I get sleepy when I study too. Try to study in an environment that wont allow you to go to sleep... For example, don't lay or sit on your bed to study. Have a snack while you study, If you're hungry you can't concentrate. Also pay attention to the temperature of the room you study in, if its too warm or cold it will make you sleepy. I usually study in my studio with classical music.. No words as it's distracting... And I have something to drink and a snack. And I make sure I have a fan on low to circulate the air. I have come to the conclusion, it's a subconscious way to "get out of" studying....lol. Good luck.
The brain needs energy to function, so it's possible that if you haven't eaten or slept enough before-hand that it will send signals through your body saying,"Hey, I need more fuel up here!" Also, if this tends to be a frequent reoccurrance, it will become a habit to your body to feel this way whenever you sit down to study. Try eating healthy and taking a nap before sitting down to study more often.
I think that you sat down and then you notice which made a habit !! Break the habit GL