We've all heard of people going gray or white-haired almost overnight
after sustaining a great shock or sorrow. What biological process
makes this happen?

Hello mccook-ga,

The biological process is called "alopecia areata".  As the following
site explains, this condition affect hairs that are not gray.  The
non-gray hairs fall out, leaving the gray hairs behind.  It can be
triggered by "a period of high stress such as the loss of a loved one
or loss of job."

"Gray hair overnight"
Sound Medicine
http://www.soundmedicine.iu.edu/archive/2002/mystery/alopecia.html

See also:

"Can hair turn white overnight from fright?"
The Straight Dope
http://www.straightdope.com/classics/a2_315b.html

There is a good deal of information about this condition on the Web. 
Indeed, it's got its own Google Directory category:

"Health > Conditions and Diseases > Skin Disorders > Hair Loss > Alopecia Areata"
Google Directory
http://directory.google.com/Top/Health/Conditions_and_Diseases/Skin_Disorders/Hair_Loss/Alopecia_Areata/?il=1

A useful site listed near the top of that category is:

National Alopecia Areata Foundation [including information on "What is
Alopecia Areata?]
http://www.naaf.org/default2.asp

- justaskscott-ga


Search terms used on Google:

inurl:straightdope.com gray hair
"alopecia areata" gray
"alopecia areata"

Man, are you good.

Wow, I always thought that was a myth. Thanks to justaskscott, there's
some good reading to do.(But, I'm a bit weary, because I don't want to
wake up with white hair.)
Yesmam