Depressive symptoms status and nighttime sleep duration in relation to cognitive performance in Chinese elderly: Insights from a large cross-sectional inquiry

BACKGROUND: The combined associations of nocturnal sleep duration and depressive symptoms with older adults' cognitive function (assessed by the Mini-Mental State Examination, MMSE) remain undercharacterized, with unclear mechanistic pathways and sleep duration-cognition non-linear relationships. This study explored the independent, joint, and mediated associations of depressive symptoms (measured via the 15-item Geriatric Depression Scale, GDS-15) and self-reported nocturnal sleep duration with cognitive function/impairment (MMSE <24) in community-dwelling older Chinese adults. METHODS: Data came from the 2019 HAELAS baseline (4145 adults ≥65 years). A three-stage analysis was used: 1) restricted cubic spline (RCS) for non-linear associations of sleep duration/depressive score with MMSE; 2) multivariable logistic regression for independent/joint associations with cognitive impairment; 3) 4-way decomposition for mediated pathways, stratified by sleep-cognition U-shaped inflection point. RESULTS: Of 4145 participants, 28.0% (n = 1160) had cognitive impairment and 24.92% (n = 1033) had depressive symptoms. Independent associations: GDS-15 score per 1-point increase raised cognitive impairment risk by 10.5% (adjusted OR = 1.105, 95% CI:1.079-1.131, P < 0.001); RCS showed a sleep-cognition U-shape (P < 0.001), with short sleepers (<6 h/night) having 27.4% higher risk vs. normal sleepers (6-7.9 h/night, OR = 1.274, P < 0.05) and long sleepers (≥8 h/night) also showing significant association. Joint associations: All 6 subgroups (depression × sleep) had higher ORs vs. reference (non-depressed +6-7.9 h/night), with Group F (depressed + ≥8 h/night) having the highest risk (OR = 2.62). Mediation: Stratified by sleep inflection (7 h), for sleep ≤7 h, extending sleep exerted a protective total effect on cognition (37.91% indirectly mediated by reduced depressive symptoms, 62.06% direct protective effect), while for sleep >7 h, longer sleep showed a harmful total effect on cognition (mainly driven by direct effect, with depressive symptoms no longer serving as a meaningful mediator). Depressive symptoms exerted a harmful effect on cognition primarily through direct effect, with sleep duration playing a negligible mediating role across both sleep strata. CONCLUSIONS: This study clarifies the sleep-depression-cognition interplay in older Chinese adults, providing an empirical basis for targeted interventions (optimizing sleep to 6-7.9 h/night, alleviating depression) to reduce cognitive impairment risk.