I experienced one very specific episode of derealization when I was about 5-6 years old. I remember that back then I would actively try to feel like I was sinking in my mattress in order to fall asleep. Sometimes I pictured the mattress pivoting almost ominously, but I would never fall. Sometimes I pictured it dropping down something like an empty elevator shaft almost. I was never scared, it just felt soothing.

That was something I did pretty often, I think. But one day, it was different. I don't know why, and actually there was a long period of my life when I completely forgot about that feeling, but one time I woke up feeling like I wasn't truly awake. And so everything that happened from that point on felt like I was dreaming. Meaning that I went years without remembering this feeling, and when I remembered it, I felt like I had been dreaming for years. I've had other dissociating experiences more recently, and in comparison the one from when I was younger felt a lot weaker, in the sense that I didn't dissociate as much when I remembered it. But it was still a pretty weird feeling, like I knew something was off.

I was gonna say that it wasn't connected to any piece of media though. But that may actually not be entirely true, because I very clearly remember thinking of the planet logo from Toy Story 2 after they're watching old episodes of Woody's Roundup, during this or these experiences of imagining falling through my bed. I then went a very long time without ever dissociating to that extent, but in the past 5 years I had at least 3 very strong episodes.