REVIEW OF: Sabia S et al, Nat Commun 2021;12(1):2289

STUDY TYPE: Prospective cohort study

How is sleep related to dementia? Prior studies among middle-aged and older adults found that both longer and shorter sleep durations are associated with an increased dementia risk. However, as the follow-up duration of these studies was less than 10 years, it is challenging to determine whether sleep irregularities contributed to dementia or reflected its early symptoms. 

This study used data from 7,959 participants of the Whitehall II cohort study in the UK to examine the association between sleep duration at ages 50, 60, and 70 with incident dementia over the following 25 years. 521 participants developed dementia over this time period at an average age of 77. 

The researchers found that sleeping less than six hours over the span of ages 50, 60, and 70 was associated with a 30% increased dementia risk compared to those getting normal sleep (defined as seven hours). The study controlled for sociodemographic, behavioral, cardiometabolic, and mental health factors (such as depressive symptoms or central nervous system drugs) that might affect sleep and dementia risk. 

The number of long sleepers (eight or more hours) was too small in this study to associate with dementia risk. Additional limitations of this study include the use of electronic records to assess dementia cases, which might misclassify milder cases, and incomplete data on the types of dementia that participants developed.

Carlat Take

We know there is an association between sleep and cognitive function, likely related to the role of sleep in learning and memory, synaptic plasticity, and waste clearance from the brain. This study supports the association between a short sleep duration in middle life and an increased risk for dementia. This is only a correlation, and a randomized clinical trial assigning patients to poor vs normal sleep and following them for 10–20 years would not be possible. Nonetheless, these findings are suggestive, and we should encourage our patients to practice good sleep habits, particularly those at higher risk for dementia.