Do mild exercise early morning and take shower.., go to bed in time at evening (no facebook, youtube after 10 pm ;), ...  eat in time, never escape breakfast. It might be biological issue or result of depriviation of essential minerals in body. Try having quality multivitamins. Avoid energy drink, sugar or coffee.

Some people who snore (loud) during sleep  dont go through all 4 sleep-cycles during their night sleep, and actually they are sleep deprived. This might also be the reason.

Or if you are weak in study  of the particular subject, your mind tries to ignore it and you feel sleepy. ;)