Improving actigraphic sleep estimates in insomnia and dementia: how many nights?

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.