I sent four today. My current rotation requires me to wake up at 5 AM (west coast time) so I wrote them all the day before and had them ready to go, hit send when I woke up. I received one invite from one of my top choices (east coast) within an hour. Nothing from the other three. I think there is something to be said for sending early in the morning so that your e-mail is there bright and early on the top of the inbox. However, I know someone who sent out one on a Friday afternoon and heard back today with an invite from a different program, so yes, we are all probably overthinking this. Send when you are ready.