Most of the time, I break everything up into small chunks. I think the most I can actually do a certain activity is about two hours max. After that, I absolutely need a break. It used to also be this way for studying. I could read a textbook for about two hours, and I often had to highlight important stuff even if it was only just to have some kind of interactive element to the reading. Otherwise, I'd knock myself asleep.

I also CANNOT do any kind of activity while lying in bed. Reading a book... psh, forget it, I'll knock myself asleep. Even playing a 3DS game can also have the same result.