You can train yourself pretty easily to wake up without an alarm. Set intention before sleep to wake up five minutes before you get up for your day in the morning, fall asleep thinking about it. Most people see results the first morning but if you don't give it three nights of effort. Once you can consistently wake up before your alarm just intend to wake up for your WBTB, whenever that is. Your body's internal clock is a pretty fine tuned thing, you should be able to do this with little effort.