If inadequate sleep is bad for Alzheimer's, what about too much sleep?

Too much sleep is probably bad too.  So says a study just published in JAMA Neurology, "Association of Short and Long Sleep Duration With Amyloid-β Burden and Cognition in Aging."

We know that a history of inadequate sleep may promote the development of Alzheimer's disease, so the Alzheimer's Association and other authorities tell us to ensure we get enough.  But the older we get, the harder it is to sleep.  We discussed "the glymphatic system" in Beating the Dementia Monster, explaining how deep sleep enables the brain to clear cell waste and debris -- waste and debris that promote Alzheimer's disease if left in the brain.  The waste and debris include beta amyloid, which is ever present in Alzheimer's disease.  In his book Why We Sleep, Dr. Matthew Walker discusses what we have learned about the fascinating phenomenon of the glymphatic system.  Some say that ensuring you get adequate sleep is second only to exercise among the tools available to you for supporting brain health.

Dr. Walker also noted that the difficulties older people have in sleeping has led to the misconception that older people don't need as much sleep.  The reality is that older people need just as much sleep as younger people.  We just have a harder time getting it.

I continue to be quite the insomniac, so getting above a minimum number of hours of sleep is something I am always working at.  But for a long time I have been often asked, is there harm in too much sleep?  This study proposes some answers.

The study drew from data accumulated for the clinical trial of the monoclonal antibody treatment, solanezumab.  (Say that three times fast.)  Solanezumab didn't fare well in it's clinical trial as an Alzheimer's disease treatment, but it nevertheless left us with a pile of useful data to help with other investigations.  It this case, we can correlate loading of beta amyloid in the brains of thousands of normal people with their performance on cognitive tests as well as with how much they slept each night.  ("Normal people," means that none of the subjects had Alzheimer's disease.)

The researchers reviewed data for about 4,500 participants from the US, Canada, Australia, and Japan.  The participants had a mean age of 71.3 years, with a 59/41% split between women and men.  All were cognitively normal at the beginning of the trial as measured by cognitive tests, and all had their beta amyloid loading measured by PET scans.  All self-reported their experience with sleep.  Subjects were then grouped by those who got six hours or less sleep per night, those who got seven to eight hours, and those who got nine hours or more.

So what did they find?

One:  Short sleep duration was associated with higher beta amyloid loading.
Two:  Short sleep duration was associated with reduced cognition that was mostly in memory domains.
Three:  Beta amyloid loading was similar between normal and long duration sleep.
Four:  Long sleep duration was associated with worse test performance across multiple cognitive domains.
Five:  Both short and long sleep durations were associated with higher body mass index, depressive symptoms, and daytime napping.

The moral of the story?  Do everything you can to get between seven and eight hours of sleep every night.  But don't overdo it.