When the sun is over the cross it's time to go to church... which is so darn early. And if you want to get to church early, you need to be fast, otherwise you'll be too late.
To remember the そう reading we use the word "soul."
When you're forced to get up fast and early you can feel it in your soul (そう). You're so tired and it's so early, your soul cries out, "Please don't make me go this fast this early. I'm gonna leave your body if you keep this up!"