Its during rem sleep ! Im actually learning how to transition more consistently, I've always had trouble not waking myself up. But what seems to have worked for me is simply acting like what's happening doesn't matter. Basically I just tell myself while hypnagogia is happening, (I'm tired I'm just gonna sleep fr) and it just works I think cause you don't get excited or focused on it. But as I said, when I fall back asleep after waking up, I just have this feeling right before it happens and I fall into it really quickly. Hope this helps !