At some points in your life you’re going to realize you’re starting to fall into routines, into monotony. I’ve got to be honest, I can’t follow routines even if I wanted to. I can’t say everyday starting tomorrow I’ll dedicate 3pm-4pm time to study, because I won’t; I can’t break that rule, because my “system” breaks itself in some way.
But many people can fall into monotony very easily. They start to do the same things everyday so eventually those actions become “automatic”; they no longer require too much thinking. So monotony leads to lack of productivity and in some way slowly turns off your brain cells. By that I mean that getting those cells working again (a.k.a. getting out of the monotony) becomes hard with time.