Obama age photo: President goes gray, ages in 2009 to 2013 photo comparison

Bill Clinton did it. So did George W. Bush. Ronald Reagan kind of did it and it looks like President Barack Obama is doing it, too.

What do all these men have in common besides holding the highest office in the United States? Their hair turned gray during their presidencies.

Why? One word: Stress.

"When people have stressful situations, what happens is there's a condition known as telogen effluvium, and all that means is that your hair is shedding more rapidly than it should," says Dr. Howard Brooks, a dermatologist and director of SKIN: Cosmetic Dermatology Center of Georgetown in Washington.

Every time a follicle loses a hair and a hair grows back, it uses up pigment. When each hair is replaced, pigment is used. Once the pigment runs out, the hair turns gray or white.

When Bill Clinton entered office at age 46, he already had salt-and-pepper hair, but he came out of his term with a head of nearly white hair.

After George W. Bush's eight years in office, he noted his looks had changed.

"When I go home," Bush said in 2009, "I'm going to look in the mirror and like what I see, except maybe for the gray hair."

As Obama enters his second term, his jet-black hair is speckled with white. With more issues awaiting him, doctors say that's just a sign of things to come.

"There's so much stress," notes Brooks, "That his hair is starting to shed more readily than it should -- about three to four, five times than it should."

And even though Obama will leave office in his mid-50s, physicians say the gray is something he'll have to live with.

"Usually once you're gray, " says Brooks, "You are stuck with the gray."