Sleeping Longer Than 6.5 Hours a Night Associated With Cognitive Decline

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.

Source: The Conversation

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. 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.

A good night’s sleep

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 shows an older couple sleeping
Does too much sleep really increase your risk of cognitive decline? Credit: The Conversation

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.

Funding: Greg Elder receives funding from Alzheimer’s Research UK.

About this sleep and cognition research news

Author: Greg Elder
Source: The Conversation
Contact: Greg Elder – The Conversation
Image: The image is credited to The Conversation

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  1. El paciente con narcolepsia puede tener dificultad para dormir durante la noche. Pero en cambio a lo largo del día tiene ataques de somnolencia agudos, que le hacen dormirse en cualquier situación, por lo que es especialmente peligroso.

  2. This feels like corporations trying to comvince us we dont need sleep to justify making us work long hours. 6.5 is not enough, thats normally what i get and i cant function until im 600MG deep in caffeine

  3. The article is fine. The headline is terrible and misleading. It should read “Much More Research Needed On Sleep and Mental Decline in Elderly, But Here’s One Study With 100 Participants”. Someone that read just the headline and sleeps 8 hours might start sleeping less than 6.5 because of a tiny study in which the findings were inconclusive.

  4. The comments on correlation not being proof of cause and affect are spot on. The problem with the publication of most studies is that the reader is never told how the numbers were analyzed or how the data was gathered. The analysis to show cause and affect is much more complicated than showing simple correlation, and each data point must to show that the hypothetical cause happened prior to the observed affect in order to be valid. This leads to misleading articles and broadcast “news” that we are constantly bombarded with.

    1. Desperate to write more articles.
      I agree with persons comment as to the morons. Also if this article has any merit it should be done with way more than 100 participants . also another disturbing thing was the age of participants are not most of these people in cognitive decline anyway? What this all mean …nothing…

    2. Omg such hypocrisy all over everywhere about everything ugh so tired. Last week they said I was killing myself over getting less than 6 hrs now too much sleep? Should we just not?

      1. * The study (and this article) are quite useless since adequate scientific co trols and processes were not used. The worst being the assumption that measuring 1 night of sleep 3 years after the initial cognitive test and then assuming that 1 night can be used as the basis for how the participant sleeps every night. Next, 100 participants is not a statistically significant sample size.

    3. There it is, they should be required by law to bold it. The dreaded “MAY” strikes again.

      I also may get younger and better looking, I may.

    4. Seems like correlation and not causation but presented and titled like causation. Does lack of sleep or too much sleep cause Alzheimer’s … Or the other way around … Or some third thing common with both but not studied at all?

  5. The authors need to be a lot more careful here. Why isn’t the title “Cognitive Decline Associated With Sleeping Longer Than 6.5 Hours a Night”? Technically it’s exactly the same thing—correlation does not mean causation, but there can be subtle hints. Or not so subtle. Like when they say “The researchers also took into account other factors that can affect cognitive decline”. OTHER factors that CAN AFFECT? As if sleeping longer was already established to be one of them. And “the impact of sleep duration … similar to the effect of age” again assumes causation. I won’t bother reading the original paper, but I hope that it makes no such claims.

    1. Weirdly misleading article. Study was for those in 70-80 yr old population. Kind of ignored their habits before age 70. Using the term older adults is very a broad term for a science site. Sleep requirements are a lot less after retirement. Misleading title for clicks. Block you guys

  6. Whenever I see articles warning that we’ll all get dementia if we don’t get enough sleep, or too much sleep, or sleep on our stomach or sleep standing on one leg or whatever, I think about the people we all read about every day that tell us they sleep 4 hours a night or whatever, and they are fine. Think Politicians, inventors, billionaires and on and on ad nauseum. I’ve been reading articles like this for 50 years. Give me a break.

