Sleep-related and diurnal effects on brain diffusivity and cerebrospinal fluid flow

The question of how waste products are cleared from the brain, and the role which sleep plays in this process, is critical for our understanding of a range of physical and mental illnesses. In rodents, both circadian and sleep-related processes appear to facilitate clearance of waste products. The purpose of this study was to investigate whether overnight changes in diffusivity, brain volumes, and cerebrospinal fluid flow measured with MRI are associated with sleep parameters from overnight high-density sleep EEG, and circadian markers. In healthy adults investigated with MRI before and after sleep EEG, we observed an increase in water diffusivity overnight, which was positively related to the proportion of total sleep time spent in rapid eye movement (REM) sleep, and negatively associated with the fraction of sleep time spent in non rapid eye movement (NREM) sleep. Diffusivity was also associated with the sleep midpoint, a circadian marker. CSF flow increased overnight; this increase was unrelated to sleep or diffusivity measures but was associated with circadian markers. These results provide evidence for both sleep related and diurnal effects on water compartmentalisation within the brain.