It's not just you, I hear this often, but I am always suprised people can read for so long in bed. No matter how interesting a book is, I can rarely read more than 20-30 minutes before the urge to fall asleep becomes too strong.

But I can sometimes code until like 4AM. Weird.

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.