For/since are less used than "because" you could use because if you wished in your example. It´s also more formal.
I was sleeping, for I was tired.
Man cannot remake himself without suffering, for he is both the marble and the sculptor. - by Carrel, Alexis.
(pois, uma vez que, porque )http://dictionary.reference.com/browse/because
1. Because, as, since, for, inasmuch as agree in implying a reason for an occurrence or action. Because introduces a direct reason: I was sleeping because I was tired. As and since are so casual as to imply merely circumstances attendant on the main statement: As (or since ) I was tired, I was sleeping. The reason, proof, or justification introduced by
for is like an afterthought or a parenthetical statement
: I was sleeping, for I was tired.
Inasmuch as implies concession; the main statement is true in view of the circumstances introduced by this conjunction: Inasmuch as I was tired, it seemed best to sleep.