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Sleep duration and daytime napping are not strongly linked to dementia risk, although there may be a small increase in risk associated with short sleep duration.
In the Million Women Study, Angel Wong, BSc, MSc, Cancer Epidemiology Unit, Nuffield Department of Population Health, University of Oxford, and investigators looked at how total sleep duration and daytime napping related to the risk of dementia. While no association was found between dementia risk and long sleep duration or daytime napping, there is evidence of a slightly higher risk with short sleep duration.1
After screening 866,421 women, a total of 830,716 were included in the analysis (mean age: 60 years). The initial assessment of the participants’ sleep duration and daytime napping habits took place in 2001 with a follow-up period spanning 17 years, during which they were monitored for the first hospital record with any mention of dementia.
The majority of women reported normal sleep duration and rarely or never napping during the day.
Sleep duration was classified into three categories: less than 7 hours, 7-8 hours, and more than 8 hours. Daytime napping was categorized as rarely/never, sometimes, and usually. The researchers then estimated the dementia detection risk ratios (RRs) using Cox regression during the 17-year follow-up in 5-year intervals.
Women who reported short or long sleep duration and those who reported sometimes/usually napping were more likely to be older, have a higher BMI, smoke, and have poorer self-rated health and current treatment for various illnesses.
Among the population of women who met the inclusion criteria for analysis, 67% reported normal sleep duration (7-8 hours), 23% reported short sleep duration (<7 hours), and 10% reported long sleep duration (>8 hours).
When assessing the habits of daytime napping, data indicated 55% of women reported rarely/never napping during the day, 39% reported sometimes napping during the day, and 6% reported usually napping during the day.
Investigators observed that those who reported short or long sleep duration and those who reported sometimes/usually napping were more likely to be older, have a higher BMI, smoke, and have poorer self-rated health and current treatment for various illnesses.
The study found little evidence to suggest that long sleep duration and regular napping are associated with long-term dementia risk. The adjusted risk ratio (RR) for dementia detection was 1.01 (95% confidence interval [CI] 0.96-1.07) for women reporting long sleep duration compared with those reporting normal sleep duration (7-8 hours).
Similarly, there was no significant association found between regular daytime napping and dementia risk compared with rarely or never napping. The adjusted RR for dementia detection was 1.00 (95% CI 0.96-1.05) for women reporting usually napping and 1.01 (95% CI 0.97-1.06) for those reporting sometimes napping, compared with those who reported rarely/never napping.
However, a modest association was observed among those who reported short sleep duration and a slightly higher risk of dementia (RR = 1.08, 95% CI 1.04-1.12).
According to the study, the risk of dementia was about 8% higher for women who reported short sleep duration compared with those who reported sleeping 7-8 hours per night, after accounting for other factors such as age, BMI, smoking, and self-rated health. Investigators noted the increased risk was modest and residual confounding cannot be excluded.
“The primary strengths of this analysis are the long and virtually complete follow-up, and the extremely large number of dementia cases included (>34,000), which is more than all cases combined in the latest meta-analysis," investigators wrote. "Although the findings were based on women only, there is no strong evidence for a difference in the association of sleep with dementia risk by sex.”