What you have to do when that happens is physical exercise. I keep a hand exerciser on my desk. When I feel drowsy I pick up the exerciser and start squeezing it. When you do physical exercise your lungs take in more oxygen, your heart rate and blood pressure increase pushing fresh, oxygen rich blood into your brain allowing it to function much more efficiently. The point is, you have to wake your body up.
Physical exercise is the way to do it.
You do not have to use a hand exerciser, you can do push ups, sit ups, or whatever it takes. When you study, you will find you retain information the longest when you attempt to integrate and apply it. Whenever you read a fact or piece of knowledge, in your mind link it to what you already know. If you are reading about the differences between men and women in terms of eye contact during dating, think about other things you might know about how men and women communicate differently. Think about other things you might know about eye contact.