We measured 24-hour circadian-rhythm patterns of activity and sleep/wake activity in a group of nursing-home patients (58 women and 19 men with a mean age of 85.7 years). Severely demented patients were contrasted with a composite group of moderate y, mild, or not-demented patients. Sleep/wake activity and light exposure were recorded with the Actillume recorder. Cosinor analyses were computed to determine the mesor, amplitude, acrophase, and circadian quotient of the activity rhythms. The diagnosis of dementia was based on the Mini Mental Examination and on examination of medical records. Sleep was extremely fragmented in both groups of nursing-home patients. Severely demented patients slept more both at night and during the day, but there were no significant differences in the number of awakenings during the night or in the number of naps during the day when compared to the composite group of moderate, mild, or no-dementia patients. The severely demented group had lower activity mesor, more blunted amplitude, and were more phase delayed (i.e. had later acrophases) than the other group. In addition, the severely demented patients spent less time exposed to bright light. These results confirm that circadian rhythms in nursing-home patients are disturbed with more disturbance in the severely demented. Much of the disturbance may be related not just to age but to mental status.