Hard time fall asleep for bedtime

I think my 15-month-old has been having some issues with bedtime sleep.

My daughter has been needing about 30 mins to fall asleep and sometimes 50 mins (she used to fall asleep in about 10 mins..). And wakes up around 5 am (but will do 'fall asleep and wake' circle until around 6:15 ish). She's also now way below the average percentage like around 70% ish (she used to be always over 88%) according to Nanit. All these have been showing for about 2 weeks.

We didn't change anything in the house. Her daily routine is also almost the same every day. We also tried to put her down early but it even takes longer to fall asleep.

She's still on 2 naps - 1st nap 9:30-10:15 (we shortened her first nap from 1.5 hr because she had fought for 2nd nap for a couple of days). 2nd nap 1:30-3 (she normally wakes up around 2:30). Bedtime at 6:45-7pm.

However, one thing I do want to mention (or actually ask) is that we put her down for bedtime based on her 2nd nap. And tried to keep the wake time for 4 hours in between (we used to do 3.5 hrs but noticed it's been really difficult for her to fall asleep). And if she skips her 2nd nap (which she does now about 1 to 2 times a week), we will let her sleep much earlier at around 5:45pm (she actually sleeps muuuch better for about 11.5 hr through the night and easily falls asleep). Therefore, her bedtime actually changes.

I wonder if we should let her sleep at the same time every night? Do the changes in bedtime time confuse her and disturbed her sleep?

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