Erratic sleep patterns over years or even decades, along with a patient’s age and history of depression, may be harbingers of cognitive impairment later in life, an analysis of decades of data from a large sleep study has found.
Samantha Keil, Ph.D., a postdoctoral fellow at the University of Washington, Seattle, and her team analyzed sleep and cognition data collected over decades on 1,104 adults who participated in the Seattle Longitudinal Study. Study participants ranged from 55 to over 100, with almost 80% of the study cohort aged 65 and older.
The Seattle Longitudinal Study first started gathering data in the 1950s. Participants in the study cohort underwent an extensive cognitive battery, which was added to the study in 1984 and gathered every 5-7 years, and completed a health-behavioral questionnaire (HBQ), added in 1993 and administered every 3-5 years. The HBQ included a question on average nightly sleep duration.
The study used a multivariable Cox proportional hazard regression model to evaluate the overall effect of average sleep duration and changes in sleep duration over time on cognitive impairment. Covariates used in the model included apolipoprotein E4 (APOE4) genotype, gender, years of education, ethnicity, and depression.
The demographic variables of education, APOE status, and depression were significantly associated with cognitive impairment.
When evaluating the duration, change, and variability of sleep, the researchers found that increased sleep variability was significantly associated with cognitive impairment. Identification of sleep variability as a sleep pattern of interest in longitudinal studies is essential, Dr. Keil said, because simply evaluating mean or median sleep duration across time might not account for a subject’s variable sleep phenotype.
Further evaluation of sleep variability with a linear regression prediction analysis found that increased age, depression, and sleep variability significantly predicted cognitive impairment ten years downstream.
Longitudinal sleep variability is perhaps for the first time being reported as significantly associated with the development of downstream cognitive impairment.
What makes this study unique, Dr. Keil said in an interview, is that it used self-reported longitudinal data gathered at 3- to 5-year intervals for up to 25 years, allowing for the assessment of the variation of sleep duration across this entire time frame. “If you could use that shift in sleep duration as a point of therapeutic intervention, that would be exciting.
Future research will evaluate how sleep variability and cognitive function are impacted by other variables gathered in the Seattle Longitudinal Study over the years, including factors such as diabetes and hypertension, diet, alcohol, tobacco use, and marital and family status.
Follow-up studies will investigate the impact of sleep variability on neuropathologic disease progression and lymphatic system impairment.
Parvathy Sukumaran 
