Divide your work into manageable pieces and set up a schedule in which you do a small part at a time. Make a chart of your energy levels. Study the most difficult subjects then. Work on the ones you find easy at times when your energy levels are low. Make sure you have adequate light.
Direct light gives off glare, tiring the eyes.
So, use indirect lighting. Sit up straight to aid concentration. Sprawling out induces sleep. Sit at a study table, not on your bed. Don't use an easy chair.