Rule reminder
CRITICAL DISTINCTION: 'Perché' has two functions. (1) CAUSAL: 'perché' + INDICATIVE = because (states a real reason). 'Non vengo perché sono stanco' (I'm not coming because I'm tired). (2) PURPOSIVE (= affinché): 'perché' + SUBJUNCTIVE = so that (expresses purpose). 'Te lo dico perché tu sappia la verità' (I'm telling you so that you know the truth). This is a common B2-level error source.
What is the function of 'perché' in: 'Non ho dormito perché faceva caldo'?