Certain levels of sleep are important for maintaining cognitive function, study says

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Sleeping fewer than 4.5 hours and more than 6.5 hours per night is associated with cognitive decline, according to researchers. 

In a recent study published in Oxford Academic's Brain, a team from the Washington University School of Medicine examined the relationship between sleep and cognitive function in preclinical and early symptomatic Alzheimer's disease. 

To reach these conclusions, the team monitored 100 participants' sleep-wake activity over a period of four to six nights. 

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The participants, who The Conversation reported were in their mid-to-late 70s on average, were tracked for four to five years and underwent standardized cognitive testing, APOE genotyping and measurement of Alzheimer's disease biomarkers. 

Additionally, at each clinical visit, individuals completed a neuropsychological testing battery to assess cognitive function.

A sleeping couple (Credit: iStock)

Performance on the tests was graded with a standard score and averaged to determine a preclinical Alzheimer cognitive composite score.

Estimating the effect of cross-sectional sleep parameters on longitudinal cognitive performance using generalized additive mixed-effects models, the researchers found that "longitudinal changes in cognitive function measured by the cognitive composite decreased at low and high values of total sleep time."

This was true even after adjusting for factors like age, gender, years of education and Alzheimer's disease biomarkers.

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At the time of the study, 88 people did not show signs of dementia, while 12 showed signs of cognitive impairment, The Conversation reported. 

"Cognitive function was stable over time within a middle range of total sleep time, time in non-REM and REM sleep and <1 Hz slow wave activity, suggesting that certain levels of sleep are important for maintaining cognitive function," the authors wrote. Slow wave sleep is often referred to as "deep sleep."

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Although the group said that further studies are needed, the researchers posited that diagnosing and treating sleep disturbances to optimize sleep time and slow wave activity could have a "stabilizing effect" on cognition in preclinical or early symptomatic Alzheimer’s disease.

While previous research has shown that lack of sleep is linked to cognitive decline, a link between over-sleep and cognitive performance is more mysterious. 

The U.S. Centers for Disease Control and Prevention (CDC) recommends seven hours or more for people ages 18-60, seven to nine hours for people ages 61-64 and seven to eight hours for people 65 years and older.

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A National Health Interview Survey from 2011 to 2014 found that 31.6% of adults 18 years and over get insufficient sleep.

People who regularly get poor sleep may be at an increased risk for obesity, Type 2 diabetes, high blood pressure, heart disease and stroke, poor mental health and even early death.