Why Sitting May Be Bad for Your Brain - The New York TimesIt was equally apparent when people broke up their sitting after two hours, although blood flow rose during the actual walking break. It soon sank again, the ultrasound probes showed, and was lower at the end of that session than at its start. But brain blood flow rose slightly when the four hours included frequent, two-minute walking breaks, the scientists found. Interestingly, none of these changes in brain blood flow were dictated by alterations in breathing and carbon dioxide levels, the scientists also determined. Carbon dioxide levels had remained steady before and after each session.