Am I falling asleep or am I deepening my concentration?

I use the Tara mantra as my focus during Samatha meditation prior to beginning Vipassana but lately I am beginning to wonder...am I falling asleep? I can't tell, really. I don't get drowsy but my mantra becomes so quiet that it begins to merge with the thought-stream that it is resting on, and then I lose awareness of both until my consciousness returns to the fact that I am still chanting, but without any awareness of what, if anything, I was actually thinking about-the thoughts being too quiet to catch or to get caught in. But just as there is no going, there is no returning either (no waking up) with this experience, just very subtle ebb and flow. When I began a regular meditation practice about five months ago discursive thought would become loud enough to break my concentration. It still does, of course, more often than not, and I thoroughly enjoy the challenge of "getting caught up in the flow" only to return to the mantra. But lately it is like meditation has lost its urgency. It has become so much a part of my life. It has become like that ebb and flow and I just want to rest in the stillness of that. It is just so beautiful...but then I start to think that maybe it is an escape from being present and that my mind is playing with me. I would really like to know what is going on, and if there is somewhere I should be going with this, some way that I should be practice with it skillfully.