I think I might have heard this at some point in the past.
Anyway, I found a reference to support your claim:
Also consistent with the conclusions of Torrey1 and Saha et al,2 our analyses shown in table 3 found a strong tendency for prevalence to increase with latitude.[1]
I'm not aware from the article of how the illness "schizophrenia" foreshadows all of mental health. My illness being Gender Dysphoria.
Vitamin D deficiency from what I've read is common everywhere. It does make sense that longer winters would impact a person such as staying inside more. Yet I doubt any evidence can confirm lack of Vitamin D is associated with the illness schizophrenia.
Anyway, I found a reference to support your claim:
Also consistent with the conclusions of Torrey1 and Saha et al,2 our analyses shown in table 3 found a strong tendency for prevalence to increase with latitude.[1]
1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2669590/