Napping is an activity typically associated with people at either end of the age spectrum. In young children, naps are considered important for brain development and function. But in the elderly, there is conflicting evidence as to whether napping similarly boosts cognitive function, or if it is a sign of cognitive decline. It turns out that not all naps are created equal, and the answer depends on whether or not the naps are intentional [1].
A recent study following older adults for up to 14 years found that with advancing age there was an associated increase in the duration and frequency of naps [2]. This age-associated increase in napping was exacerbated in the context of dementia, such that the degree of daytime napping more than doubled in individuals with Alzheimer’s disease. Naps became increasingly longer and more frequent with the progression of cognitive impairment. Multiple studies suggest that increased daytime sleepiness in older adults may be an indicator of future cognitive decline, and that excessive napping is associated with increased risk for Alzheimer’s disease [2; 3; 4; 5].
Unintentionally nodding off during the day after a bad night’s sleep is a normal biological response, and not a cause for concern. It becomes a potential warning sign when it happens with increasing frequency in a manner that is unrelated to the amount of sleep at night. In many of these studies, the individuals who napped the most also slept longer. Research suggests that the inability to stay awake is due to the loss of cells in the part of the brain that controls sleep- wake activity [6]. Excessive sleepiness may be evident prior to cognitive impairment because this is one of the earliest brain regions to be affected by Alzheimer’s pathology.
It should be noted that excessive daytime sleepiness is also associated with chronic conditions other than dementia [7]. Inflammation is a potent inducer of sleep, such that excessive sleepiness is a common feature of inflammatory conditions, and many of these same conditions, such as cardiovascular disease and depression, are themselves associated with increased risk for dementia.
In contrast to the connection between unintentional napping and cognitive decline, there is ample evidence to indicate that a regular intentional napping practice can improve cognitive performance in adults of all ages. A grouped analysis of 11 studies examining the effect of a short mid-day nap on cognitive performance found that napping boosted performance on all tested cognitive measures, including memory, attention, and executive functions (i.e. planning) for at least two hours after the nap [8]. These studies were all performed in young adults, but observational studies have found similar benefits to cognition in adults aged 60 and older [1; 9]. An intentional short napping practice, less than 30 minutes, has been associated with a decreased risk for cognitive decline in older adults [10]. However, similar benefits have not been seen in this population with long naps [4; 5]. This is likely because the need for long naps may be an early sign of dementia, and long naps may interfere with evening sleep.
The biological mechanism of napping’s cognitive benefit is not fully understood. It is thought to involve many of the same brain restoring processes of evening sleep, such as clearing away the buildup of materials in the brain generated by cognitive activity, and reducing stress [10].
THE IDEAL NAP
As mentioned, not all naps are created equal, and following best practices will help ensure maximum cognitive benefit from napping.
How Long: Naps less than 30 minutes have consistently shown cognitive benefits. There are four stages of sleep [11]. Within this 30-minute time frame, an individual will stay within the first two stages of sleep, while naps that are 60 to 90 minutes can go through all four stages. These later stages are important for learning and memory. A longer nap after learning a new task may benefit task performance, but unlike the immediate boost from a short nap, these learning-related benefits aren’t apparent until the next day [9]. Naps that go into these later stages increase the chance for grogginess, leading to a temporary decrease in cognitive performance immediately following the nap. Additionally, naps longer than two hours may disrupt the sleep cycle, resulting in insomnia.
When: The body has a natural rhythm, and the restorative capacity of a nap is determined by that rhythm. While the exact time may vary from person to person, an early afternoon nap between 1 to 3 PM has been shown to be the most cognitively restorative for most people [8]. Naps taken outside of this window are more likely to result in grogginess. Additionally, napping after 4 PM can negatively impact evening sleep quality.
While naps cannot substitute for a quality night’s sleep, they can help enhance the brain boosting powers of sleep to stay mentally sharp.
Betsy Mills, PhD, is a member of the ADDF's Aging and Alzheimer's Prevention program. She critically evaluates the scientific evidence regarding prospective therapies to promote brain health and/or prevent Alzheimer's disease, and contributes to CognitiveVitality.org. Dr. Mills came to the ADDF from the University of Michigan, where she served as the grant writing manager for a clinical laboratory specializing in neuroautoimmune diseases. She also completed a Postdoctoral fellowship at the University of Michigan, where she worked to uncover genes that could promote retina regeneration. She earned her doctorate in neuroscience at Johns Hopkins University School of Medicine, where she studied the role of glial cells in the optic nerve, and their contribution to neurodegeneration in glaucoma. She obtained her bachelor's degree in biology from the College of the Holy Cross. Dr. Mills has a strong passion for community outreach, and has served as program presenter with the Michigan Great Lakes Chapter of the Alzheimer's Association to promote dementia awareness.
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