Because, as, since are used as conjunction to introduce a clause that 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 used to state merely circumstances
As (or since) I was tired, I was sleeping.
‘since’ can also used as adverb and preposition but is generally used with a perfect tense in the main clause, toexpress an event from a particular time in the past until the present, or from a particular time in the past until a later time, or until now :as:
We've been waiting here since two o'clock.(Preposition)
It was exactly five years since her father had died (conjunction and perfect tense)
He lost his job five years ago, but has since found other work. (adverb and perfecr tense)
Student or Learner