If one were to compile a list of worries in the industrialized world, two would surely be near the top: that we don’t get enough sleep, and that as we age we are on an inexorable path to cognitive decline and neurodegenerative disorders, including Alzheimer’s disease.

It turns out the two worries are related, but the relationship between sleep and cognitive health is a complicated one.

Poor sleep is an enemy of mental sharpness, but a study recently published in the journal Neurology makes the case that getting lots of sleep may not be a path to cognitive health. In fact, the opposite may be true.

Simply put, the quality of sleep matters much more than the quantity, the researchers suggest.

The study focused on older women who were at least 82 and hadn’t yet experienced cognitive impairment. Researchers recruited women for the sleep study who had been dealing with fractures caused by osteoporosis. The participants then wore small sensors called actigraphy devices on their wrists to measure their movement and their periods of sleep and wakefulness. They had their cognition measured at the beginning of the study and then in follow-up tests.

The study identified three “sleep-wake change profiles”: stable sleep, declining nighttime sleep and increasing sleepiness. The women in the increasing sleepiness group were at twice the risk of dementia than those in the stable sleep group. Significantly, those in the high-risk group had “large increases in daytime and nighttime sleep duration.”

To understand the seemingly tangled relationship between sleep and cognitive decline, we spoke with Dr. Amy Amara, professor in the Department of Neurology at the University of Colorado School of Medicine. Dr. Amara, who sees patients in the UCHealth Neurosciences Center – Anschutz Medical Campus, was not involved in the Neurology study, but she has done extensive research in the relationship between sleep and neurodegenerative disorders, especially Parkinson’s disease.

What piques your interest in the study of sleep and dementia in older women?

“I’ve always been interested in the relationship between sleep and cognitive dysfunction, or cognitive decline,” Amara said. “In my own work, we found relationships between certain stages of sleep and Parkinson’s disease.”

The study published in Neurology notes a ‘potential bidirectional relationship between sleep and cognition.’ What does that mean?

It’s a “chicken-and-egg” relationship, Amara said. “We know that in neurodegenerative disease, brain changes happen long before we detect any clinical symptoms.”

The question: Does that neurodegeneration trigger disruptions in sleep, or do disruptions in sleep trigger neurodegeneration?

Which one happens first is a difficult question to answer, as the two may be intertwined, Amara said. As the study authors noted, “growing evidence” suggests that “sleep disruption may not only stem from but also contribute to neurodegeneration.”

The authors went on to write that “concurrent declines in sleep health and cognition may exacerbate one another, creating a vicious cycle and accelerating deterioration in both domains.”

What are some of the important brain changes caused by neurodegeneration that can disrupt sleep?

One example is changes in the brainstem, which is responsible for regulating arousal. That can lead to insomnia and fragmented sleep (alternating periods of sleep and wakefulness, rather than a solid stretch of uninterrupted, deep sleep), Amara said. Disrupted sleep patterns, like these, are common in people with Alzheimer’s and Parkinson’s diseases, she said.

Another important cause of sleep disruption is neurodegeneration of the suprachiasmatic nucleus, a brain structure that, among other responsibilities, helps us know when to wake up and when to go to sleep.

“It is our central clock, which is responsible for the circadian rhythm,” Amara said. “Neurodegeneration in that area can lead to abnormal sleep patterns.”

If neurodegeneration can lead to disrupted sleep, how can healthy sleep help to protect against cognitive decline?

Simply put, healthy sleep includes slow-wave or deep sleep, Amara said. She conducted her own studies in 2020 and 2021, investigating how exercise might help people with Parkinson’s disease to increase time sleeping deeply, and thus, sharpen their cognitive function. She leads another study, currently underway, that further explores the therapeutic possibilities in this area.

The idea is that slow-wave or deep sleep is tied to the work of the glymphatic system, a part of the central nervous system responsible for removing from the brain waste materials that jam nerve cell communication and interfere with cognition, Amara said.

The Neurology study noted that napping might be a contributor to cognitive decline. Isn’t it healthy to take naps?

“There are plenty of patients I encourage to nap if they are sleepy because we want people to have adequate amounts of sleep,” Amara said. “But there also certainly can be detrimental effects of napping, because if you nap a lot during the day, then you have less drive to sleep at night. It is important to limit naps to 20 to 30 minutes in length and to avoid napping late in the day (after about 2 p.m.).”

In addition, a sleep pattern fragmented by excessive napping can disrupt a person’s circadian rhythm and lead to insomnia, Amara said.

“You then nap more during the day, and the pattern perpetuates itself.”

The recent study found that the women at greatest risk of dementia had increased sleep times both during the day and at night. In that case, more is likely not better, Amara noted.

“It may not be just that they have an abnormal pattern of sleep because they are sleeping during the day, but maybe there is something disrupting their sleep at night,” Amara said. “So even though they are sleeping for many hours, they are not getting good, quality sleep.”

Were there questions that the Neurology study left unanswered?

Yes. One area of uncertainty centers on the use of actigraphy to measure sleep times, Amara said. She noted that the devices simply measure movement. That’s important, but the devices don’t tell the whole story of what happens while a person tries to sleep.

