Well let's see here ...

To quickly answer one question, most men are in the mood first thing in the morning. The bulk of our testosterone is produced at night so we're swimming in it in the morning and that puts us in the mood.

Depression can really kill your libido and that would be the first thing I would suspect. It won't be really predictable either. One depressive episode may have zero effect on your libido, but the next can kill it. So that's where I would start.

And like Frankbtl was saying, there are newer antidepressants out there. For me, Zoloft is like children's aspirin; it's worthless for depression and just make me numb. Maybe something like Pristiq or Viibryd?