A) Example timeseries of the cortical gray matter BOLD signal and the mean CSF signal from one subject. During wake, signals are low-amplitude and synchronized to respiration (0.25 Hz). B) During sleep, a large-amplitude BOLD oscillation appears, and its timecourse is coupled to the ventricle CSF signal (~0.05 Hz). C) The mean cortical gray matter BOLD signal power increases during sleep (n=11 subjects for pairwise test). D) The mean cross-correlation between the zero-thresholded negative derivative of BOLD and CSF signals shows strong correlation (n=176 segments, 13 subjects). Shaded blue is standard error across segments; black dashed line is 95% interval of shuffled distribution. E) Example timeseries showing the correlation, suggesting that CSF flows up the fourth ventricle when cerebral blood volume decreases.