In fact, if you're doing straight memorization, don't spend more then twenty to thirty minutes, first, when you are under an imposed time restriction, you use the time more effectively. Second, psychologists say that you learn best in short takes. The specialists say you'll get your most effective studying done if you take ten minute breaks between subjects. Pavlov's dogs were taught to respond on cue by being reward with tidbits. Dr Walter Pauk, director of the reading and study center at Cornell University, suggests you take a short break whenever you feel you need one, so you don't fritter your time away in clock watching and anticipating your break.
Brain waves are like radio waves.
If there isn't enough space between inputs, you get interference. The more similar the kinds of learning taking place, the more interference. So separate your study periods for courses with similar subject matter. Follow your hour of German with an hour of chemistry or history, not with Spanish. Psychologists have found that everyone has a certain time of day when he or she gets sleepy.