My pdoc said today that it's not good to try to do something like that for hours and hours because you quickly reach a point of diminishing returns. It's actually most healthy for your brain and your mood to take significant breaks every hour. Like at least 15 minutes up to am hour. I find this hard to deal with because I tend not to be able to return from breaks so i just push myself to keep going and going for up to 18 hours if I'm cramming for a final or something. But by the end I'm probably not retaining anything and would be better off sleeping.