Go for a blood check up. You might be having iron deficiency. People tend to get lethargic when they have iron deficiency. Some, even faint. So, best is to go to the doctor for a thorough check up.
Also, exercise to keep yourself fit. Swimming, yoga and aerobics are great. Since you are a student, I would suggest some stretching exercise at home that can do wonders before you start studying.
Hope that helps !