I'm sorry but I can't see how this is applicable for my situation. What kind of studies are you talking about? University (First degree)? Second degree ? High School? I've just started my first year of computer science and mathematics. I study all day long and still don't find time for anything else. Also, try adding a girlfriend or a spouse to the equation. With all the positive sides of it, it adds a lot of distractions during the day that you can't control.
I doubt you are being productive. You have to isolate yourself from distractions (on a college campus this is most likely people).
If you really are studying with perfect productivity, then you need to ask yourself if you are triaging your time properly. For instance, say you have three tests: one easy, one medium, and one hard. Do you study equal amounts for all of them? Probably not. If you have already spent seven hours studying for the hard test, how much will one more hour help you? Maybe an 85 to and 87? But then if you spent that hour on the medium subject maybe your grade would jump up from an 80 to a 90. Thus your time would better be allocated to the medium test.
There is an economic concept about this though I cannot remember its name. Marginal Returns and the Law of Diminishing Returns, I think.
As a first year CompSci student I find this method works remarkably well for me. Workloads are most likely different where you are but I find I'm able to complete most (often not all) of my schoolwork within this time constraint. It's all about productivity. I never even realized how much time I spend on HN or reddit or whatnot when I still have work to do. I have been following this sort of method for a month now although I find I have to break my rules almost every week, I mostly manage to get everything done between 8 and 5. The author makes a good point on sleep too, I can't stress enough how important it is to get a good nights sleep for productivity!