Why is insomni@ masculine in Spanish?

[archaicos]

I'm really curious about this recent finding of mine...

The word is feminine or ends in -a in these languages:
insomnia: English, Latin
insonnia: Italian
insônia: Portuguese
İnsomnia: Turkish
инсомнија: Serbian
insomnie: French, Romanian, Czech, Serbo-Croat (not sure about the gender in the last two)

The word is masculine or ends in -o in these languages:
insomnio: Spanish, Galician

Also found:
insomni: Catalan
insomniu: Asturian

So, what happened in Spain, in Castile and Galicia?

Sorry if there're mistakes in the above. I obviously don't know all of the mentioned languages.