Sometimes if I spend too long playing a specific game in one day, or over a span of multiple days for multiple hours, I kinda of get the feeling of almost complete immersion.
It's never more than a moment, it's usually after I'm done playing, and it's typically something relating to common sense in the game kind of not transfering to real life.
However that's just some hours over a few days, imagine absolute immersion over a few years. That'd frick up your mind in incomprehensible ways.