A new study finds a surprising bidirectional connection between excessive daytime napping and Alzheimer’s disease.
Interview by Alyx Arnett
Older adults may doze off unexpectedly, and this tendency toward napping is more frequent in people with Alzheimer’s disease. Are excessive naps only a consequence of dementia, or has it happened prior to cognitive symptoms? More mechanistically, asks Peng Li, PhD, is napping induced by the pathological changes in the brain, or does the excessive sleep also promote such pathological changes? “These are all questions that await answers,” says Li, of the medical biodynamics program in Brigham and Women’s Hospital’s Division of Sleep and Circadian Disorders.
Li and co-investigators have now provided some answers: They found a bidirectional, longitudinal link between excessive daytime napping and dementia.
Li discussed the study with Sleep Review over email. The transcript has been lightly edited for clarity and style.
[Editor’s Note: Read the full study, Daytime napping and Alzheimer’s dementia: A potential bidirectional relationship, in Brain Communications.]
What inspired you to study the connection between sleep and Alzheimer’s disease?
Sleep is a universal need of all higher forms of life. It serves a vital function. The active neurobiological processes during sleep directly influence our next day’s energy levels and mental activities. This gives a short-term connection between sleep and cognitive activity. Then, how about the long-term? This is not a new question but has been a hot research topic since decades ago. Scientists have tried to answer the long-term interaction between sleep and Alzheimer’s development and pathology in animal models and humans, and the knowledge and understanding have been progressed quickly. We would like to further advance the understanding of the connection between sleep and Alzheimer’s from a perspective of the circadian or daily oscillation that lives on our planet have evolved with. We’d like to separately study how sleep at different times of a day link differently to the risk of Alzheimer’s or cognitive trajectory in the aging process.
What surprised you most about your findings?
Based on many prior studies, we can speculate that daytime napping may be negatively linked with cognition. Our study supported this speculation. What is more unique of our study is that we modeled the longitudinal profile of objective daytime napping in older adults across an up to 15-year life span. We observed that daytime napping became longer and more frequent in these adults, and what is more interesting to us was that these aging-related changes were dramatically accelerated by the progression of Alzheimer’s dementia. What also surprised us was that daytime napping and cognition drove each other’s changes bidirectionally, the first evidence—to the best of our knowledge—that directly proves this relationship in a human cohort of older adults.
Can you explain how the bidirectional, longitudinal relationship between daytime sleep and Alzheimer’s dementia is a “vicious cycle”?
We found that daytime napping and cognition mutually predicted each other’s changes in the future. Specifically, older adults tended to nap longer and more frequently as they grew even older, and these changes sped up dramatically with the progression of Alzheimer’s dementia. Longer and more frequent daytime naps in cognitively normal adults predicted an elevated risk of developing Alzheimer’s dementia in the future. By modeling their longitudinal changes simultaneously, with a consideration of directionality, the results suggested that longer/more frequent daytime naps were associated with worse cognitive performance in the following year, and worse cognitive performance predicted even longer/more frequent daytime naps in the subsequent year. Basically, this gives the reason why we called it a “vicious cycle”—the two processes drive each other’s changes in this bidirectional manner.
However, can we draw a causal relationship that they “cause” each other’s changes? The short answer is that it’s too early to say so. It is possible that changes in daytime napping behavior may lead to or be a result of disruptions in nighttime sleep or our circadian clocks, as occurred in shift workers. Circadian and nighttime sleep disturbances are both known to link with Alzheimer’s disease. Excessive naps may also be a sign of other underlying pathophysiological changes that cause cognitive decline and dementia. Interventional studies are required to determine whether daytime naps have any causal impacts on cognitive changes.
Also, let’s just be a bit broader. What further fuels the development of Alzheimer’s? A well-accepted mechanism of Alzheimer’s disease is the abnormal buildup of toxic proteins in and around cells in the brain. These include β amyloid and hyperphosphorylated tau. The deposition of β amyloid creates plaques around the cells, and tau creates tangles within the cell. Plaques and tangles will destroy the functions of neurons by affecting, for example, the metabolism of cells and the communications between cells that are vital roles. There are many processes that may trigger the accumulation or affect the normal clearance of these proteins. It is multifactorial, involving genetics, lifestyle factors, and environmental factors. And these processes may further be affected by the deposition of β amyloid and tau, resulting in a loop that worsens each other—“vicious cycles” that work together toward adverse trajectories with aging in all these factors.
Based on these findings, what should people know about napping?
If napping is a long-term habit that boosts your vigilance and is restorative, and it does not affect one’s sleep during night, it should be fine to continue this habit. However, if one identifies drastic changes, they may want to seek medical attention.
It also depends on what time of the day and how long one naps. Many sleep scientists or doctors recommend power naps, naps that are short—less than 30 minutes. A short sleep during the early afternoon hours may be enough to be refreshable and boost alertness for afternoon jobs. However, if one wakes up with grogginess from a nap and feels even more drowsy, it is highly possible that they have napped too long and woken from a deep sleep. This is called sleep inertia is also a sign that people may think of shortening their daytime napping.
Daytime napping prevents the buildup of sleep drive. If one finds it difficult to fall asleep during nighttime, which seems to link to their napping behaviors, this may also be a signal that they may want to cut their daytime napping first.
Many behaviors can feed back to our circadian clocks, so it is better if people keep a regular daily schedule, including both nap and sleep behaviors.
What are the clinical implications of your research?
This study calls for closer attention and reinsures the importance of sleep health, not only the typical referred sleep during nighttime but also daytime naps. Primary care doctors may want to suggest their patients pay more attention to changes in their sleep behavior. They may also want to recommend their patients maintain a regular daily behavior pattern, including naps and sleep, and seek further medical advice if they or their close family members recognize drastic changes in the sleep behaviors.
What further research should be done?
The gold standard for sleep assessment is polysomnography, which should be used for a better understanding of the sleep architecture during daytime and its link with Alzheimer’s. Our study also calls for technical advances to increase the accuracy of actigraphy-based sleep scoring to promote the use of actigraphy in long-term monitoring of sleep patterns, especially in older adults.
Other limitations of this study, which implies what other research needs to be done, include the uncertainty of whether increased napping is the result of nighttime sleep changes and whether planned and unplanned naps have different implications.