If you find yourself nodding off while reading a book, but you weren't especially tired when you started reading, there is a word or words just prior to the nod-off point that you don't understand or you misunderstand. Grab a dictionary and clear up the word(s) and you'll be good to go.
If you find yourself re-reading the same passage over and over, there is also a word or words just prior that you don't fully know the meaning of or misunderstood. Or there is a concept that you don't fully grasp. Grab a dictionary and clear up those words and you'll be good to go.
A nice side effect of this practice is that it gets easier over time. Reading gets easier over time. And it raises your IQ.
Tips from a rabid reader. You can thank me later.