Hi Ujjwal Kumar Singh,

                             I had also faced similar problems when I was preparing for my entrance exams. I used to sleep a lot and if I tried to control it my concentration levels started to decrease. But it is possible with practice. You need to try to sleep less after lunch. As in 4 hours of  afternoon sleep is not good for your health also. If you try to go outside and play for an hour or so you will be refreshed and not sleepy. Try this for a month with efforts. It will automatically become your own habit. But try hard it is not easy. All the best!!