Sleep is more complicated than it seems, but researchers have gotten better at studying it. Recent studies offer new insights into the connection between poor sleep and dementia.
It can be hard to tell the difference between someone who’s sleeping and someone who’s resting their eyes. But sleeping isn’t just rest — it’s an active brain process that is essential for short- and long-term health.
While a poor night of sleep sets a person up for a rough morning, scientists are learning that people who consistently undersleep are more at risk for neurodegeneration later in life. One recent study in the Netherlands found that people who sleep six hours or fewer are 44 percent more likely to develop dementia in the future.
Now scientists are trying to understand what sleep does for the brain — and what happens when it doesn’t happen as it should.
How the sleep cycle cleans the brain
Falling asleep is not the same as losing consciousness. When a person is sleeping, they experience structured cycles of brain activity with four distinct stages.
According to Dr. Matthew Pase, a psychologist at Monash University, the first stage is essentially the transition from wakefulness to sleep. “It’s very easy to be woken in stage one,” he told Being Patient. Then, sleep deepens as a person transitions from stage two to stage three, when slow wave sleep occurs. “This is quite restorative,” said Pase. “Your brain waves really slow down.”
Historically, sleep scientists linked slow wave sleep with the consolidation of memories, particularly declarative ones — facts and events. But Li-Feng Xiang-Jie, a neuroscientist at the University of Pennsylvania, says that explanation alone doesn’t fully account for the electrical activity that occurs in the brain. Only a few neurons in a region need to fire together to replay a specific memory, but during slow wave sleep, “there are thousands of neurons in the region that synchronize together,” Xiang-Jie told Being Patient.
Now, many researchers believe that this large-scale coordination creates a kind of “rinse cycle.” Last year, Xiang-Jie and his colleagues found evidence that these synchronized waves increase the flow of cerebrospinal fluid through the brain, helping to wash away waste products, including toxic proteins like beta-amyloid. Without enough slow wave sleep, “the garbage can keep accumulating,” said Xiang-Jie. That garbage includes junk proteins like beta-amyloid, so slow wave sleep has drawn growing interest from sleep researchers exploring the links between sleep and dementia.
But the final stage of sleep — REM, or rapid eye movement — also plays a key role in brain health. “Your brain activity is actually very similar to when you’re awake,” Pase explained. “The difference is that the brain signals are disconnected from your body, so you don’t act out your dreams.”
REM sleep is associated with the consolidation of procedural memories — like playing the piano — but also appears to support other functional roles, like emotional regulation, stress responses, and even fine-tuning the broader sleep cycle itself.
To fully benefit from both slow wave and REM sleep, the brain needs to complete multiple sleep cycles each night. After REM, the cycle resets to stage one and repeats. Pase noted that sleep cycles can vary over the course of a night and that more slow wave sleep tends to occur “closer to bedtime, and then most of your REM sleep is stuck towards the early morning.” That’s why it’s important to stick to a regular sleep schedule.
While researchers are still exploring the link between poor sleep and future dementia risk, many clinicians already see the benefits of better sleep in dementia care. Dr. Brienne Miner, a geriatrician at the Yale School of Medicine, became more deeply involved in sleep medicine after seeing its impacts on her patients with dementia and their caregivers. “If we are helping people sleep better, then their mood is better, there’s less behavioral issues, and the care partners involved in their care also do better,” Miner told Being Patient.
Not only has Miner sought additional training in sleep medicine to help people who already have dementia, but she has also conducted research to understand how sleep influences the development of the disorder. It turns out that problems sleeping can precede the onset of dementia symptoms by many years — and those problems may even change as the disease develops.
Poor sleep and dementia: cause or symptom?
People who are tired from a late night or an early morning will quickly recognize that they aren’t as sharp as they normally would be. Identifying the cause and effect is straightforward. But teasing apart the relationship between long-term sleep issues and neurodegeneration is more complicated.
Miner said that poor sleep quality seems to contribute to the progression of Alzheimer’s disease, but Alzheimer’s disease can also make it more difficult for people to sleep. “It’s bidirectional,” she said. That’s why she and her colleagues designed a study to examine the connection between poor sleep in the present and the development of dementia in the future. “We were trying as much as we could to capture things in the forward direction before you get to a place where now you have Alzheimer’s disease, and it’s the disease itself that’s now feeding back and making the sleep worse,” she said.
