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Studies examining levels of illumination in adult populations have demonstrated that the level and amount of light exposure are lower in the elderly compared with younger adults, particularly in institutionalized patients with dementia. Although insufficient light exposure has been implied as a cause of sleep fragmentation, evidence for such a relationship is scant. Sixty‐six institutionalized elderly had their activity and light exposure monitored for a 3‐day period. Mean and median light levels, minutes spent over 1000 and over 2000 lux, percent sleep and wake, and number of naps were computed for daytime intervals, defined as 07.00–18.59. Percentages of sleep and wake, number of awakenings and mean duration of wake periods were computed for night‐time intervals, defined as 22.00–05.59. Mesor, amplitude and acrophase of activity and of light were determined by cosinor analysis. A mixed linear model was …