Reliable measures of rest-activity rhythm fragmentation: how many days are needed?

Background A more fragmented, less stable rest-activity rhythm (RAR) is emerging as a risk factor for health. Accelerometer devices are increasingly used to measure RAR fragmentation using metrics such as inter-daily stability (IS), intradaily variability (IV), transition probabilities (TP), self-similarity parameter (alpha), and activity balance index (ABI). These metrics were proposed in the context of long period of wear but, in real life, non-wear might introduce measurement bias. This study aims to determine the minimum number of valid days to obtain reliable fragmentation metrics. Methods Wrist-worn accelerometer data were drawn from the Whitehall accelerometer sub-study (age: 60 to 83 years) to simulate different non-wear patterns. Pseudo-simulated data with different numbers of valid days (one to seven), defined as < 1/3 of non-wear during both day and night periods, and with omission or imputation of non-wear periods were compared against complete data using intraclass correlation coefficient (ICC) and mean absolute percent error (MAPE). Results Five days with valid data (97.8% of participants) and omission of non-wear periods allowed an ICC >= 0.75 and MAPE <= 15%, acceptable cut points for reliability, for IS and ABI; this number was lower for TPs (two-three days), alpha and IV (four days). Overall, imputation of data did not provide better estimates. Findings were consistent across age and sex groups. Conclusions The number of days of wrist accelerometer data with at least 2/3 of wear time for both day and night periods varies from two (TPs) to five (IS, ABI) days for reliable RAR measures among older adults.