Interesting observation. Are you prepared to reconsider the merits of astrology? It is, after all, a system of correlations between human temperaments and seasons of birth. The underlying theory invokes the positions of celestial bodies at birth as formative influences on temperament. A physical explanation might cite seasonal variations in the proportions of electromagnetic wavelengths reaching Earth, as well as varying gravitational influences, as the various celestial bodies fall into and out of alignment.

Seasonal variations in visible light might be only part of the story. Solar output includes nonvisible wavelengths as well.

You might look at where on the calendar autistic birthdays clump, and then compare autistic temperament with the personality characteristics associated with the corresponding zodiacal signs. Might reveal some surprising correlations.