I think for me being around the same time is important,but there is no way in heck I am setting an alarm, so I tend to temp at whatever random time in the morning I find myself up with the baby without her really being up for the day yet (if that makes any sense), which is typically somewhere between 5-6:30. How much sleep I've gotten doesn't seem to play in. You could write down all the variables for a little while and see if you notice any patterns.
this is what i did and it worked for me. i saw a clear pattern every month.