Lonely days: Linking day-to-day loneliness to biological and functional aging.

Lonelier people age more quickly than their less lonely counterparts. Nevertheless, loneliness has been conceptualized as a stable trait rather than a fluctuating experience. The current study examined whether two markers of loneliness in daily life-average daily loneliness and loneliness susceptibility, that is, day-to-day fluctuations with changing circumstances-were associated with poorer biological, phenotypic, and functional aging outcomes. Adults who participated in the National Study of Daily Experiences and Biomarker Project of the Midlife in the United States study (N = 1,008) reported their daily loneliness on eight consecutive evenings, provided blood samples assayed for interleukin (IL)-6 and insulin-like growth factor-1 (IGF-1), and completed assessments of gait speed and grip strength. Self-reports captured difficulties with instrumental activities of daily living, demographics, health conditions, and trait measures of depression and social connection. Contrary to its traditional treatment as a trait, loneliness varied substantially day to day (intraclass correlation coefficient = .57). Controlling for age, gender, comorbidities, body mass index, education, and time between projects, higher daily loneliness was associated with lower IGF-1, weaker grip, slower gait, and more self-reported functional limitations. Those who were more susceptible to daily loneliness also had higher IL-6 and slower gait. Trait measures of social connection did not predict these outcomes, and daily loneliness measures were largely robust to the effects of depression. Two signatures of daily loneliness highlight its dynamic nature and show its unique importance for unhealthy aging, underscoring the value of daily approaches for assessing and intervening on loneliness to offset aging-related decline. (PsycInfo Database Record (c) 2025 APA, all rights reserved).