Interesting. Sorry, I can't answer your question specifically, except to say that a "normal" preschooler will be going in and out of naps at that age, depending on growth spurts, level of activity, etc. ... and as part of phasing out naps it's normal for kids to push them later into the day. How that all fits for a child who hasn't napped for years, however, is a mystery to me.
I had one child who stopped napping at a year and a half and never looked back (although we never tried melatonin, either) and one who drove us crazy from 3 1/2 through K not being able to decide if she wanted to take a nap and when.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).