Did you choose this subject on your own or did someone else guide you? See, if you are really interested in this subject and yet find it difficult, clarify your subject related difficulties with your course mate or your teacher as and when required. You may also keep an extra tuition for the same. With interest and motivation, everything can be achieved. Do you feel sleepy only while studying, or at other times also? Poor motivation and lack of interest generally leads to poor concentration, which may induce sleep.
But if you are not interested to do any of the above, then maybe you are interested in some other subject or extra curricular. It is most important to analyze your interest related area and achieve in that. Once you are able to recognize and start achieving in that field, your concentration will be renewed. Be positive.