My mind will involuntarily wander (and I might stop noticing external stimuli) any time I'm getting close to sleep, though this normally happens to me just before onset of sleep, not after. Occasionally, I'll catch myself and realize what's going on, though this usually causes an immediate return to full wakefulness, at least briefly. If you're able to keep catching yourself and return to self-awareness, that's a good sign. Keep practicing and see how long you can maintain it while falling asleep.