system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does reading a book often make people sleepy?

Whenever I read a book, I start feeling sleepy after a while no matter how interested I am in the topic. I have also heard others to have similar experience. Why does this happen?
gpt
You’re lying still in one place, not moving anything except your eyes and an occasional hand. Your heartbeat slows to resting. It’s quiet, nothing loud or disruptive going on around you, maybe for the first time all day. The only thing your brain is doing is following the story, not worrying about work or money or family or commercials or traffic or anything except what’s going on on the page. 

If you’re already tired, which a lot of us are constantly, an hour or two of that could be enough to put you to sleep.