Singer, in her book about cults referenced in my previous post, describes problems some people have after doing meditation for long periods -- both long periods every day and over years. One man writes, "'Suddenly I became one with the air conditioner. I just dissolved, and it seemed that when the air conditioner started up it just took me out of my body. There wasn't any me on the bed -- I was "at one" with the motor sounds. It was unspeakable terror. I had dissolved and melded with a motor sound.'" Cults in Our Midst by Margaret Thaler Singer (1995 edition), pp. 144-145.
"Unspeakable terror"? I remember how it was for me when I first started to notice that the objects I used to think of as outside myself weren't really outside at all. There was a mystery to it, a curiosity, but certainly no terror. It was what I had sought, in fact, back in those old days when I did Zen in Japan and read all of those marvelous stories about people who had become one with trees and birds and such. (No climate control in Zen temples in Japan, so becoming one with an air conditioner wasn't likely!)
So why was this man terrified? I think this has to do with preparation. Many of the techniques that were used in a monastic setting in Asia came to America with the idea that they could apply to anyone (especially if they could be taught for a good price). When they work, the ego begins to dissolve -- the ego being, as I think of it, just an arbitrary thought wall separating oneself from the outer world. And I would guess that if someone were not prepared for this to happen, he would imagine he was going crazy.
So can there be too much meditation? I am hardly qualified to speak about this since I don't meditate, but my guess is that yes, for some people, there can be too much of at least some kinds of meditation. The ego does help us navigate the world and it can't be bullied into submission. If it feels that it is going to be annihilated, it will rebel by manifesting various symptoms, including terror. In the end, in my experience at least, the ego doesn't have to be annihilated -- it just becomes seen as provisional, not absolute. But one doesn't know that it will work out that way in the beginning, and for some people, that could be terrifying.