What is your theory on that? I read my emails when I have time and when it is convenient for me. The time something comes in doesn't have any bearing on when I might actually read it.
I was checking around and it seems to be a combination of times when people read mails on phones and right before and after started 9-5 EST work hours.
I guess I'm still confused as to why the time you send them would really matter since people are just going to read them when it's convenient anyway. (unless you're worried if they aren't ready to read it when they receive it they may never get around to it, which makes sense)