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Sleep duration's association with future dementia could be a cause or consequence, or both. We searched electronic databases on 14 th April 2023 for primary peer-reviewed, longitudinal studies examining the relationship between sleep duration and dementia with any follow-up duration. We meta-analysed studies examining brief (≤6 h/night) and extended sleep duration (≥9 h/night) separately and divided the studies into those with follow-up periods of less or more than 10 years. The quality of evidence was assessed using the Newcastle-Ottawa scale. 31 studies fulfilled the inclusion criteria. For brief sleep duration, a meta-analysis of short follow-up studies (≤10 years) found a 46 % increased risk of future dementia (relative risk RR – 1·46; 95 % Confidence Intervals CIs 1·48–1·77; I 2 = 88·92 %, 6 studies). Studies with long follow-ups (>10 years) did not show a significantly increased risk (RR – 1·12; 0·95–1·29; I 2 = 65·91 %; 5 studies). For extended sleep duration, a meta-analysis of short and long follow-up studies reported an increased risk of dementia (respectively RR - 2·20; 1·11–3·3; I 2 = 94·17 %; 4 studies and RR – 1·74; 1·30–2·18; I 2 = 86·56 %; 4 studies). Our findings suggest that brief sleep duration might be a prodromal symptom but not a risk factor of dementia. Extended sleep duration may be a risk factor. However, our results had high heterogeneity limiting external validity and generalisability. • • Brief sleep duration is only associated with dementia in studies with short follow-up periods (10 years.
Howard et al. (Sun,) studied this question.