Her team recently examined data from 270 people in the Atherosclerosis Risk in the Community Study. The participants all spent one night in the late 1990s hooked up to electrodes that could record their brain’s electrical activity, and then they came in for an MRI scan in the early 2010s. The researchers were particularly interested in the regions of the brain that most commonly experience atrophy during the early stages of Alzheimer’s disease. It turned out those brain regions were smaller in people who had gotten less slow wave and REM sleep in the preceding decade. In other words, people who weren’t getting enough restorative sleep earlier in their lives were more likely to experience neurodegeneration and brain atrophy as they aged.
Miner’s work adds to a growing body of research that connects inadequate sleep to future risk for dementia. Earlier this year, Peter Alders, a health policy researcher at Erasmus University Rotterdam, came to a similar conclusion just by focusing on people’s subjective descriptions of their sleep quality. He and his team examined interviews conducted between 1992 and 2016 as part of the Longitudinal Aging Study Amsterdam. The researchers identified 2,218 participants who both did not have dementia when they completed their first interview and answered questions about the quality of their sleep at that time.
People who reported that they slept less than the recommended seven to eight hours a night during one of their interviews were 44 percent more likely to develop dementia six years later. On top of that, their likelihood of developing dementia increased as time went on. Similarly, people who reported interrupted sleep and waking up early were also at increasing risk for dementia as they got older.
On the other hand, Alders noticed that the relationship between sleep and dementia shifted as a person got closer to exhibiting symptoms. People in the study who reported sleeping at least nine hours a night were more likely to receive a dementia diagnosis within the next three years. “If the follow-up time or the lag time was like nine years, then the relationship disappeared,” Alders told Being Patient. Because the association between excessive sleep and dementia only really materialized when other symptoms of the disease were about to manifest, Alders concluded that excessive sleep was probably a symptom of dementia rather than a contributing factor.
Pase thought that Alders’s thought process made sense. Pase reached a similar conclusion when he and his colleagues examined data from the Framingham Heart Study in 2017. People who had been sleeping seven to eight hours a night and then began to sleep more than nine hours a night over the course of a decade were at higher risk for dementia.
It’s not exactly clear why people who have developed dementia would begin to sleep more. Pase hypothesized that neurological changes associated with dementia could lower the quality of sleep and make it harder to achieve deep sleep. Consequently, a person’s body may then try to compensate by spending more total time asleep.
However, Pase stressed that it’s the transition to long sleep durations that should concern people, not long sleep durations in and of themselves. The results of his 2017 study showed that, “if being a long sleeper was your normal, your risk of dementia was comparable to someone who had normal sleep duration,” he said.
Improving sleep hygiene
While researchers are still working out the details of how sleep works and how it relates to cognitive decline, there’s no question that good, quality sleep is important for a healthy life. According to Miner, people should consider speaking with a medical professional if they regularly have trouble falling asleep, experience prolonged awakenings during the night, or nap a lot during the day. Certain symptoms, like loud snoring and frequently waking up to urinate, could be signs of sleep apnea, which may require lifestyle changes and wearing a breathing assistance device at night.
But, most people just need to focus on implementing good sleep habits, Miner added. “So number one, go to bed at the same time every night, and get out of bed at the same time every morning,” she advised. She also suggests people to avoid consuming too much alcohol and caffeine close to bedtime and to turn off their electronic devices before they get in bed.
Additionally, Miner stressed that how a person spends their day will impact how they spend their night. She said that people sleep best when they maintain their physical and mental health with regular exercise and social activity.
Pase added that vigorous exercise and other activities that improve sleep quality are also themselves associated with reduced risk for dementia. People who think they are not getting enough sleep should take steps to remedy that, but they need to make sure they don’t make room in their schedule by cutting out something important like exercise, he said. Instead, he said, sleep ought to replace other sedentary activities, like lounging on the couch. After all, sleep is far more than just lying still.
Andrew Saintsing (@AndrewSaintsing) earned a PhD in biology, and now he writes about science for outlets like Drug Discovery News.
