Sleep regularity is associated with cognitive function and shows an inverted U-shaped relationship with serum brain-derived neurotrophic factor

Day-to-day sleep patterns have been hypothesized to influence cognitive health and Alzheimer's disease. However, it remains unclear whether sleep irregularity is associated with cognitive function and its key marker, brain-derived neurotrophic factor (BDNF), especially in individuals with subjective concerns about cognition and sleep but who are otherwise relatively healthy. A total of 458 participants (aged 65.0 ± 11.1 years, 51.1 % female) participated in the Tsukuba Happiness Life Study between 2023 and 2024 in Japan. Day-to-day sleep patterns were measured using the Sleep Regularity Index (SRI) via 24/7 actigraphy. Cognitive function was evaluated using a comprehensive assessment battery. Serum BDNF was measured in a subgroup of 232 participants. Multivariate regression analysis was used for the main analysis. Overall, the SRI ranged from 10.86 to 85.59; after adjusting for demographic factors and total sleep time, participants with higher SRI (range, 65.35-85.59) exhibited higher global cognitive function (B = 0.13, 95 % CI = 0.02, 0.24) than the lower SRI group (range, 10.86-54.08). Notably, a nonlinear relationship (inverted U-shaped) was observed between SRI and serum BDNF levels, whereas balanced SRI (around 60) and the middle SRI group exhibited significantly higher serum BDNF levels (B = 0.17, 95 % CI = 0.04, 0.30) than the lower SRI group. Higher SRI serves as an important factor for cognitive function. Maintaining a balanced SRI, which is neither too rigid nor highly irregular, might benefit Alzheimer's disease progression.