The optimal solution for me has been to combine both approaches. It’s very simple, and many early risers do this without even thinking about it, but it was a mental breakthrough for me nonetheless. The solution was to go to bed when I’m sleepy (and only when I’m sleepy) and get up with an alarm clock at a fixed time (7 days per week). So I always get up at the same time (in my case 5am), but I go to bed at different times every night.