Feeling sleepy while studying usually comes from a mix of causes, and fixing a few of them together works best. The big ones are lack of proper sleep, studying lying down or in a too comfortable spot, a warm stuffy room, heavy meals, and boredom from passive reading.
Start with your setup. Study sitting upright at a table with good lighting, not lying on your bed, because your body links bed with sleep and will happily oblige. Keep the room cool and well aired if you can, since warmth and stale air make anyone drowsy. Splash water on your face, keep a water bottle handy, and avoid studying right after a heavy meal, because digestion pulls energy away and makes you sleepy.
Then change how you study. Passive reading is the fastest way to nod off, so make it active. Write notes by hand, say things out loud, solve problems, or quiz yourself, because doing keeps you far more awake than just looking at a page. Study in focused blocks of about twenty five to forty minutes with short movement breaks, and stand up or walk during those breaks. If you are still fighting your eyelids every day despite all this, the honest fix is more sleep at night, because no trick beats an actually rested brain.
Example: Swapping silent reading for writing summary notes out loud can turn a sleepy half hour into an alert, productive one.
One practical tip: Never study lying down. Sitting upright at a desk with a cool room and active note making keeps sleep at bay far better than caffeine.