DD is 19 months and since last March has been waking at 5am or earlier

(((. We tried allsorts and never seem to get more than 10 hours at night ,unless Ot then drops to 9. We have been sticking consistently to one set nap at 12pm for a few months now with bedtime pushed out til 7.30pm. Naps tend to be no longer than 1.5hrs. Earlier bedtimes don't work as we just get 4-4.30 wake ups, so I started to think she is just LSN.
On holiday at Christmas we managed to get longer naps which enabled us to go with a later Bt but still only got 10 hours at night. Then about 3 weeks ago over night she started sleeping until 6am

. We stuck with the same time re: naps and Bt and although we got 2 Ew towards the end of week she managed to reset herself and catch up, so for the majority I was 6am which I am so happy with. Then it changed over night back again to 5am, she was still doing 1.5 nap although the last 2 days she has done 2hrs! So now I am at a loss what to do, not sure if she is just in an OT cycle and to let her nap longer, cap nap, bring BT forward or


Please help I was enjoying my extra hour xxx