The deterioration of nocturnal sleep and increase in daytime napping that accompany aging may represent changes in homeostatic or circadian aspects of sleep regulation. We addressed these possibilities by examining patterns of unintended sleep episodes, subjective sleepiness and mood during 36 hours of constant wakeful bed rest in 17 healthy 20-30-year-olds (n = 9 men, n = 8 women) and 18 healthy 80-year-olds (n = 11 men, n = 7 women). Subjects had continuous electroencephalographic (EEG) monitoring and core body temperature measurements and completed visual analog scale ratings of sleepiness and mood each hour. Young and elderly groups both showed significant linear and circadian trends in sleep episodes across the interval. Group x time interaction effects revealed a stronger linear trend in the young vs. elderly group, but no difference in circadian trends. Analysis of age groups by sex demonstrated more complex interactions, with elderly men showing weaker rhythmic (circadian) trends than elderly women. Subjective ratings of sleepiness and mood showed no difference in temporal trends between young and elderly groups. These data support the hypothesis that age-related sleep changes result primarily from decreases in homeostatic, rather than circadian, sleep propensity. They also confirm the presence of sex differences in sleep patterns among the elderly and support the distinction between objective and subjective measures of sleepiness.