Like Backwoodsman, I began by delivering newspapers at 5-6 AM... then I spent most of my working years getting up early and punching someone's time clock. During my college years, I often had early morning classes before going to work. This continued as I worked full-time.
   Then, unlike Backwoodsman, when I retired I discovered that my comfort zone meant staying up till 11 or 12 PM (maybe later) and sleeping until eight or nine AM. I try very hard not to schedule a doctors appointment or any such thing like that before 11 AM, and preferably in the early afternoon. I can still cope with early morning if absolutely necessary, but it requires setting the alarm clock.
   Here's a kind of technical explanation of all..
diurnal cycles