If you know that you sleep at 12 and wake up at 6-7am, you should plan your preparation according to your sleeping schedule. Might sound strange but that acually works. Be specific what you want to study within the hours that you are awake. Sketch a timetable and stick to it during the hours you are awake. If you think too much of preparation scares you or distracts you, divide the information to small and manageable parts. Let's say Mathematics is a more demanding subject for you, prepare for it when you think you feel the most active as in after your breakfast or cup of tea. Watch what you eat or drink because tea and coffee might keep you alert but iin the long run, they might cause anxiety, headaches and dizziness when taken in higher doses. And if you have less concentration span and if you think you more of tend to day dream, take frequent breaks. If you think you can only study for 30minutes at a stretch, do that and after every half an hour, take a 5-10 minutes break. And on those days that you think concentration is difficult, plan a mock test. Frequent review of material is important. And always make sure that you practise those formula or theories within 24 hours of learning them.