Why am I suddenly dozing off?

In the last week or so, I have been literally dozing off at my desk while working. I simply cannot keep my eyes open. I don't know if this is a part of menopause (I'm 49), or if it is a side effect from the Zoloft and Wellbutrin I'm taking -- though I've been taking it for months without this happening. My doctor says it's part of my anxiety. I'm not so sure about that.