When my kids lived at home, they flew off on adventures often, and their flights always seemed to leave at 4, 5, or 6 AM. I know this because I had to drive them to the nearest commercial airport 90 minutes away. Now I know why- early morning flights tend to be less expensive than those later in the day. But that's not the only reason you might want to book an early morning flight -and annoying the person who gets up super early to drive you to the airport is not a sufficient reason.
Another reason to fly early in the day is that the odds of flight delays or cancellations are lower for flights leaving first thing in the morning, and there are a bunch of different reasons why later flights can run into problems. There are other reasons why morning flights are a better idea all around, which you can read at Mental Floss. It helps if you are able to sleep on a plane.
(Image credit: Dmitry Avdeev)