The study, led by Associate Professor Matthew Pase, from the Monash School of Psychological Sciences and the Turner Institute for Brain and Mental Health in Melbourne, Australia, and published today in JAMA Neurology, looked at 346 participants, over 60 years of age, enrolled in the Framingham Heart Study who completed two overnight sleep studies in the time periods 1995 to 1998 and 2001 to 2003, with an average of five years between the two studies.

These participants were then carefully followed for dementia from the time of the second sleep study through to 2018. The researchers found, on average, that the amount of deep sleep declined between the two studies, indicating slow wave sleep loss with aging. Over the next 17 years of follow-up, there were 52 cases of dementia.

Even adjusting for age, sex, cohort, genetic factors, smoking status, sleeping medication use, antidepressant use, and anxiolytic use, each percentage decrease in deep sleep each year was associated with a 27% increase in the risk of dementia.