Yes, especially as one "gets older" (which for a 20-something might mean approaching 30, while for other of us it means something else!), one notices that one's mind and body both have daily rhythms that are hard to change, hard to ignore, hard to "overcome".
Thus, yes, it is wise to observe where/when one has the right kind of mental+physical energy for various things, whether they be "tasks" or something more positive.
I disagree with people who claim that study has to be painful... Maybe certain classes in school are unpleasant, but that's a not-necessarily-related complaint. Classes are made painful by other people. True, bad writing can make reading unpleasant, but my advice is to not subject oneself tooooo passively to bad writing. Or to boring writing. If you are studying for your own interest/pleasure, an unhelpful author is... unhelpful. (Don't listen to gossip/mythology about must-do tests of your machismo. Srsly... )
I work intermittently all day long, but there are certain points, correlated with time-of-day and amount-of-caffeine, in which the most productive things happen. Correlated with my exercise-time, too.