Study objectives: Previous studies with limited follow-up times have suggested that sleep-related traits are associated with an increased risk of incident dementia or cognitive decline. We investigated the association between midlife sleep characteristics and late life cognitive function.
Design: A follow-up study with a median follow-up time of 22.5 (range 15.8-25.7) years assessing the association between midlife sleep characteristics and later cognitive function.
Setting: Questionnaire data from 1981 were used in the assessment of sleep characteristics, use of hypnotics, and covariates at baseline. Between 1999 and 2007, participants were assigned a linear cognitive score with a maximum score of 51 based on a telephone interview (mean score 38.3, SD 6.1). Linear regression analyses were controlled for age, sex, education, ApoE genotype, and follow-up time.
Participants: 2,336 members of the Finnish Twin cohort who were at least 65 years of age.
Measurements and results: Baseline short (< 7 h/day) and long (> 8 h/day) sleepers had lower cognitive scores than participants sleeping 7-8 h/ day (β = -0.84, P = 0.014 and β = -1.66, P < 0.001, respectively). As compared to good sleep quality, poor or rather poor sleep quality was associated with a lower cognitive score (β = -1.00, P = 0.011). Also, the use of hypnotics ≥ 60 days per year was associated with poorer cognitive function (β = -1.92, P = 0.002).
Conclusions: This is the first study indicating that midlife sleep length, sleep quality, and use of hypnotics are associated with late life cognitive function. Further confirmation is needed, but sleep-related characteristics may emerge as new risk factors for cognitive impairment.
Keywords: Cognition; cohort studies; hypnotics and sedatives; risk factors; sleep.