The mediating role of low-grade inflammation on the prospective association between sleep and cognitive function in older men and women: 8-year follow-up from the English Longitudinal Study of Ageing.

Suboptimal sleep patterns predict poorer cognitive function in older adults and induce inflammatory responses. Inflammation could also adversely affect cognitive function. This study explored whether systemic inflammation may be one biological mechanism through which sleep influences cognitive performance. Participants were 4877 men and women from the English Longitudinal Study of Ageing who were followed-up for 8 years starting at wave 4 (2008-09), through wave 6 (2012-13), and until wave 8 (2016-17). Sleep quality and duration were measured with self-reported questionnaires. Cognitive function was assessed objectively with tests of verbal fluency, memory (immediate and delayed recall) and time orientation. Analyses were stratified by sex and adjusted for socio-economic circumstances, health behaviours, limiting long-standing illness, medication, depressive symptoms, and baseline inflammation and cognition. In men, in comparison with optimal sleep duration, short sleep (≤6 h: β = -0.343, C.I. -0.611 to -0.076; >6-7 h: β = -0.263, C.I. -0.506 to -0.020) and long sleep (β = -0.536, C.I. -1.019 to -0.053) measured at baseline predicted lower scores in delayed memory recall at follow-up. In women, sleep duration was unrelated to cognitive performance at follow-up, and in both sexes, there was no relationship between sleep quality and follow-up cognitive performance. There was no evidence of mediating effects of inflammatory markers in the relationship between sleep measures and cognitive performance in both sexes. In conclusion, baseline short and long sleep duration is associated with follow-up cognitive performance in older men, but we found no evidence of any mediating effects of inflammation.