Exposure to light was recorded from 10 healthy elderly adults and 13 age-matched subjects with senile dementia of the Alzheimer's type (SDAT). Data were recorded in the home, for an average of 5 days, while subjects continued their normal daily activities. Subjects were exposed to remarkably small intervals of illumination exceeding 2000 lux. Subjects with SDAT were exposed to bright light significantly less than healthy controls (0.5 vs. 1.0 hr). Whether or not they had SDAT, males were exposed to illumination exceeding 2000 lux significantly more than were females. Healthy elderly received about two-thirds the duration of bright light received by healthy younger subjects. These findings suggest an association between decreased exposure to bright light and the declines in sleep quality which typically accompany normal and pathological aging.