I never understood falling asleep while reading...reading definitely keeps me awake. I have a friend, though, who falls asleep whenever she reads. It doesn't matter if she's tired, if it's the middle of the day, what she's reading, anything. It's like a conditioned response, as soon as she starts to read she starts to doze off. She likes reading, but this makes it difficult for her. It is so bad that she literally has to kneel on a hard floor while reading to stay awake, and even that doesn't always work! We were roommates for about a year, and there were definitely times I'd find her asleep on the kitchen floor next to a book.