Well that is a time where a lot of people lose their hair color.
People lose their hair color due (primarily) to a lack of catalase, which normally removes hydrogen peroxide. It is now thought that much of this work is done during sleep. So, assuming this theory is true, a lack of sleep (an effect every president has mentioned) does cause people to go gray.
That makes sense. Several of my friends in college developed grey patches in their hair that went away shortly after graduation.
It's an exaggeration, but maybe if the 2nd term doesn't go so well, this is what he could look like in 2016.
If that happened to me, I'd bow out of a second term.
Stress is a contributor too, even if you sleep well. Clinton was known as a night owl but Bush, by most reports, got plenty of sleep and he still aged a lot.