During the aging process, the amplitude of the circadian rhythms of many physiological variables is reduced. It has been hypothesized that increasing light intensity during the light phase of the light-dark cycle might result in a reduction of age-related changes in the circadian rhythms. Indeed, in the present sleep-wake study in young and old rats it was found that (a) various parameters, such as the light-dark differences and total amounts of each behavioral state responded positively to changes in environmental light intensity (i.e., age-related trends were reversed), (b) in both age groups, the logarithm of light intensity appeared to have a linear dose-response relationship with light-dark differences of the sleep-wake states, (c) the light-dark difference of active wakefulness and quiet sleep of old rats under high light intensity were comparable to those of young rats under low light intensity. The results of the present study suggest that, under appropriate conditions, light could be of clinical use in reducing age-related circadian sleep disturbances in humans. This may, in turn, reduce the use of sedatives in elderly people.