It’s apparently based on the Circadian rhythm which is based on sunrise and sunset. I too was wondering however how different timezones, latitudes and day light savings fit into this. I think it’s safe to say that you should go to bed early, 10:00 is probably a good time to aim for because if you go to bed super late you’ll miss the physical restoration regardless of what time it exactly starts and stops.
I’ve experimented with this. I went to bed around 1:00 - 2:00 and woke up at 10:00 am for a couple of months and then I tried going to bed around 10:00 and waking up around 6:00 or 7:00 am and I feel better when I sleep earlier.
Of course getting into a routine definitely helps. If you’re going to bed and waking up at a different time every day your body has no idea what’s going on.