A pooled cohort study was done in 20.065 English (9.254) and Chinese (10.811) aged people with a mean age of 64.6 years (UK) and 57.8 years (China) and a median follow-up of respectively 8 and 4 years. The proportion of females and males was approximately equal. During 100.000 person-years of follow-up global cognitive sccores in individuals with 4 hours or less and 10 hours or more of sleep per night declined faster than in the reference group (7 hours per night). The sleep duration per night was self-reported according to face-to-face interviews. An inverted U-shaped association between sleep duration and global cognitive decline was also observed.
It is concluded that cognitive function should be monitored in individuals with insufficient (≤ 4 hours per night) or excessive ( ≥ 10 hours per night) sleep duration. Future studies should examine the mechanism of the association between sleep duration and cognitive decline.