I would vote because here. But I won’t forget an archaic use of the word "so" to mean "if" or "on the condition that" as a conjunction.
I think u can't say ''so'' because u gave us a reason why u r tierd !!
It is because
It's because, for it gives a reason for going to bed.
It should be because coz you are telling the reason
it should be because as you are telling the reason
I would say "because".
yes it`s is because
hehehe.... offcourse itz because...