I just came back from a retreat with Yongey Mingyur Rinpoche where he discussed sleeping meditation. He mentioned two types of sleeping meditation which have bearing on this topic. The first is that you can meditate in lucid dreams. It's possible what you experienced was related to that. I know nothing more about that than that it's possible. One of the other methods of sleeping mediation he discussed involved taking a state of open awareness into sleep. Honestly I was skeptical of this.

I've been doing meditation adjacent practices for 20 years (self hypnosis and mindfulness) and once I got through some original misinterpretations about terms his style for everything else he taught really clicked so I thought I'd give sleeping meditation a try. It worked. It's crazy weird. You can feel the sleep paralysis take hold. You're aware of all of the sounds around you and can still control your breathing and your eye movement. You maintain a meditative state of open awareness the entire time and his description of blue skies, clear, and open for that meditative state made perfect sense with what I felt. He said stuff about a number of visual effects which can occur in this state but I've yet to experience any.

So far in my experiments with it I've found 5 stages in sleep. Honestly I don't know what to call them as he barely touched on sleep meditation at the retreat so I'll call them paralysis 1, paralysis 2, REM, deepening 1 and deepening 2. The first state of paralysis is just a heavy feeling kind of like drifting off to sleep. Relatively comfortable. The second stage is much more intense, also felt hot despite the room being a reasonable temperature and was a bit uncomfortable to relax into because of how odd it is. My muscles also kept wanting to twitch in this state but couldn't really and that increased the feeling of discomfort. This is where REM comes in. I found that my moving my eyes I felt a corresponding feeling in my limbs and could relieve the discomfort. I sat in this state for a while then continued deeper. Deepening one has a bit of a sleepy feel to it, I don't know how else to describe it. It was still clear and open, just more relaxed and with a hint of the feel of sleepiness. Again, I sat here a while. This in turn passed to deepening 2. Here I could feel a lot of the positive and restorative effects of sleep as they occurred. Again, really odd sensation. At one point in here I smiled which a) made me realize I could smile in this state and b) allowed me to bring lovingkindness into this headspace. I had tried bringing that in at earlier stages unsuccessfully. In all, the experience is absolutely surreal and serene. An amazing meditative experience I'd definitely recommend to advanced meditators.

Word of warning, I messed up my sleep for about a week learning this process, thinking to much on any sensation stops you from progressing, you need to focus past the sensation in such a way to let it grow and intensify.

I've been doing this instead of normal sleep for a few days with no ill effect. Mingyur talked about multiple people he's been on three year retreats with as well as his own experience of doing this at least that long again with no ill effects. Really restful.