▎ WuXi AppTec content team editor
Global burden of disease data shows that there were 57.4 million people with dementia worldwide in 2019, and this figure is expected to reach 150 million by 2050. Alzheimer's disease (AD), which accounts for 60%-70% of dementia cases worldwide, is one of the major health challenges of the 21st century.
Neurodegenerative lesions that cause dementia, especially Alzheimer's disease, are usually progressive and have developed for decades before the patient's disability. Previous studies have confirmed that patients experience 10 symptoms more frequently in the first 15 years of diagnosis, including major depressive disorder, anxiety, severe stress and adaptation disorders, hearing loss, constipation, spondylosis, abnormal weight loss, malaise and fatigue, memory loss, syncope, and prostration, long before the onset of Alzheimer's disease.
A recent study published in Alzheimer's & Dementia confirms that early progression in Alzheimer's disease accelerates changes in daytime sleep in older populations, and that the relationship can be two-way. The results corroborate another large-scale study published in Nature Communications that people who often don't sleep less than 6 hours in middle age have a higher risk of Developing Alzheimer's disease and dementia later in life.
Screenshot source: Alzheimer's & Dementia
Sleeping more during the day is at high risk of dementia!
Daytime sleep supplementation, such as napping, is a common habit in the elderly, but there is insufficient research evidence on the impact of daytime sleep on cognitive health. A paper published recently in Alzheimer's & Dementia reports the association between daytime sleep and Alzheimer's disease in older populations.
The researchers analyzed data from a Chicago study called the Rush Memory and Aging Project, which included a total of 1401 subjects (median age at baseline was 81 years old) and were followed up for more than 14 years. Subjects recorded their daily activities by wearing watch-like devices. Daytime sleep is defined as prolonged inactivity from 9 a.m. to 7 p.m.
The results of the analysis show that:
Subjects who slept more than 1 hour a day had a 40% higher risk of Developing Alzheimer's disease than subjects who slept less than 1 hour per day;
Subjects who slept at least once during the day had a 40% higher risk of Developing Alzheimer's disease compared to subjects who slept less during the day;
The correlation between daytime sleep and Alzheimer's disease persists after correcting for factors such as sleep quality, daily rhythms, comorbidities, therapeutic drugs, and APOE4 allele carrying status;
A two-way relationship was shown between daytime sleep and cognitive health: longer and more frequent daytime sleep was associated with poorer cognitive function after one year, and poorer cognitive function was associated with more daytime sleep after one year.
Overall, as subjects age, the length and frequency of daytime sleep tended to rise, and the progression of Alzheimer's disease accelerated this change, more than doubling the time and frequency of daytime sleep year by year. In addition, daytime sleep and Alzheimer's disease may be pushing each other to produce changes.
The researchers stress that the study is "the first to confirm a two-way relationship between daytime sleep duration (based on objective records) and Alzheimer's disease."
Although there is currently insufficient evidence to establish a causal relationship between the two, i.e. whether daytime sleep has an impact on cognitive health, or whether excessive daytime sleep itself may be a signal of accelerated aging or accelerated decline in cognitive function. However, after adjusting the length and quality of nighttime sleep, the association between excessive daytime sleep and dementia still exists, suggesting that the role of daytime sleep itself is very important and that this effect is not related to nighttime sleep.
Possible mechanism: pro-awakening neuronal injury
The findings of this study dovetail with the findings of the latest study published in JAMA Neurology. According to the latter study' hypothesis, daytime sleep is associated with the risk of developing dementia, and the underlying mechanism may be that under the combined effect of amyloid-related injury and obstructive sleep apnea, individuals begin to fragment at nighttime sleep, and poor nighttime sleep quality causes the corresponding individual to always feel not fully awake during the day and choose to supplement sleep at some point during the day.
▲"Awakening neurons" have begun to degenerate in the early stages of Alzheimer's disease (Image source: 123RF)
In addition, the study authors emphasize that because "pro-wake neurons" have begun to degenerate in the early stages of Alzheimer's disease, this part of the population may not be tired during the day because they do not sleep well at night, but because the system in the brain that keeps them awake disappears.
Based on objectively evaluated sleep data and autopsy results, the team confirmed their suspicion that the reason Alzheimer's patients need daytime sleep is that the disease has damaged "arousal neurons", and the cause of the damage is most likely due to the accumulation of tau protein, not because of amyloid deposition.
The dangers of lack of sleep in middle age
A large observational study published last year in Nature Communications suggests that the relationship between sleep and the onset of dementia in later life may still exist even in relatively young people.
Screenshot source: Nature Communications
After following nearly 8,000 UK adults for up to 25 years, the researchers found that those who often slept less than 6 hours in middle age were at higher risk of developing Alzheimer's disease and dementia in later life. Overall, people who slept less than 6 hours per night in middle-aged and old age (between 50-70 years) had a 30% increased risk of developing dementia in old age compared to those who slept 7 hours a night for a long time.
The study also analyzed a number of possible influencing factors and found that the association between sleep duration in middle age and the risk of late-onset dementia was not related to psychiatric disorders, cardiometabolism or other known dementia-related chronic diseases. In addition, there was no significant association between nighttime sleep duration ≥8 hours and the risk of developing dementia.
Although the study, as a correlation study, cannot yet establish a causal link, the data tells us that sleep may be important for middle-aged brain health. Future studies may further determine whether improving sleep habits can help prevent dementia, but even if we don't have definitive results right now, there's always nothing wrong with staying asleep.