Why is sleep so important?

In Beating the Dementia Monster, I emphasized the importance of getting a good night's sleep every night.  For younger people, this is a challenge, because so much is going on in their lives.  For us older folks, our physiology seems to get in the way.  Even if we allocate enough time to sleep, getting to sleep and staying asleep is increasingly difficult.

The quality of sleep is important.  REM sleep -- when you dream -- does not appear to provide the benefit of deep sleep.  (REM stand for "rapid eye movement."  Your eyes are moving around when you are in REM sleep, and you are dreaming.)  Analysis of amyloid concentrations in people who have been sleeping indicates that deep sleep does more to control amyloid than REM sleep or no sleep.

Sleep is so important!  Why?

The first question is, why do we sleep in the first place?  For a long time, it was assumed that there was some sort of repair process for the body that occurs during sleep, but no one could find any repair processes going on.  This led to speculation that it was primarily to provide an opportunity for consolidation of mental activity or to keep diurnal animals safely sleeping in hiding places during the night.  Or, for that matter, nocturnal animals would find a safe place to sleep during the day.

These ideas struggled for confirmation, so more recently a better explanation has appeared.  Researchers increasingly believe that a vital function of sleep, at least in higher order animals, is to allow the removal of toxic wastes, including amyloids, from the brain.

As recently as 2015, we discovered a cerebral-spinal fluid circulatory process in the brain which has been named the glymphatic system.  During sleep, especially during deep sleep, the cells in the brain shrink, expanding the space between the cells by as much as 60%.  Cerebral-spinal fluid enters the brain to fill the space.  Expansion and contraction of the cells promotes the movement of the fluid within the brain and its eventual exit from the brain.

As a natural part of cellular metabolism, cells expel waste products, including amyloids.  In other parts of the body, blood and the lymphatic system carry the waste away, but this is not the primary mode of waste removal in the brain.  Instead, these wastes are removed by the movement of the cerebral-spinal fluid of the glymphatic system. 

Amyloids expelled from brain cells accumulate in the space between the cells.  They are then either removed from the brain, or they congregate as plaques on the dendrites of brain cells.  The presence of amyloid plaques is a hallmark of AD, and they play some kind of role in the disease process.  It's  not entirely clear to everyone what that role is, but removing amyloids from the brain has been a focus of developing methods for stopping AD.

The bottom line:  Deep sleep promotes the removal of amyloids from the brain.  This may be a factor in resisting AD.  Failure to sleep well and often enough may be a factor in the development or aggravation of AD.

Here is a National Institute of Health article discussing what we have been learning about the glymphatic system.