The point is that the hours between 5 am to 9 am should be more productive for the majority of people (assuming everyone else you know is sleeping at that time)

The point is that the hours between 5 am to 9 am should be more productive for the majority of people (assuming everyone else you know is sleeping at that time) than say the hours between 9 pm to 1 am when most of us can hardly get anything done. But I agree with your premise that maximum productivity is achieved when one finds his/her best schedule and sticks to it.