Reading is usually more passive than coding. I'm often never sleepy if I'm actively coding something late at night but reading a book (no matter how engaging) or watching a tv show can very easily make me sleepy. That said, everyone's brains work very differently.
This is just so weird. In general coding won't let me fall asleep but a book 100% will never let me sleep until I finish.
I also find the idea of "forcing" yourself to read rather peculiar, but we're all different people. I wonder if there's genuinely something different in how the brain reacts.