The purpose of the current study was to examine differences in hour-by-hour sleep/wakefulness profiles between severely and mild-moderately demented patients, and to assess how many elderly patients remain almost fully asleep or nearly fully awake in each hour of a 24-hour period. Sleep/wakefulness patterns of 67 demented nursing home residents (mean age = 85.7 years) were recorded using Actillume recorders. One 24-hour period was used, and numbers of minutes spent asleep or awake were computed for every hour. There were 46 severely demented patients, and 21 mild-moderately demented patients. The amount of sleep and wakefulness recorded for each hour was compared between the two groups. In addition, the frequencies of patients who remained asleep for more than 90% of each hour, and of those who sustained wakefulness for more than 90% of each hour were computed for every hour, and comparisons were again made between the two groups. Multivariate analysis of variance showed a significant effect of dementia group on the percent of sleep/wakefulness over 24 hours (p = 0.028). Subsequent t-tests performed separately for each hour revealed significant differences between the two dementia groups in 13 out of the 24 hours. Significant differences in the frequencies of patients asleep > 90% or awake > 90% of each hour were centered around the early night and early morning hours. Patients with mild-moderate dementia showed a disproportionate amount of wakefulness during the night, whereas, in addition, patients with severe dementia showed a disproportionate amount of sleepiness during the day. With the progression of dementia, both the capacity to maintain sleep and the capacity to maintain wakefulness are impaired, and result in complete fragmentation of sleep/wakefulness during the night and day.