Hi, there can be various reasons( both physiological and psychological) for you feeling unusually sleep while studying. The former can be related to your biology( what kind of food you are eating? How many hours of sleep are you getting on average? Do you have any other physical symptoms like headache, cold etc.). The latter can be related to any stressful event you are experiencing in your life( any recent fights with a friend, academic pressure, dislike of the subject etc)
Next Steps
Answering the above questions might help you in understanding your issue better. In general, physical exercise, meditation, light meals( avoid junk food) and taking proper breaks while studying can help you focus and concentrate more.
Health Tips
If you have any other questions, please feel free to contact me. Good luck!