Sometimes when I'm lying on my back, trying to fall asleep, I create an image of something very small like a floating dust particle, marble, pinpoint, or a tiny steel ball. My brain automatically imagines an extreme weight or massive density of the tiny items. Then suddenly, a very odd feeling overcomes my entire body as if one of these small items weighs 100's of tons and my entire body "experiences the weight" or "becomes the weight". Imagine a marble or tiny pellet that weighs as much as a Navy ship. It doesn't make sense. Similarly, It's like imagining and realizing the weight and density of an entire mountain shrunken in the palm of your hand. The result makes me feel physically sick and I have to sit up on my bed or go walk to the bathroom to shake off the sick feeling. I like to call it the "neutron star sensation" (not "dark matter") since it's the only thing I could think of that has such striking density. This all started when I was 9 years old when I was diagnosed with mono. The sensations were very strong and happened almost every night for 3 months.The "neutron star sensation" brought on some fear because it was as if I could "activate" the feeling just by thinking about a single small particle. Then somehow my brain makes it weigh 100 million tons. Yes it sounds crazy but it's hard to try to "not think" about something, especially while lying still in bed trying to go to sleep. So it took a while to avoid activating the sensation but eventually I was able to blot it out of my thoughts. Every couple of years, I'll hesitantly make the attempt to see if I could still "activate" the experience and once it starts, I immediately get up and try to walk it off and release it. Here I am, age 30. I can still experience this phenomena If I willingly do so. It's not a problem. I just have to be careful. When I was young, it happened so easily but now, it thankfully takes a concerted effort. I am very curious as to what in the world this could be. There is nothing like it. It typically happened when I layed on my back. So sleeping on the side helped a lot in avoiding it. I was searching around online for some concrete answers to this phenomenon and found an entire thread on these physics forums at this link: https://www.physicsforums.com/threads/strange-sensation-starting-to-fall-asleep.159608/ They relate it to microsomatognosia and macrosomatognosia. I don't think that is what it is. I am afraid that after people read this they may experience it too when they go to sleep. Please don't get caught up in it if you do. It is interesting at 1st but can become very unsettling, very quickly, and hard to break out of once it happens. I hope that the many others who have had this or similar experiences will be able to share some info as it is hard to get any good info on it online. I would like to know more about this phenomenon. Thanks ahead of time for any feedback. Sorry for such an extremely long post!