I can only recall one such instance. It happened a few years ago, during a period of time when I had been practicing meditation more strongly than usual. It was not intentional, although I do recall having a quiet interest in the back of my mind about the subject.
It happened like this. I had this moment when I almost
fell in to deep sleep, the first stage of the sleep cycle (I know this was not a dream, it felt different) - and just for a moment, had the perception of falling backwards in to unlimited, all-encompassing, gentle white light. I felt no embodiment, and it's hard to explain how I was falling in backwards and yet not perceiving myself as embodied, but that was the perception. When I came to and realised how I had just been on the cusp of deep sleep, I had two emotional reactions: that was simultaneously a scary
experience, and a blissful
experience. I still recall it as both. It was beautiful yet unnerving.
I have not attempted to repeat this much since then, but the times that I have I've not been able to replicate it. Last night I tried to fall asleep mindfully, but used anapanasati. I went through the first tetrad, just like in a real meditation. I really noticed how, lying in bed, when the mind wanders it sure does wander, like mindfulness went 'missing in action' a few times, and the mind had episodes of being totally 'lost in thoughts' but due to strength of acquired habit, awareness would return again. I tried to notice the body beginning to calm down, the breathing process naturally calming and slowing. I tried to adjust my mindfulness to allow for this state; not trying to be so alert that I won't be able to fall asleep - but still keeping just a loose, relaxed kind of 'basic knowing' present, to see if I could observe the mind / body enter sleep. Alas, sometime after this, I did
fall asleep and as usual, I don't recall it.
I'm interested to hear of others' experiences with falling asleep, and how I might be able to do so in awareness, and not by accident, but intentionally (if possible).