Sleeping longer than 6.5 hours a night associated with cognitive decline according to research - what's really going on here?

November 5, 2021

A good night's sleep is important for many reasons. lt helps our body repair itself and function as it should, and is linked to better mental health and lower risk of many health conditions - including heart disease and diabetes. lt has also been shown that not getting enough sleep is linked to cognitive decline and conditions such as Alzheimer's disease.

However, more is not always better, as one recent study found. Researchers from lhe Washington University School of Medicine have published a paper that indicates that just like getting too little sleep, sleeping too much may also be linked with cognitive decline.

The research team wanted to know how much sleep was linked to cognitive impairment over time. To do this, they looked at 100 older adults in their mid-to-late- 70s, and tracked them for between four and five years. At lhe time of their study, 88 people did not show any signs of dementia, while 12 showed signs of cognitive impairment ( one with mild de mentia and 11 with the pre-dementia stage of mild cognitiv.a. impairment).

Throughout lhe study, participants were asked to complete a range of commonplace cognitive and neuropsychological tests to look for signs of cognitive decline or dementia. Their scores from these tests were then combined into a single score, called the Preclinical Alzheimer Cognitive Composite (PACC) score. The higher the score, lhe better their cognition was over time.

Sleep was measured using a single-electrode encephalography (EEG) device, which participants wore on their forehead while sleeping, for a total of between four to six nights. This was dane once, three years after people first completed their annual cognitive tests. This EEG allowed the researchers · to accurately measure brain activity, which would tell !hem whether someone was asleep (and for how long), and how restful that sleep was.

Sleep was only measured atone period during the study, but this still gave the research team a good indication of participants' normal sleep habits. While using an EEG to measure brain activity may be somewhat disruptive to sleep on the first night, as people get used to the equipment, sleep tends to return to normal the following night. This means that when sleep is tracked from the second night onwards, · it is a good representation of a person's normal sleep habits.

The researchers also took into account other factors that can affect cognitive decline - including age, genetics and whether a person had signs of lhe protein beta-amyloid or tau, which are both linked to dementia.

Overall, lhe researchers found that sleeping less than 4.5 hours and more than 6.5 hours a night - alongside poor quality sleep - was associated with cognitive decline over time. lnterestingly, the impact of sleep duration on cognitive function was similar to lhe effect of age, which is lhe greatest risk factor for developing cognitive decline.

(Adapted from https://theconversation.com/)