Since then, I've been sitting on the couch watching Netflix, so here the time referred to by since then is 6 p.m. I finished work at 6 p.m. Today since then, since that time from that point in time, until this conversation, I've been sitting on the couch watching Netflix, so we do not use since then to introduce a reason, as we do with because or because of or since rather since then is referring to a point in the past relative to now.