And a new study published September 21 in the Journal of the American Geriatrics Society has provided further insights into sleep’s role in dementia.
Sleep can influence physical and mental health and is linked to conditions from heart disease and stroke to depression and obesity.
Researchers in China, Sweden, and the UK looked at sleep data of 1,982 Chinese individuals with an average age of 70 none of whom presented with symptoms of dementia at the start of the study.
An average of 3.7 years later, 97 participants (5%) had been diagnosed with dementia according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) criteria. Those primarily impacted were aged between 60 and 74. Men were also at higher risk, which contradicts what many other dementia researchers have previously found.
“In most studies, women are known to have a two-fold greater risk of dementia than men. Unusually, this study found the opposite,” shared Dr. Alex Dimitriu, double board-certified in psychiatry and sleep medicine and founder of Menlo Park Psychiatry & Sleep Medicine and BrainfoodMD.
The study found that longer time spent in bed (TIB) was associated with significantly increased dementia risk. Those in bed for more than 8 hours were far more likely to show a cognitive decline during a Mini-Mental State Examination (MMSE) — a test used to measure cognitive impairment.
“As we get older, we see a fragmentation of the sleep states,” Dr. Michael Breus, a sleep specialist, and clinical psychologist told Healthline. This means “that we don’t seem to get the same type of physically restoring sleep (stages 3/4) as we did when we were younger.”
As such, “it’s possible that people with poorer quality sleep may require more sleep time to compensate,” added Dimitriu.
Other factors can play a role, too, explained Dr. Carl W. Bazil, Ph.D., and Caitlin Tynan Doyle professor of neurology at Columbia University College of Physicians and Surgeons.
Depression (for which older adults are at greater risk) can make sleeping difficult. But there are also many other medical conditions (such as heart disease or diabetes) and the medications taken for them that can increase tiredness and sleep requirements.
The study authors hypothesized that earlier bedtimes could be driven by disrupted circadian rhythm.
“Parts of the brain responsible for managing sleep start to change as we age. This impacts our circadian rhythm cycles,” said Dr. David Rabin, Ph.D., a neuroscientist, board-certified psychiatrist, and co-founder of Apollo Neuro, a wearable device for stress relief.
Age-related factors, such as having to use the bathroom more frequently during the night, also impact us getting good quality and deep sleep, continued Rabin. Accumulative sleep deprivation results in a change in brain structures that regulate circadian cycles. Other influences might also be at play, stated Dimitriu
“It is possible that people with early stages of dementia experience earlier brain fatigue in the day, leading them to want to sleep earlier,” he said. “‘Sundowning’ is a well-known effect in older people prone to dementia, where they can become confused and disoriented in the evenings.”
One of the main drawbacks to the research is that TIB doesn’t necessarily reflect the time spent asleep. Sleep duration has been noted by scientists as a crucial factor in cognitive health and dementia risk. Breus stated that longer TIB could indicate an underlying sleep-related issue, such as insomnia, which “could be affecting this situation and making it worse.”
A recent Canadian study also highlighted that those with insomnia were at higher risk of memory loss. Furthermore, TIB doesn’t consider the quality of a person’s sleep — also regarded as important in cognition and dementia. For instance, not getting enough deep sleep can greatly impact memory (more on this later). There’s one final consideration to bear in mind.
“This study, as well as many others like it, are association studies and, as such, do not show cause and effect,” explained Bazil. “So it is never clear whether the observed association (in this case, short or long times in bed or sleep onset time) actually causes dementia, or is indirectly related to it,” he added.
A key signal of dementia is memory loss. However, in all stages of life, we know that quality sleep is required for many, if not all kinds of memory, Bazil explained. With regard to memory, two main actions occur during sleep.
The first is the processing and ‘storing’ of memories.
Short-term memory is initially stored in the hippocampus when it comes into the brain, which is the area where information is stored for short-term recollection and use, explained Rabin. When we sleep, information from the hippocampus gets passed into the higher cortical structures of the brain that allow it to become long-term memory and integrate with memories, he continued.
Rabin revealed this process is called memory reconsolidation and is particularly impacted by poor quality REM sleep or shorter sleep duration.
Second, sleep is when our brains clear out harmful toxins that, over time, can impact memory.
“When the brain is active during the day, it produces a lot of what we call ‘reactive oxygen species’ or inflammatory waste products,” stated Rabin. “When the brain is asleep and able to recover, especially in deep and REM sleep states, [it] detoxifies and removes inflammatory waste products.”
A build-up of toxins ultimately puts extra stress on the brain and prevents it from achieving memory reconsolidation. In summary, sleep quality, as much as sleep quantity, may be important, said Dimitriu.
This study monitored dementia onset in older individuals — the time of life when its symptoms are most likely to arise.
“Dementias, like Alzheimer’s Disease, often present with symptoms [among people] in their 60s, although early onset in the 40s or 50s may occur,” Dr. Sandra Petersen, senior vice president of health and wellness at Pegasus Senior Living. She continued: “Dementia is an ‘umbrella’ term for a group of diseases, of which Alzheimer’s is the most prevalent, in which progressive changes take place in the brain.”
Petersen explained that common signs and symptoms of dementia are:
- Persistent and pervasive difficulty with memory, cognition, and ability to perform everyday tasks
- Loss of focus
- Inability to pay attention
- Loss of language skills
- A decrease in visual perception
- A loss of problem-solving skills
- Impaired reasoning and judgment
“Researchers have considered a number of possible causes of dementia,” said Petersen. “We don’t know for sure, but it is likely a combination of factors that contributes to [its] development and progression.”
She revealed scientists hypothesize dementia may arise from:
- Inflammation resulting from poor sleep, poor diet, lack of exercise, and other unhealthy habits
- The occurrence of abnormal ‘tau’ proteins in the brain
- Genetics
- Untreated and prolonged depression
- The brain’s inability to use insulin correctly
Sleep has long been linked to dementia. Poor slumber is believed to increase risk, while those with dementia often struggle to obtain a sound and restful night of rest.
More research is required into how TIB and bedtimes may influence dementia onset.
Parvathy Sukumaran 
