For starters, it is intensely relaxing. Returning to the real world afterwards was a shock — but more on that in a second.
With nothing to entertain it, your mind can play tricks on you. For some people, it can even induce hallucinations.
I found myself hovering in that twilight state between awakeness and sleep. My mind ran in a game of free association — castles, clouds, music, shapes! It definitely wasn't a hallucination — not how they're normally defined. But unconstrained by reality, my imagination was vivid and wild.
At one point, it felt like I was hearing Hillary Clinton's voice right of the edge of my hearing, not quite distinct. (I had watched one of the presidential debates live the night before.) I wasn't really hearing it, and on an intellectual level, that was obvious. If I focused, it would instantly dissipate, and I'd be lying alone in the darkness. But the moment my mind began to wander... there it was again!