Sleep protects the brain against Alzheimer’s by washing it

Researchers have found that when in deep sleep, the brain waves within can start off a cleaning system within the brain that can protect it against Alzheimer’s and other neurodegenerative brain diseases. The study titled, “Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep,” was published this week in the latest issue of the journal Science.

Researchers explained that there are several forms of brain waves especially slow ones that appear during the sleep and have been recorded using EEG (Electroencephalogram). The authors wrote, “Sleep is essential for both cognition and maintenance of healthy brain function. Slow waves in neural activity contribute to memory consolidation, whereas cerebrospinal fluid (CSF) clears metabolic waste products from the brain. Whether these two processes are related is not known”.

The new study reveals that before each of such slow waves occur in the sleeping brain there is a pulse of CSF that washes all over the brain to remove the toxins from it. This means that approximately every 20 seconds or so, the brain gets a wash down. CSF normally is the fluid flowing around the brain and spinal cord.

Laura Lewis, lead author of the study and assistant professor in the department of biomedical engineering at Boston University said that this study reveals the link between sleep deprivation and neurodegenerative disorders such as Alzheimer’s disease. She said, “Some disruption to the way sleep is working could potentially be contributing to the decline in brain health.”

Researchers believe that these findings point to the fact that individuals can reduce their risk of getting Alzheimer’s disease by getting good quality sleep each night. Experts have said that people with Alzheimer’s tend to get poor sleep. Now this study links the two conditions. Lewis said, “It's been known for a long time that sleep is really important for brain health...but why it is was more mysterious.”

To study the effect of sleep on the brain the team used functional MRI or fMRI imaging to study the brains of 13 participants who were sleeping. They wrote that they used, “accelerated neuroimaging to measure physiological and neural dynamics in the human brain.” They looked at several parameters including the brain waves as well as the flow of the CSF. Lewis explained, “And that's when we discovered that during sleep, there are these really large, slow waves occurring maybe once every 20 seconds of CSF washing into the brain.” She added that it was like a large slow washing machine that was coming in pulses.

Lewis said that another important finding from this study was that before the CSF washed the brain there was a brain wave - an electrical wave that covered whole of the brain. She said, “Before each wave of fluid, we would actually see a wave of electrical activity in the neurons. This electrical wave always happens first, and the CSF wave always seems to follow seconds later.”

They noted that it was the electrical wave that triggered the CSF flow. The wave was the slow brain wave that is seen in the NREM (non rapid eye movement) stage of sleep or deep stage. Authors wrote, “During non–rapid eye movement sleep, low-frequency oscillations in neural activity support memory consolidation and neuronal computation.”

Lewis added, “It's already known that people with Alzheimer's disease have less of these electrophysiological slow waves, so they have smaller and fewer slow waves.” Now it is known that less of these waves also means less washing out of the toxins. Lewis explained, “It would make sense that if there's large waves of fluid, of CSF, that that might in turn cause mixing and dispersion with other fluids in the brain and help with this waste removal process.”

Another finding from the MRI scans revealed that CSF flow rose with the decrease in blood flow. They noted that lower blood flow allowed the CSF to clear out the brain better and would allow removal of toxins better. The team wrote, “Slow oscillatory neuronal activity thus leads to oscillations in blood volume, drawing cerebrospinal fluid into and out of the brain.”

The authors concluded, “These results demonstrate that the sleeping brain exhibits waves of CSF flow on a macroscopic scale, and these CSF dynamics are interlinked with neural and hemodynamic rhythms.” Lewis says that this study could be the beginning of understanding of how decline in removal of toxins from the brain could be contributing to Alzheimer’s and other diseases and how sleep waves and CSF flow could help. This in turn could also mean that therapeutic management of neurodegenerative disorders could focus on this aspect of development of the conditions, the team explained.

Journal reference:

Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep, Nina E. Fultz, Giorgio Bonmassar, Kawin Setsompop, Robert A. Stickgold, Bruce R. Rosen, Jonathan R. Polimeni, Laura D. Lewis, Science 01 Nov 2019: Vol. 366, Issue 6465, pp. 628-631, DOI: 10.1126/science.aax5440, https://science.sciencemag.org/content/366/6465/628