  7. What is the direction of causality (which is also not implied by any correlation in itself, as is well known)?
    Although I admit to reading this summary quickly (and I would need to read the full science article to be honest), is it not possible that cognitive decline could in itself lead to people sleeping more? What is known about that aspect? This may well be addressed somewhere or even in the full article, just a comment… but this does seem like there could be enough other variables one needs to try to control for (as commented on by others), so firm conclusions may be tough to come by. Speculative on my non-expert part of course… Articles should include the reviewers’ comments as publicly available, so readers can be more fully informed regarding how controversial certain publications might be; this is done by some journals in some fields but not everywhere. Keep up the research though! Cognitive decline is an important topic.

  8. I don’t think one nights sleep is accurate to someone’s sleep behaviors.. and I’m also curious what the control in this study was.. what is the normal cognitive decline over 5 years for someone over 70?

    1. A study of tribes of people still living hunter gatherer indicated that they consistently slept about 6.5 hours.

      1. Nobody lives forever!
        If I want to sleep 4 hours a night on weeknights and 10 hours a night on weekends to catch up then regardless of what anyone or any study says, I’m going to f’ing do it!!!

  9. I sleep very poorly and I am losing my brain function I miss pronounce words correctly and drawing complete blank on simple words when speaking with my friends. They very kind by saying, “I do that too.”

  10. This is a small but critical step in understanding the relationship between sleep and health. Prior studies were longitudinal self-reports that are useful but limited. This is longitudinal measured. I teach in this field and have monitored the research closely. Quality of sleep is as or more important than length of sleep. That is a complex subject and this study just scratches the surface.

    People tend to report the time from going to bed to getting up. They may be in bed 8 hours but only sleep 5 hours or have sleep apnea and have little or poor quality sleep. My main criticism is the headline of this article and the omitted critical information, not the one used by the authors of the study or the content they covered. The headline is accurate but misleading. The authors of the study point out that 4.5 to 6.5 hours of EEG measured sleep is the equivalent of 5.5 to 7.5 hours of self-reported sleep. Even that is an average range with significant variation. People reading this article, which I realize you copied with credit, will be misled about what it says about sleep in the way most of us think about it.

  11. I am bone-weary of all of the contradictory information from all these scientifically trained smart cookies. What if we sleep long enough (or short enough) so that we can feel tested and cope with life. That amount may vary with individuals, but at least we can test in a way that suits each of us individually. Because (gasp!) we’re not all the same.

    1. I am a 70 year female living in N.J. ☆ frustrated that I can NOT locate how to get an official written record of my covid19 vaccinations. (Resident of senior retirement home in Orance county), N.J. thank you

  12. I find this study to be very misleading. All REAL science points to a full eight hours to be best for everyone

  13. Too many maybes. Needs more variable factors to be measured. A very small group study for drawing those kind of generalizations. Fundamentally a flawed study.

  14. Maybe that’s why it’s hard for me to sleep more than 7 hours. Would that 6.5 hours include an afternoon nap? Then with the idiotic time changes, it makes it more difficult to understand how much sleep you need or how much quality sleep you might be getting. Since I usually don’t agree with the one-size-fits-all mentalty for much of anything, these studies are worthless. I sleep what I sleep and the experts can go take a hike.

  15. This is a misleading study, with such a small group of participants, and countless uncontrolled independent variables, it is unscientific to claim that sleeping more than 6.5 hours a night leads to cognitive decline.

    1. Agreed, there are SO MANY factors at that age that contribute to cognitive decline (Weight and physical fitness, diet, social support, mental activities (do they like to read or just sit in front of a TV…. even age itself as a variable)… also the small sample size and the unscientific time of 6.5…. we know that one sleep cycle is 90 min and 90 min does NOT divide into 6.5 hours, and waking a person in the middle of a sleep cycle is proven to have negative effects (recommended by almost all neuroscientists that we need 5 complete sleep cycles a night = 7.5 hours sleep)…. the recommendation of 8 hours is usually due to most people taking a little while to fall asleep each night. This article makes me question the quality of this site.

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