Too much of a good thing? Rip Van Winkle was at risk for dementia.

As we age, otherwise healthy adults sleep progressively less. This graph (Mindell & Owens, 2015) models typical time awake versus asleep over the human lifespan. Combined total time asleep (consisting of REM and non-REM sleep) decreases from 8 to 6 hours, as we progress from 20 to 60 years old. This is not to concretely state that an older brain would not benefit from a return to 8 hours of sleep (in fact, many older individuals rely on afternoon naps to probably make up for the decline). Dr. Matt Walker argues in his recent book "Why We Sleep," that this phenomenon may be akin to bone density declines in older age, it is typical, yes, but not desirable - thus we attempt to intervene and supplement.

In concert with this change in sleep behavior, as adults reach their mid-to-late 60's, the incidence of mild cognitive impairment (MCI) begins to increase. MCI can be thought of as a "pre-dementia" condition, an important diagnosis because of the significant likelihood of these patients developing a full dementia syndrome (most commonly due to Alzheimer's disease). Researchers have noted that sleep characteristics are different in people with MCI. The chart below (D'Rozario et al. 2020) summarizes these changes: people with MCI, compared to their peers without MCI, tend to have less sleep efficiency (i.e., less time in bed is spent asleep) and their sleep consists of more "light" non-REM sleep and less REM sleep.

The common interpretation of this finding is that the neuropathology causing MCI is also disrupting the brain's sleep-wake system. This is in fact supported, given that in large longitudinal studies, adults between 50-80 have almost 3x the risk of developing MCI over 5 years if they sleep 5 or less hours each night (Brachem et al., 2020). However, more recent research has suggested the reverse could ALSO be true, i.e., sleep disruption may be contributing to the build up of the neuropathology.

How? A recently discovered waste-clearance function in the brain appears to be primarily active during sleep and shuts down during wakefulness (see figure below, Hauglund et al. 2020). This waste-clearance has been credited to the GLYMPHATIC system, from "glial" (type of neuron) + lymphatic (the body's waste and immune transport system). So, while in the short-term a night of poor sleep might cause you to have trouble concentrating the next day (which could be fixed with a good nap), over the long-term (say, years or decades even) chronic sleep disturbance could result in a backlog of metabolic waste accruing in the brain - we think this increases the risk of neuropathology like Alzheimer's disease.

So, does that mean the goal in middle-age is to sleep more? If you are sleeping 9, 10, or even 11 hours each night, are you at lower risk of developing MCI (and becoming at-risk for dementia)?

Counter-intuitively, the opposite may be true - at least when observing the more extreme "sleep overachievers." Researchers (Low et al., 2019) have found that in middle-to-late adulthood, 9+ hours of sleep per night is associated with cognitive deficits that look like MCI. (note: while those with very little sleep, say 2-5 hours, also performed poorly, this finding disappeared after accounting for confounds like sleep disorders, depression, and certain medications, i.e., fixable factors).

Why is this the case? The likeliest answer is that middle-to-older adults who are sleeping for long periods at night are experiencing worse *quality* of sleep, they are likely not getting enough slow-wave sleep ("deep sleep") and/or REM sleep. This can be due to untreated sleep disorders (e.g., sleep apnea, insomnia), poor sleep hygiene (e.g., pets in the room), medications that interfere with sleep physiology, and potentially the early signs of certain degenerative neuropathologies (e.g., MCI due to very early signs of Alzheimer's disease). The result is the person sleeps longer to obtain the same restorative effects as their peer with good quality sleep.

So, while sleep is critical to brain health, for the average middle-to-older adult, a "Goldilocks" sleep goal is best, ~6.5 to 8.5 hours a night. More than 9 hours or less than 5 hours suggests something about your sleep may be amiss (especially if accompanied by excessive daytime sleepiness, acting out dreams, and/or heavy snoring), and talking with your physician should be considered.