Counting Zs: Determining the Monitoring Days Needed to Obtain Reliable Sleep Metrics in Older Adults with Mild Cognitive Impairment

OBJECTIVE: This study aims to determine the minimum number of days required to obtain reliable actigraphy-measured sleep metrics in older adults with Mild Cognitive Impairment (MCI). METHODS: One hundred and fourteen older adults (61 with MCI, 53 cognitively unimpaired) wore wrist-worn actigraphy devices for at least 60 days. Total sleep time (TST), sleep efficiency (SE), and wake after sleep onset (WASO) were calculated over cumulative windows (2 - 59) and compared to 60-day reference values using linear mixed-effect regression models. Sensitivity analyses were conducted using 30- and 45-day references. RESULTS: Across all participants, reliable estimates required a minimum of 8 days for TST and 6 days for SE and WASO. Participants with MCI required more days than controls (9 vs. 2 days for TST, 7 vs. 4 days for SE, and 6 vs. 2 days for WASO). Sensitivity analyses showed that shorter monitoring periods (30 and 45 days) could still provide reliable metrics. CONCLUSIONS: To obtain reliable sleep metrics, at least 9 days of actigraphy data are needed for older adults with MCI, compared to 4 days for controls, suggesting the need for extended monitoring in cognitively impaired individuals.