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Sleeping Longer Than 6.5 Hours a Night Associated With Cognitive Decline

Original study’s abstract.

Sleep monitoring may provide markers for future Alzheimer’s disease; however, the relationship between sleep and cognitive function in preclinical and early symptomatic Alzheimer’s disease is not well understood. Multiple studies have associated short and long sleep times with future cognitive impairment.

Since sleep and the risk of Alzheimer’s disease change with age, a greater understanding of how the relationship between sleep and cognition changes over time is needed. In this study, we hypothesized that longitudinal changes in cognitive function will have a non-linear relationship with total sleep time, time spent in non-REM and REM sleep, sleep efficiency and non-REM slow wave activity.

This basically means that the researchers think that those who sleep more, will be more likely to suffer from cognitive decline.

LLIF Commentator

To test this hypothesis, we monitored sleep-wake activity over 4–6 nights in 100 participants who underwent standardized cognitive testing longitudinally, APOE genotyping, and measurement of Alzheimer’s disease biomarkers, total tau and amyloid-β42 in the CSF. To assess cognitive function, individuals completed a neuropsychological testing battery at each clinical visit that included the Free and Cued Selective Reminding test, the Logical Memory Delayed Recall assessment, the Digit Symbol Substitution test and the Mini-Mental State Examination.

Performance on each of these four tests was Z-scored within the cohort and averaged to calculate a preclinical Alzheimer cognitive composite score. We estimated the effect of cross-sectional sleep parameters on longitudinal cognitive performance using generalized additive mixed effects models. Generalized additive models allow for non-parametric and non-linear model fitting and are simply generalized linear mixed effects models; however, the linear predictors are not constant values but rather a sum of spline fits.

We found that longitudinal changes in cognitive function measured by the cognitive composite decreased at low and high values of total sleep time (P < 0.001), time in non-REM (P < 0.001) and REM sleep (P < 0.001), sleep efficiency (P < 0.01) and <1 Hz and 1–4.5 Hz non-REM slow wave activity (P < 0.001) even after adjusting for age, CSF total tau/amyloid-β42 ratio, APOE ε4 carrier status, years of education and sex.

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.

Although longitudinal and interventional studies are needed, diagnosing and treating sleep disturbances to optimize sleep time and slow wave activity may stabilize cognition in preclinical or early symptomatic Alzheimer’s disease.

Sleeping Longer Than 6.5 Hours a Night Associated With Cognitive Decline For Older Adults

Summary: Older adults who sleep less than 4.5 hours, or more than 6.5 hours per night and who experience sleep disruptions are at greater risk of cognitive decline, researchers report.

A good night’s sleep is important for many reasons. It 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. It’s also been shown that not getting enough sleep is linked to cognitive decline and conditions such as Alzheimer’s disease.

But more isn’t always better, as one recent study found. Researchers from the 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 on average, and tracked them for between four and five years.

Take specific note of the sample size for this study. It’s incredibly important to understand how a study is limited by its sample size.

LLIF Commentator

At the time of their study, 88 people did not show any signs of dementia, while 12 showed signs of cognitive impairment (one with mild dementia and 11 with the pre-dementia stage of mild cognitive impairment).

Throughout the 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, the 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 done once, three years after people first completed their annual cognitive tests. This EEG allowed the researchers to accurately measure brain activity, which would tell them whether or not someone was asleep (and for how long), and how restful that sleep was.

Although sleep was only measured at one period during the study, 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’s 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 the proteins beta-amyloid or tau, which are both linked to dementia.

Overall, the 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. Interestingly, the impact of sleep duration on cognitive function was similar to the effect of age, which is the greatest risk factor for developing cognitive decline.

What is a “good night’s sleep” and is it generalizable?

Let’s address this in a small aside. Yes. There are individual differences in sleep quality. However it is generally accepted that high quality sleep meets these criteria.

Sleep latency: quick to fall asleep.

This is a measurement of how long it takes you to fall asleep. Drifting off within 30 minutes or less after the time you go to bed suggests that the quality of your sleep is good.

Sleep waking: stay in deeper sleep cycles through the night.

This measures how often you wake up during the night. Frequent wakefulness at night can disrupt your sleep cycle and reduce your sleep quality. Waking up once or not at all suggests that your sleep quality is good.

Wakefulness: stay fully asleep through the night.

This measurement refers to how many minutes you spend awake during the night after you first go to sleep. People with good sleep quality have 20 minutes or less of wakefulness during the night.

Sleep efficiency: sleep to time in bed ratio.

The amount of time you spend actually sleeping while in bed is known as sleep efficiency. This measurement should ideally be 85 percent or more for optimal health benefits. 

We know from previous research that lack of sleep is linked to cognitive decline. For example, one study showed that people who reported sleep disturbances, such as insomnia or excessive daytime sleepiness, have a greater risk of developing dementia compared to people who don’t. Other research has shown that people who have short sleeping times have higher levels of beta-amyloid in their brain – which is commonly found in the brains of people who have Alzheimer’s disease.

Researchers don’t know for certain why lack of sleep is linked to cognitive decline.

One theory is that sleep helps our brain flush out harmful proteins that build up during the day. Some of these proteins – like beta-amyloid and tau – are thought to cause dementia. So interfering with sleep might interfere with our brain’s ability to get rid of these. Experimental evidence even supports this – showing that even just one night of sleep deprivation temporarily increases beta-amyloid levels in the brain of healthy people.

But it’s less clear why long sleep is linked with cognitive decline. Previous studies have also found a link between over-sleep and cognitive performance, but most relied upon participants self-reporting how long they sleep nightly – which means the data is less accurate than using an EEG to measure brain activity. This new study therefore adds weight to such findings.

This is one of the most important reasons to use fitness and sleep trackers to monitor your own health over time. You can take control of your health and future using data available already.

LLIF Commentator

What’s surprising about this study’s findings is that the optimal sleep duration is much shorter than that which previous studies have suggested are problematic. The study showed that sleeping longer than 6.5 hours was associated with cognitive decline over time – this is low when we consider that older adults are recommended to get between seven and eight hours of sleep every night.

It could be the case that it isn’t necessarily the length of the sleep that matters, but the quality of that sleep when it comes to risk of developing dementia. For instance, this study also showed that having less “slow-wave” sleep – restorative sleep – particularly affected cognitive impairment.

What we also cannot tell from this study is if long sleep durations can independently predict cognitive decline. Essentially, we can’t rule out that participants who slept longer than 6.5 hours every night might not have already had pre-existing cognitive problems of brain changes suggestive of dementia that weren’t picked up on the tests.

And although the researchers were careful to adjust for dementia-related factors, longer sleepers may also have had other pre-existing conditions that might have contributed to their cognitive decline which weren’t taken into account.

For example, this could include poor health, socioeconomic status or physical activity levels. All of these factors together may explain why longer sleep was linked to cognitive decline.

There are many factors which can impact on both our sleep quality, and whether we experience cognitive decline. While some factors aren’t preventable (such as genetic predisposition), there are many things we can do alongside getting a good night’s sleep to help reduce our likelihood of developing dementia – such as exercising and eating a healthy diet. But while the researchers of this study seem to suggest there’s an optimal sleep duration – between 4.5 and 6.5 hours every night – the occasional weekend lie-in is unlikely to do your brain any harm. 

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