Frequent nappers – people who said they napped every day or most days – had a 48 percent higher rate of dementia than people who never or rarely had naps.
Unintentional napping was also associated with a greater risk of dementia (39 percent higher rate than non-nappers).
Length of naps was also a factor, with people who napped for 30 minutes or less having a 40 percent higher rate of dementia than non-nappers in the study period.
Frequent, unintentional, and shorter (30 minutes or less) napping were also found to be associated with higher risks of developing impairment in memory during follow-up, the researchers found.
It's worth noting the researchers also found signs of elevated dementia risk associated with infrequent napping, intentional napping, and napping for periods of 30 minutes or longer, but the effects were not statistically significant after adjustment in the final analysis.
Other studies have previously explored the relationship between napping and cognitive health in older adults, but the results have been mixed, with some studies showing signs of elevated cognitive risks associated with napping, and others finding little evidence of a link.
Due to the observational nature of the new study, the findings can't be interpreted as evidence that frequent, unintentional, or short naps cause dementia, and the researchers acknowledge that the links they've found are complicated.
"As with sleep disturbances, the association between napping and dementia may be bidirectional," they write in their paper, "such that neurological changes that adversely affect cognitive function may also lead to increased daytime napping."
In other words, if older adults start to nap a lot during the day, it could be a sign of worsening brain health – but that worsening brain health might also be making them sleepier, prompting them to doze in waking hours.
Another complicating factor is that daytime napping may be a compensatory mechanism, making up for sleep disturbances at night. But just as poor rest at night might induce catch-up napping in the daytime, so might napping disrupt nighttime sleep.
In the study, the researchers accounted for this by adjusting for other sleep-related factors (nighttime sleep duration, daytime sleepiness, and sleep apnea risk), and the results suggest that frequent napping, unintentional napping, and naps lasting 30 minutes or less remain significantly associated with an increased risk of dementia, even after accounting for these other sleep characteristics.
