Sleep quality modifies the association between nocturnal sleep, daytime napping, and mild cognitive impairment: A population-based dose-response study.

Objectives: This cross-sectional study, based on the two-process model of sleep regulation, explored the nonlinear relationships and interactions among nocturnal sleep duration, daytime napping, sleep quality, and mild cognitive impairment (MCI) in middle-aged and older Chinese adults. Methods: Data from 7480 participants in the 2018 wave of the China Health and Retirement Longitudinal Study (CHARLS) were analyzed. Restricted cubic splines modeled dose-response relationships, and stratified logistic regression assessed subgroup variations. Models were adjusted for sociodemographic, lifestyle, and clinical covariates. Results: The lowest odds of MCI were observed among participants reporting 6-8 h of nocturnal sleep and 30-60 min of daytime napping. Short (<6 h) and long (>8 h) sleep durations, along with prolonged napping (≥90 min), were linked to higher odds of MCI. Sleep quality modified these associations: poor sleep quality elevated the odds of MCI, especially among long sleepers (OR = 1.88 vs. 1.33 for good sleep quality). Among short sleepers (<6 h), 30-60 min naps were associated with lower odds of MCI (OR = 0.72). Conclusions: These findings indicate that, in this cross-sectional sample, 7-8 h of nocturnal sleep combined with 30-60 min of daytime napping were associated with better cognitive status, whereas long sleep with poor sleep quality was associated with higher odds of impairment. These observational associations, considered within a circadian-homeostatic framework, are hypothesis-generating and warrant confirmation in future longitudinal and interventional studies.