter than during the summer, although activity levels did not differ for the two seasons.