Actigraphy-derived sleep fragmentation index: convergent validity and associations with clinical outcomes

Wake after sleep onset (WASO) and sleep efficiency (SE) derived from actigraphy are common assessments of sleep fragmentation (or continuity). The sleep fragmentation index (SFI), measuring the frequency of sleep-wake transitions, is less understood. This study examined (1) the convergent validity between SFI and other sleep metrics obtained by actigraphy and polysomnography; and (2) associations of SFI with sleep symptoms, obstructive sleep apnea (OSA), periodic limb movement index (PLMI), and cognition (Digit Symbol Coding test (DSC)). Cross-sectional analysis using logistic and multiple regression analyses adjusted for potential confounders. 1,908 participants in the Multi-Ethnic Study of Atherosclerosis study who underwent 7-day actigraphy and polysomnography. The sample was 53.9% female; age 68.3 ± 9.1 years (mean ±SD); apnea-hypopnea index (AHI) 19.5 ± 17; and SFI 20.09 ± 6.99. Higher SFI was associated with older age, male sex, Black race, smoking, body mass index, OSA, and polysomnography-based metrics of sleep architecture. SFI was strongly correlated with actigraphy-measured SE (r= -0.75; p<0.0001) and WASO (r= 0.63; p<0.0001), and modestly correlated with polysomnography-WASO, AHI, and arousal index (r's=0.23 to 0.27; p's<0.0001). In adjusted analyses, each standard deviation unit increase in SFI was associated with 1.1 to 1.4 higher odds of insomnia symptoms, sleepiness, OSA, an elevated PLMI, and with lower DSC scores (p<0.05). The results support the convergent validity between actigraphy-estimated SFI and actigraphy-WASO and SE. SFI showed modestly stronger associations with clinical symptoms compared to other fragmentation variables, supporting its utility as a marker of sleep continuity.