In order to investigate how the duration of actigraphic recordings affects the reliability of actigraphic estimates of sleep and 24-h activity rhythm variables, two to 3 weeks of actigraphy were recorded, from which pairs of variables derived from two periods of increasing length (1-10 days) were compared. Two groups were studied: (1) 10 subjects suffering from primary insomnia; and (2) 12 demented elderly subjects living semi-independently in group care facilities of homes for the elderly. Actigraphic estimates of primary measures of sleep (duration and efficiency) and of the 24-h activity pattern (interdaily stability, intradaily variability and amplitude) were calculated on variable lengths of the actigraphic recordings. The average absolute difference of two estimates decreased - and reliability increased - strongly with an increasing number of days analysed. An acceptable reliability of the interdaily stability estimate required more than 7 days of recording. It can be concluded that a valuable improvement in the reliability of actigraphic sleep estimates can be obtained by simply increasing the number of recording nights. The results support the importance of day-to-day variability in insomnia and dementia that has already been previously noted by others, and even suggest the presence of 'week-to-week' variability. This variability may have been involved in the equivocal results of treatment studies in insomnia and dementia where outcome measures were based on a limited number of nights. Such studies could profit from extension of the recording duration to, e.g. 2 weeks, and from the inclusion of variability measures as measures of clinical interest.