This is my attempt to explain a particular thing that I've experienced sometimes which I think falls under the category of derealization/depersonalization. This thing involves intrusive thoughts about the nature of reality and consciousness, and certain physical feelings, which tend to occur together. I've experienced this on-and-off I think at least since middle school. When I first experienced it I called it "worrying", since that was the only term I had to express thinking about uncomfortable things.
(I'm writing this in case it's helpful for anyone else to understand their own experience; however, I'm only writing about my own experience, and I can't say how common or rare anything I've described here is. I'm not asking for help dealing with this.)
(By the way, while I've seen other reports of derealization connected to drug use, I've never used any sort of recreational drugs, so this can't be due to that for me.)
I don't remember when exactly these feelings started. My first real explicit memory of this was basically telling my mom, "You know that song, 'Row, Row, Row Your Boat', how it ends with 'Life is but a dream'? I'm worrying that that's actually true". I think that was when I was in middle school (age 11-14), although I'm not sure. However, I have memories from earlier (age 7-11) of being uncomfortable with the question of whether space is infinite, and at one point with the question of whether things continued to exist when not observed, which might be related.
Since then, there have been times in my life when the feelings have gotten stronger or weaker. I remember at some point in high school noticing that these feelings were much less strong than they had been, but they came back really strong around the middle of summer after I graduated high school (I think this is when it was the worst), sort of gradually got a bit better through college but started becoming stronger shortly before I graduated from college. As of January 2018 (age ≈ late 20's), these feelings don't seem that bad recently (not entirely sure, but I think the feelings got better around when I realized I was trans). Also, sometimes I take week-long vacations with my family, and derealization often seems worse, or at least more noticeable, during those. (This seems consistent with the idea that it gets worse in unfamiliar situations or major life changes.)
There are some things that will sometimes cause these feelings to get worse short-term (like, for the rest of the day or shorter):
The time it got really bad right after high school started somewhat suddenly while I was watching a video called something like "Many Worlds Mario" (this or something similar), which caused me to think about quantum mechanics (if it was video I just linked, the varying volume of sound effects might have also been part of the issue). Normally that would just cause short-term feelings, but for some reason that time the feelings didn't stop.
Somewhat ironically, I tend not to feel like this during my dreams. I remember a couple dreams involving thoughts related to this, and at one point when derealization got really bad my dreams became weird and abstract and dark, like just some colored splotches on a black background and nothing happening, but otherwise derealization hasn't seemed to affect my dreams.
I have no control over when I experience derealization, nor does it seem to be something that has a purpose; it just happens, and I don't know why.
This feeling has generally involved various uncomfortable intrusive/obsessive thoughts. These thoughts have changed somewhat over time.
Like I mentioned previously, one of my first thoughts was a fear that I was in a dream. I don't think that specific thought was that common for me, though; however, I've had plenty of similar thoughts about not being real, like "What if I'm in a simulation?" (simulation hypothesis), and "What if I'm in a fictional story?"
Other thoughts I had—particularly in middle school—were about the nature of consciousness. It seems like some part of my brain interprets my sensory experience as the universe seeing and hearing stuff, which raises the question, why my senses in particular—and will it always be this way? I wondered if maybe I would at some point switch to experience someone else's life, and maybe what life I'm experiencing has changed many times and I don't know because memories stay with the brain and not the consciousness. I also worried a lot that the thing that was experiencing this stuff (and having these thoughts about consciousness) might not be the same as the thing that was controlling my body.
After high school, my thoughts centered more on the question of "How does anything exist?" Sort of a first cause type question, combined with the question of what distinguishes this universe in particular as existing, as opposed to all the other logically possible universes. This led to fears about metaphysical nihilism, and thoughts about the mathematical universe hypothesis. The mathematical universe hypothesis and the quantum mechanics many worlds interpretation were particularly unsettling, because they implied that there's some universe where I'm thinking what I'm thinking, and then something extremely unlikely but not absolutely impossible happens, like I fall through the floor or go through a wall. At some level I think I actually did expect to fall through the floor.
There are a few sensory changes associated with this feeling.
First of all, it sometimes kind of feels like there's a specific physical feeling associated with derealization itself, but since I'm not good at remembering physical feelings, I'm not sure if it's the same each time, and I'm not sure I can describe it here. From what I can remember it's something like pressure in my forehead, maybe something involving my eyes, and perhaps similar to feeling tired? (It's also possible that it's just something I was coincidentally feeling anyways.)
Sometimes there were feelings that certain sensations were weaker than they should be, or, somewhat counterintuitively, that certain sensations were unpleasantly strong (compared to how my senses are when derealization isn't that bad). Some examples of things I've felt when derealization was bad (though my memory of this sort of stuff might not be 100% reliable):
Negative emotions. Anxiety, fear. Constantly being afraid that the next moment the world as I know it will cease to exist, or that my ability to experience things will stop. Never feeling safe, even in places where I would normally feel safe.
I don't remember noticing my emotions being weaker (unlike some descriptions of derealization I've seen) associated with this. I do think my emotions are weaker than they should be, and it's possible that it's related, but if so, it's not obvious.