“We can’t tell anything about the quality of the sleep,” Amara said.

The study researchers attempted to assess “sleep efficiency,” which they defined as “a measure of sleep quality calculated as the percentage of time in bed at night that was scored as sleep.”

But as Amara noted, “actigraphy can’t tell you if that’s light sleep or deep sleep,” or even if a person is actually asleep or simply lying still.

In addition, the study did not control for sleep apnea, which has been linked to cognitive decline, Amara said. Sleep apnea interrupts breathing, which may cause intermittent periods of sleep and wakefulness, and less time spent in the crucial slow-wave phase, during which the glymphatic system clears the brain of waste, she explained. (Most people who have sleep apnea don’t know they have it. Learn how to detect sleep apnea and how to get help for it.)

That’s obviously a problem, but fragmented sleep could have an upside, she added – if an individual takes the repeated interruptions as a clue to intervene.

“Maybe they need a sleep study because they have sleep apnea, or they might need to exercise because that’s known to improve sleep and cognition,” she said. “That’s one of the things that I think is exciting about this area. There are ways that we can improve sleep.”

Have other studies used actigraphy to try to measure the amount of sleep during a 24-hour period?

Yes. Amara cited a 2013 study published in JAMA Neurology that investigated sleep patterns in nearly 700 people who did not have dementia, but carried two copies of a specific form of the APOE4 gene called APOE e4 homozygotes – an important risk factor for developing Alzheimer’s disease.

Researchers used actigraphy for 10 days at the outset of the study to measure the study participants’ “sleep consolidation,” which they defined as “the extent to which sleep is uninterrupted by repeated awakenings.”

They then evaluated the study participants for Alzheimer’s disease annually for a six-year period.

The study, which included autopsies of participants who died, concluded that better sleep consolidation lessened the effects of the APOE e4 gene – as measured by the density of nerve cell tangles in the brain, which are the calling cards of Alzheimer’s disease.

The study was important for two reasons, Amara said. It showed that the participants who had fragmented sleep at the beginning of the research were more likely to have cognitive decline over time. But the study also showed that those with the APOE e4 gene who had good, consolidated sleep were “protected from that decline in cognition, even though they had that genetic risk factor.”

Do the findings of that study relate to the Neurology study, and if so, how?

Yes. Both emphasize the value of good sleep in protecting cognition. In the Neurology study, the stable sleepers were at much less risk of developing dementia than those with increasing sleepiness. In the JAMA Neurology study, those with the best consolidated sleep were at the least risk of developing nerve cell tangles and Alzheimer’s disease.

“Even the people who had that genetic risk could protect themselves if they were good sleepers,” Amara said. “Not everybody can make themselves good sleepers, but there are definitely things you can do to improve it.”

Before we get to that, what are the important risk factors for cognitive decline?

They can’t always be controlled. The biggest risk is aging, Amara said. Women tend to live longer than men and are therefore more likely to develop dementia. They account for about two-thirds of people in the United States with Alzheimer’s disease. Some people carry a genetic risk for Alzheimer’s disease, but as the JAMA Neurology study suggested, good sleep – among other factors – can mitigate that risk.

Sleep apnea, as mentioned before, is also an important risk factor for cognitive decline. Others include smoking, high blood pressure, cardiovascular disease and stress.

On the flip side, what are some of the proactive steps people can take to improve sleep and preserve mental acuity?

People can manage each of the risk factors just outlined, sometimes with the help of medications, exercise, and diet. For stress, Amara said she encourages her patients to focus their thoughts and attention on “support structures,” like family and physical activity, to healthily manage the pressure, avoid feeling overwhelmed and sleep better.

“Cognitive behavioral therapy is a great way to approach that,” either by talking one-on-one with a therapist or taking advantage of online resources and apps, she added.

“None of us can have a stress-free life, but we have to figure out the best ways to respond to it,” Amara said.

Not surprisingly, an active mind provides a bulwark against cognitive decline, she added.

“We think of mental activity as being important for cognition in general,” Amara said. “But it also increases the drive to have deep sleep.” As she discussed, it’s during that stage of sleep that the glymphatic system “prunes and cleans” the connections between brain cells that are the key to clear cognition.

“Exercise also increases deep sleep, so physical activity is important as well,” Amara said.

Amara emphasized that people don’t tread a predetermined path to dementia. Age is a risk, but many people reach old age with their mental acuity intact. The Neurology study “definitely supports that if you can improve your sleep in your 80s, you might prevent yourself from developing dementia, even that late,” she said.

Likewise, people with a hereditary predisposition to cognitive decline never suffer a precipitous mental slide.

“Even if you have genetic risk, you won’t necessarily get dementia,” Amara said.

But she offered one cautionary note.

“There is also some pretty good evidence that how you behave in your middle-age years, and even younger, is also important,” she said. “It’s never too early to get started and try to reduce your risk.”

So, younger people who may have sleep apnea or who are dealing with insomnia should try to resolve their sleep challenges in hopes of avoiding cognitive problems later in life. (Get practical advice about how to sleep better.)