Flushing the Brain While You Sleep

I had written before about the breakthrough in studies of mice that showed lymphatic  flushing of brain tissue in mice while they slept. Now this phenomenon has been confirmed in humans. During human sleep, pulses of cerebrospinal fluid (CSF) flush throughout the brain. You can see a spectacular real-time video at this site: https://www.sciencealert.com/mesmerising-video-shows-waves-of-spinal-fluid-washing-over-the-brain-during-sleep.

Midline brain scan showing flushed area in red at one instant, pulsing at about 1-2 times/sec. From Fultz, 2019.

Interestingly, the flushing seems to include most of the brain, except the brainstem and the cerebellum. These CSF waves presumably flush out unnecessary proteins and other redundant debris. It is likely that the microtubule lymphatic-like system inside of brain tissue that opens during deep sleep is part of the CSF circulatory system. CSF is generated in specialized regions of the cerebral ventricles and ultimately drains back into the bloodstream.

Another research group simultaneously reported in the same issue of Science that cerebral blood flow diminishes by about 25% during slow-wave sleep, and apparently this facilitates an increase in the volume of CSF that can flow through the brain.

Another research group simultaneously reported in the same issue of Science that cerebral blood flow diminishes by about 25% during slow-wave sleep, and apparently this facilitates an increase in the volume of CSF that can flow through the brain.

The CSF pulsing is associated with slow-wave pulsing in the field potentials (as seen in EEGs, for example) generated by brain during the initial stages of sleep. The electrical waves and CSF pulses are coincident in a shared rhythm. The amount of slow-wave electrical activity diminishes in most elderly, and this may be a cause of dementia, which results from accumulated metabolic waste products. Sleep clinics could easily determine the amount of slow-wave sleep and thus perhaps detect early warning signs of impending dementia. Research on drugs and sleep habits that promote slow-wave EEGs might forestall and even treat dementia.

Sources:

Fultz, Nina E. et al. 2019. Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep. Science. 366(6465), 628-631. doi: 10.1126/science.aax5440

Grub, Søren and Lauritzen, M. 2010. Deep sleep drives brain fluid oscillations. Science. 366(6465), 572-573. DOI: 10.1126/science.aaz5191