How Sleep Impacts Cognition, Memory, and Dementia

• Seventy million adult Americans struggle with some type of sleeping disorder.
• Studies show a link between sleep and cognitive decline.
• Approximately 15 percent of cognitive impairment or Alzheimer’s disease (AD) might be prevented with improved sleep.
• Sleep cleanses the brain.

 “Approximately 15 percent of cognitive impairment or Alzheimer’s disease (AD) might be prevented if effective interventions could be implemented to reduce sleep disorders.” That is the conclusion of a group of scientists writing in the journal Sleep. Their findings are particularly disturbing, considering that 2023 statistics show impaired sleep is reaching epidemic proportions in the United States.

Statistics from the Center for Advancement of Health (CFAH) indicate as many as 70 million adult Americans struggle with some type of sleeping disorder, such as insomnia, hypersomnia (daytime fatigue), sleep apnea, and circadian rhythm abnormalities. Insomnia is among the most common sleep disorders, with 30 to 40 percent of adults reporting symptoms. Indeed, studies link insomnia, marked by difficulties in falling asleep or remaining asleep, to a variety of cognitive and psychiatric problems, including depression and anxiety.

The CFAH numbers do not include the millions of Americans who are regularly sleep-deprived, primarily because they do not allow sufficient hours for sleep and are harvesting the same long-term, negative effects on their brain and overall health as those with disordered sleep.

Series of Studies Show Sleep and Cognitive Decline Linked

In their 2017 Sleep journal study, the scientists warned of “mounting evidence” implicating “disturbed sleep or lack of sleep as a [major] risk factor for Alzheimer’s disease.” The group undertook a “meta-analysis” of findings from 27 observational sleep studies involving more than 69,000 participants between 40 and 91 years of age and determined the relative risk of developing Alzheimer’s disease or other forms of cognitive impairment to be one and a half times greater among those who fail to get quality sleep.

Added confirmation of the link between sleep and cognitive decline comes from a series of recently published studies. In fact, authors of a 2020 article in the Journal of the American Medical Association called the “association between sleep duration and cognitive function…one of the most studied yet controversial topics.”

A 2018 National Institutes of Health project determined impaired sleep leads to a build-up of metabolic waste–proteins called beta-amyloid and tau–in the brain. These proteins play a significant role in the development of Alzheimer’s disease. In one of the latest research reports, appearing just last year (2022) in Sleep, investigators from Massachusetts General Hospital found that brain wave patterns during sleep are strongly associated with cognition.

Research also has demonstrated that even one night of lost or deprived sleep can result in an accumulation of beta-amyloid in the brain.

So, why the connection between sleep and cognition?

Sleep a Brain Cleanser

Noted neuroscientist Maiken Nedergaard calls sleep a brain cleanser. He discovered and studied the glymphatic system, a pathway allowing cerebral spinal fluid to enter the brain and clear it of waste products, including beta-amyloid, during deep (slow wave) and non-REM (rapid eye movement) sleep. That is the stage of sleep when cerebral blood flow volume falls.

Other researchers, writing in the journal Neurology, report a lower overall percentage of REM-stage sleep and REM sleep of longer duration as having a connection to a higher incidence of dementia. The authors stated,

Each percentage reduction in REM sleep [in the study] was associated with approximately a 9 percent increase in the risk of incident dementia (hazard ratio 0.91; 95 percent confidence interval 0.86, 0.97),

REM sleep is the stage in which dreaming occurs, and the brain consolidates memories and performs emotional processing.

According to 2023 numbers from the CFAH, approximately 6.5 million Americans aged 65 and older live with Alzheimer’s disease. Alzheimer’s is a primary contributor to what health professionals sometimes refer to as dementia because the disease leads to the deterioration of cognition, memory, focus, and attention.

That individuals develop Alzheimer’s disease and other variants of dementia in the latter phases of life are not surprising, considering the connection between sleep and neurological decline. As people age, their sleep patterns change. Sleep becomes of shorter duration, and the amount of deep, slow-wave sleep decreases.

In a 2019 issue of Science Translational Medicine, researchers determined that older individuals, still “predominantly cognitively normal” but experiencing less slow-wave, non-REM sleep, have higher concentrations of tau in their brains. The authors noted such changes in sleep patterns might prove to be biomarkers for encroaching Alzheimer’s disease and allow for treatment intervention “either before or at the earliest stages of symptomatic Alzheimer’s disease.”

Should We Be More Concerned About Quality–Not Quantity?

Of course, all this discussion about sleep begs the question: how much sleep should the average individual get? In research published in an April 2022 issue of Nature Aging, scientists concluded seven hours of daily sleep is optimal for a healthy life, superseding earlier beliefs that eight hours of sleep–and up to nine hours for those between 26 and 64 years of age–was ideal.

But rather than focusing on the number of sleep hours, should we be more concerned about the quality of our sleep architecture? An individual will cycle through four to six sleep stages during a normal sleep period. But, as a person ages, more time is spent in lighter rather than deeper sleep. Does quality then override quantity if deep non-REM sleep–not the lighter stages– is needed for optimal functioning of the brain’s glymphatic waste-clearing system? Future studies may provide more answers.

Tips for a Healthy Sleep

Meanwhile, the National Science Foundation and other experts offer these general tips for achieving a healthy rest:

• Maintain a regular sleep schedule. Go to bed and get up at the same time every day.
• Stop constantly checking the time when you are in bed and worrying about falling asleep. The anxiety will only hinder sleep. Turn the clock face down or put a cloth over it.
• Improve your sleep hygiene. Darken the room, lower the temperature, move the television out of the bedroom, put the mobile phone down an hour before bedtime and place or plug it where you cannot see it, and, if necessary, buy a new mattress.
• Exercise regularly.
• Avoid alcohol consumption and caffeinated drinks several hours before bedtime.
• Keep away from bright lights shortly before going to bed. Bright lights only promote wakefulness.
• If experiencing difficulties falling asleep, get out of bed and read or do some other relaxing activity until becoming drowsy.
• Depend on natural rather than medicated sleep. Some experts contend that using sleeping pills is associated with higher risks for mortality and cancer.

Finally, contact a sleep medicine specialist or psychiatrist for help if you have a sleeping disorder. In the words of the author Arianna Huffington: “The way to a more productive, more inspired, more joyful life is getting enough sleep.”