Rest-Activity Rhythms, Social Interaction, and Depressive Symptoms in Older Adults: A Longitudinal Within- and Between-Person Analysis

Objectives: Rest-activity rhythm organization and social interaction have each been linked to late-life depression, yet their independent longitudinal contributions have rarely been examined within a framework separating between-person differences from within-person fluctuations. This study examined their independent associations with depressive symptoms in community-dwelling older adults.Method: Data from Rounds 12–14 (2022–2024) of the National Health and Aging Trends Study (NHATS) included 1,532 person-wave observations from 606 participants. A Behavioral Rhythm Index (BRI) combined mean activity, relative amplitude, and reversed day-to-day variability. Person-mean-centered random-intercept linear mixed models separated within- from between-person associations, adjusting for wave, age, sex, and education.Results: Higher BRI was associated with fewer depressive symptoms between persons (B = −0.238, p < .001) and within persons (B = −0.139, p = .013), although the within-person association was not robust across sensitivity analyses. Greater social interaction was associated with fewer depressive symptoms between persons (B = −0.714, p < .001) but not within persons. BRI and social interaction did not significantly interact (χ²(2) = 2.05, p = .359).Conclusion: Higher BRI and greater habitual social engagement were each independently associated with fewer depressive symptoms between individuals, warranting further investigation as potentially modifiable behavioral factors in late-life depression.