I was hoping to keep COVID-19 out of this piece, but new things keep flooding in, and I can’t totally ignore them. However, I’ll save them to the end of the story. They are both “good news,” more or less, and I like to leave you with a bit of cheer, when possible.
Sleep and dementia/Alzheimer’s disease
The first comment that I’m addressing was in response to the recent piece about Alzheimer’s disease: “I’d like to know Doc Gumshoe’s thoughts about proper sleep and the link to AD.” Initially I thought that lack of “proper sleep,” whatever that might be, could be lumped in with Lancet’s “population attributable factors” such as hearing loss, hypertension, obesity, smoking, physical inactivity, and others. The Lancet authors thought that those factors were the cause of more than a third of all dementia cases. Not getting enough sleep, thought I, was just another aspect of an overall unhealthy lifestyle.
But then I did a bit of digging around and what I found was something of an eye-opener. It turns out that several careful studies have been conducted, and that not only has the connection between sleep and dementia been convincingly demonstrated, but that a specific mechanism has been identified and tracked.
Here’s what I found:
The underlying reality is that there is a well-recognized association between sleep and cognitive function. Fundamentally, sleep plays a vital role in learning and memory. Apparently, we take in a torrent of information while we’re awake and then sort it and file it during our sleeping hours. Then, when our little brain cells aren’t being flooded with new incoming perceptions that we take in through our senses, they have time to process it so that we can access it when we need it. When we don’t get enough of the right kind of sleep – deep, dreamless sleep in contrast with rapid eye movement (REM) sleep – our brains just don’t do a good job of processing the information they take in.
A study quantifying this effect was published in Nature Communications (Sabia S et al; 2012; 12, no. 2289). It was a very long-term study, in 7959 participants, followed for 25 years. Of these, 521 subjects developed dementia, The overall finding was that consistently sleeping less than 7 hours per night was associated with a 30% increased risk of dementia, independent of any other factors that might cause dementia. The increased risks varied with the ages of the subjects. Subjects in their 60s had the highest increased risk of dementia (37%), followed by those in their 70s (24%) and those in their 50s (22%). The results were judged to be statistically significant (P < 0.02).
When the study subjects were divided into thirds based on their hours of sleep duration , the increased risk for dementia in the subjects whose sleep duration was in the first tertile, (under 6 hours) was 63%. At the other end of the scale in the highest tertile, there was a very slight, statistically insignificant increased risk of dementia in subjects with sleep duration of up to 10 hours.
The lowest incidence of dementia was seen in subjects whose mean sleep duration was 7 hours.
But that’s by no means the whole story. As I poked around more, I came across several studies that seem to point to the reason that short sleep duration is linked with dementia. It appears that during deep sleep, the brain washes away the waste products that increase the risk for Alzheimer’s disease. A study led by Matthew Walker, a professor of neuroscience at U. C. Berkeley, found that poor sleep led to increased accumulations of amyloid beta (Aβ) and toxic tau, both of which are associated with Alzheimer’s disease. (Curr Biol 2020;30:21-4291-4298)
In the study, Walker’s team followed 32 subjects in their 70s who had taken part in a sleep study that looked for the slow electrical waves that signal deep sleep. The study used brain scans to monitor the levels of Aβ in each study subject over the six-year duration of the study. The study demonstrated a clear correlation between the amount of deep sleep and Aβ – the less sleep, the more Aβ accumulated.
A previous study, led by Laura Lewis at Boston University, demonstrated how during sleep waves of fluid wash through the brain. During the deep sleep period, the brain behaves like a dishwasher, and these waves of fluid get rid of the waste that builds up during the day – substances like Aβ and tau. These waves are preceded by a large, slow electrical wave. Lewis notes that these waves can be induced in mice, and may be able to be induced in human subjects.