The New England Journal of Medicine, April 2012 issue, has a very thorough article on the Medical Progress of Alopecia Areata. You can read the article in its entirety online.
I saw that I have the "sudden graying"...so, essentially, all my pigmented hairs are gone and what is left is probably how much gray (actually, white) hair I would have had by now, naturally, mixed in with my former brunette hair. Hm. Should I buy a salt-and-pepper wig to show my REAL age? Ha ha.