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mysteryofdungeon I've always been incredibly deja vu prone. Ever since I can remember I just get it a lot and it lasts a relatively long time compared to others. Since I don't know what your baseline is like, I don't know how it feels when you say it won't turn off but is just constant. I may experience something akin to that but just think it's normal because pretty much everything that happens has a sense of the familiar in it to me. I'm not sure what causes me to get it more often and more intensely than other people I've asked about it, but it hasn't seemed to be detrimental or anything, so I wouldn't worry about it too much unless you're experiencing this change in conjunction with other symptoms as well. Depending on your age, a lot of your neurology can still be somewhat plastic, so it could just be things settling into place. Or, as One suggested, maybe it has something to do with some kind of change like you not getting enough sleep.
As far as the recursive nature of existence goes, I'm not so sure about the idea that things have happened exactly this way before... They certainly might have... What I do think though is that reality is built out of a handful of underlying patterns and that these patterns are predictable. I tend to feel that nothing is really random, and on some level you can sense that a certain outcome will occur a little before it happens or even as it's happening such that it feels familiar. Though saying it out loud makes it sound like I'm postulating a somewhat superhuman ability of premonition. Perhaps more likely is that for one reason or another your sense of time "lags" and your mind double or maybe even triple feeds the same experience to you before you properly feel yourself experiencing it so that by the time you do actually experience it it feels highly familiar. If we could find a way to intentionally and reliably stimulate deja vu in people, it would be very interesting to test their reaction times while they're experiencing it and then compare it to subjects' baseline reaction times. I wonder what kind of effect we'd find. I think the most likely result would be no effect, which would be unfortunate cause it wouldn't tell us much. If it was that their reaction time was delayed however then I think that might point to their sense of time "lagging" as I've said. What would be truly fascinating though is if they reacted faster. That might indicate that they're somehow either processing faster than usual, or "predicting" things more effectively, or something like that. I really want to see some kind of study like this