Time roughs up presidents. Photos of Barack Obama on election night in 2008 look like they were taken much longer than four years ago. Now his face has deeper creases and crow's feet, while his hair is salted with white.

"You look at the picture when they're inaugurated, and four years later, they're visibly older," said Connie Mariano, a former White House physician whose stethoscope checked presidential hearts from 1992 to 2001. "It's like they went in a time machine and fast-forwarded eight years in the span of four years."

That's because of the unabated, unfathomable stress that presidents face. "You see it over a term," said Ronan Factora, a physician specializing in geriatric medicine at the Cleveland Clinic. "It's a good study of chronic stress on a person's overall health."