I get this quite often, but it's only recently it's been more obvious to me. I find myself doubting my own existence and thus, that of the world around me. It's a spacey feeling.
In the most extreme form I feel invisible or non-existent one moment and in the next, realise I have a physical body and thoughts. That happens about once every two weeks.
Consider this passage on Schopenhauer
http://en.wikipedia.org/wiki/Schopenhauer%27s_aesthetics :
Schopenhauer believed that while all people were in thrall to the Will, the quality and intensity of their subjection differed:
"Only through the pure contemplation . . . which becomes absorbed entirely in the object, are the Ideas comprehended; and the nature of genius consists precisely in the preëminent ability for such contemplation. . . . This demands a complete forgetting of our own person."
The aesthetic experience temporarily emancipates the subject from the Will's domination and raises them to a level of pure perception. "On the occurrence of an aesthetic appreciation, the will thereby vanishes entirely from consciousness."
The personality of the artist was also supposed to be less subject to Will than most: such a person was a Schopenhauerian genius,
a person whose exceptional predominance of intellect over Will made them relatively aloof from earthly cares and concerns. The poet living in a garret, the absent-minded professor, Vincent van Gogh in the madhouse, are all (at least in the popular mind) examples of Schopenhauer's geniuses:
so fixed on their art that they neglect the "business of life" that in Schopenhauer's mind meant only the domination of the evil and painful Will. For Schopenhauer, the relative lack of competence of the artist and the thinker for practical pursuits was no mere stereotype: it was cause and effect.
I feel that depersonalization is akin to what Schopenhauer is saying. Some individuals escape from the demands of the will, either through excitement, anxiety, or a sheer absorption in what they are doing. The intellect, or something quite beyond the individual will, is so absorbed in something, or detached from the will, that the will to engender desire vanishes from consciousness, causing a comprehension of unique ideas.