Are you using computers? For how many hours? It may be computer vision syndrome or you must be having eye sight problem.
Use eye drops refresh tears 4 times a day for a week and and also check your eye sight ones.
Sleeping night is better than sleeping day. Try to study in day time ...in early morning.
Next Steps
consult Ophthalmologist for proper treatment with complete eye check up
Health Tips
oxygen levels in the night are less comparative to day time and your cornea needs oxygen from environment if you study more in the night the oxygen availability is not compromised if you have lot of plants around your study room.then you get tired very easily.