Avoid scheduling marathon study sessions. When possible, study in shorter sessions. When you do study in long sessions, take a planned break every hour. Work on several subjects and avoid studying similar subjects back to back. Be aware of your best time of day.
Many students learn best in daylight hours.
Observe yourself and, if this is true of you, schedule study time for your most difficult subjects when the sun is up. The key point is to determine your best learning time. If early morning doesn't work for you, find out what time is better. Use waiting time. Have short study tasks ready to do during these times.