Total sleep duration and daytime napping in relation to dementia detection risk: Results from the Million Women Study.

INTRODUCTION: There is inconsistent evidence on the associations of sleep duration and daytime napping with dementia risk. METHODS: In the Million Women Study, a total of 830,716 women (mean age, 60 years) were asked about sleep duration (<7, 7-8, >8 hours) and daytime napping (rarely/never, sometimes, usually) in median year 2001, and were followed for the first hospital record with any mention of dementia. Cox regression estimated dementia detection risk ratios (RRs) during 17-year follow-up in 5-year intervals. RESULTS: With 34,576 dementia cases, there was strong attenuation over follow-up in the RRs related to long sleep duration (>8 vs 7-8 hours) and usually napping (vs rarely/never). Short sleep duration was modestly, positively associated with dementia in the long term (RR = 1.08, 95% confidence interval [CI] 1.04-1.12). DISCUSSION: There was little evidence to suggest that long sleep duration and regular napping are associated with long-term dementia risk. Short sleep duration was modestly associated with dementia risk, but residual confounding cannot be excluded. HIGHLIGHTS: Long sleep duration was not associated with long-term dementia risk. Daytime napping was not associated with long-term dementia risk. There is some evidence for a small higher risk of dementia related to short sleep.