I have never had an especially hard head... I can get easily absorbed in alternate realities. Things that some people barely notice can be very real for me. Lucid dreams, visualizations... and that kind of thing. I need a tiny fraction of the dose of some of my friends when it comes to psychedelic drugs, I've had psychedelic expriences on nothing at all. That's probably why I get a lot out of SL and some of my RL friends don't. My head seems to meet the technology at least half way, and pulls powerful things (to me, at least) out of it.
I've been living with my avatar now for a year and a half and one of the clearest effects I've noticed has been in my dreams. Its often said it's a dream-like world. Well it is and it isn't, but I clearly remember my first SL influenced dream. It didn't look like SL but it had the same feeling of wonder and community.
Since then, dreams involving SL friends have become at least as regular as those with RL friends. A result of spending time where and forming emotional bonds I suppose, but these drerams aren't populated by avatars.
I just know who the people are with that unquestionable knowledge you get in dreams. actual appearance is fluid, like any detail if you try to fix on it in a dream, but each one is a solid presence and I know for sure who they are in my dream (unlike SL). The dreams don't take place in the same places as my RL dreams, which are often set in familiar but distorted RL environments.
The SL dreams happen in a world that's not quite like the real one real and yet it's not like SL either.
I find it interesting that I don't ever dream that I'm sitting at a computer, logged into SL. I'm always totally in-world, it shows how immersive an environment it is, maybe, that the interface totally disappears when I dream of it. Experiences in SL get filed by the brain as part of my life just as normal RL incidents do. Another interesting point... I've had dreams with different friends from SL in them but never a mixed reality dream... so there is a clear separation somewhere in there, even though parts of my life blur the line.
I think its like anything that stretches your experience I, it informs your dreams, gives you new ways of thinking and feeling which are manifested in dreams. In that respect, it's like an hallucinogenic drug. It opens you up to other ways of being and thinking and once that's done, you can't go back.