I had something like this for a while a few years ago. I was lying on my back in bed trying to get to sleep and was sort of drifting off but still awake. I then started to feel as if I was separating from my body and sinking down through my bed. It was terrifying so I jumped up and sat on the edge. I kept having to move my arms about or blink hard to stop the feeling coming back the whole night. This carried on for about a month, sometimes during the day as well until it gradually wore off. It was brought on by drugs though, I had been up all night the night before the original experience taking pills and smoking weed (which I don't touch anymore).
That was when I made this account actually, hence the name.