Reading takes some mental effort. If you're already sleepy, then it's going to be more tiring on top of it. Like running a marathon then climbing a hill after.

Sometimes I read before bed, sometimes during the day. Sometimes I can barely keep my eyes open, sometimes I can stay up all night eager to finish a book.

It doesn't really change my sleep when it happens. I just don't try to force reading when tired, no matter the time of day.

I imagine that if I wasn't tired yet and was reading, eventually I'd get tired in the same way that if I wasn't tired yet, and kept laying in bed with my eyes closed, eventually I'd get tired.