For the first time, scientists have demonstrated how the brain washes away toxic proteins during a specific phase of deep sleep. The study builds on a growing body of evidence suggesting chronic sleep disruption could play a role in the onset of Alzheimer's and dementia.
In 2012, a team of scientists from the University of Rochester discovered a previously unknown system the brain uses to remove accumulated waste in brain tissue. Called the "glymphatic system," it was revealed that the brain sporadically pumps cerebral spinal fluid through brain tissue, essentially flushing damaging waste out into the body's circulatory system. Further work by the team revealed this process primarily occurs during sleep, offering a mechanism by which to explain another important reason almost every animal needs to sleep.
Alongside these insights into the newly discovered glymphatic system, other recent research is finding significant associations between disrupted sleep patterns and the development of dementia and Alzheimer's disease. The most recent study of this kind found a specific correlation between low levels of slow-wave sleep and greater accumulations in the brain of toxic proteins generally associated with Alzheimer's.
In the search for a clearer causal understanding of this strange association between sleep disruption and dementia, a new study set out to identify if there is a certain phase of sleep specifically connected to the glymphatic system activity clearing the brain. To examine this, researchers used six different anesthetic regimens on mice to stimulate a variety of sedated neural states. While the animals were in each state, the researchers observed neural activity and the flow of cerebral spinal fluid through brain tissue.
Out of the six drug-induced states, one state in particular generated neural activity and heart rates most similar to that of a human in deep, non-REM, slow-wave sleep. This particular sleep state also generated the best glymphatic system activity and, even more importantly, suggests a neural mechanism that could explain why this particular sleep phase can stimulate the brain cleaning process.
"The synchronized waves of neural activity during deep slow-wave sleep, specifically firing patterns that move from front of the brain to the back, coincide with what we know about the flow of CSF in the glymphatic system," explains Lauren Hablitz, first author of the study. "It appears that the chemicals involved in the firing of neurons, namely ions, drive a process of osmosis which helps pull the fluid through brain tissue."
While this study isn't the single smoking gun that offers a definitive connection between sleep deprivation and Alzheimer's, it does add weight to the argument that chronic bad sleep could drive the onset of the disease. Clinical observations have revealed solid associations between dementia and disrupted sleep, but it is still yet to be proven whether bad sleep is causing the disease or is merely a consequence.
If further research can effectively verify this slow-wave sleep glymphatic process as definitively clearing toxic proteins from a human brain, then sleep therapy could potentially be an effective clinical approach to slow the onset of dementia in at-risk patients. This new study certainly concludes not all sleep is equal and a chronic lack of deep sleep could reasonably be one of the causal elements driving age-related neurological decline.
The new study was published in the journal Science Advances.