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.”

Changes in skin or hair are gradual, he said. “If you do have a stressful event, nothing is going to happen right away.” Nothing visible, anyway.

Inside the body, the pituitary gland jolts the adrenal gland, just above the kidneys. Hormones start coursing. Adrenaline cranks up the heart rate and blood pressure. Cortisol, another hormone from the same gland, causes inflammation and preps the body for converting sugars into energy.

“It’s not intended that people would be chronically exposed to these levels,” said Sherita Golden, a physician at the Johns Hopkins Bloomberg School of Public Health.

Cortisol strains the circulatory system, battering artery walls. The hormone also thins the skin and makes muscles and bones lose mass. The immune system weakens, and viruses that cause colds and cold sores take hold. Sleep turns fitful.

“Your cognition slows, you may feel more depressed, your ability to concentrate goes down,” Factora said. “And it just builds on itself — a real cascade.”

There is one known treatment for stress: exercise. “It is the best benefit a physician can recommend,” Factora said. “There is no drug that can present as many benefits as exercise can.”

Obama is a fiend for exercise. In hourlong workouts, he has been known to hit treadmills hard, weight-train with arms and legs, and build quickness through “plyos,” or plyometrics — exercises that involve explosive movements. He also throws footballs, shoots basketballs and thwacks at golf balls.

His predecessors exercised, too, some of them fiercely. George W. Bush ran till his aging knees made cycling a better option. Presidents Jimmy Carter and Bill Clinton jogged, while Ronald Reagan rode horses and split logs with such vigor, he once cut his thigh. President Gerald Ford performed a daily exercise regimen in his robe and PJs.

President George Bush “didn’t just work out, he worked out vigorously,” said his physician Burton Lee, citing 6-km runs and ball games with marines at Camp David. Once he even played tennis with Pete Sampras.

“He broke the StairMaster at Camp David; he pounded it till it didn’t work,” Lee recalled. “If I’m on it for five minutes, you have to take me out on a stretcher.”

Good exercise leads to better thinking, brain mapping has shown. “Exercise actually brings more blood flow,” explained Linda Fried, an epidemiologist and geriatrician at Columbia University. “Parts of the brain are activated, and they’re associated with complex thinking and problem-solving.”

Workouts also force a president to — truly, finally, deeply — rest. Only then can the relaxed brain start to make creative associations.

Obama had a fitness test on Jan. 12, and the White House said the results would be released by February. His previous physical was in October 2011; it showed that he had added half a kilogram since his February 2010 physical (his 2011 weight: 97 kg, very good for a man who was then 50 and 183 cm tall).

Like all presidents since 1992, he is under constant medical watch: A military physician is on hand wherever a president goes, day or night. That firsthand observation started with Mariano, and even with all her access, she recalled how difficult it was to determine on many occasions whether Clinton was just superstressed or full-on infirm.

“We were worried about Clinton when he was being impeached,” she recalled. “He looked like he had it all together, but we worried.” When she and her colleagues asked, all he would say was, “I’m tired.”

Lee, whom the first President Bush brought to the White House to monitor his health, agreed with Mariano that presidents are a special lot. They push their bodies and minds, and thus develop a greater capacity to fight off infection. They shake enough hands to fell a lesser creature, he said.

But the mental intrusions — the sense that someone needs something every moment of every day — are as insidious as the germs. “It’s just a phenomenally demanding job,” Lee said. “You never get one minute off.”

The job has compounded certain human frailties. Most famous perhaps is the lethal case of pneumonia that 68-year-old William Henry Harrison caught at his inauguration. Woodrow Wilson’s stroke certainly limited his leadership of the country, and Franklin D. Roosevelt worked around the problems related to his polio more ably than might have been expected.

But daily habits also affect presidential well-being in lesser-known ways. Dwight D. Eisenhower was so dedicated to his form of exercise that he played 800 rounds of 18 holes over eight years as president, according to Evan Thomas, the author of “Ike’s Bluff: President Eisenhower’s Secret Battle to Save the World.” Then, in 1955, Eisenhower had a heart attack, and two years later, a stroke. Intestinal surgery came in between, all as he was staving off nuclear war and realigning Southeast Asia.

“Toward the end,” Thomas said, “he was taking an extra sleeping pill at night” — the powerful, old-school kind, with barbiturates. And that was on top of a nightly scotch, never more than five ounces (140 grams), except when it was, Thomas said. “A couple of times he says to his doctor, ‘Let’s get drunk.’ “

Abraham Lincoln’s health can be examined through a series of portraits by Mathew Brady, said David Von Drehle, the author of “Rise to Greatness: Abraham Lincoln and America’s Most Perilous Year.”

While abstemious in an era when other statesmen chain-smoked cigars, Lincoln saw his legendary strength dwindle. At the start, the vigorous president was an able horseman who wrestled soldiers and challenged them to strength tests. One required a nearly 1-meter-long axe to be held out from his body with one hand, which is much harder than it sounds, Von Drehle attested.

By the time of his death, the president had been wracked by insomnia and lost interest in food. He arrived at the White House as a sinewy 190-cm, 97-kg strongman. In the course of four years, he dropped 16 kg. “He was sunken-eyed and grizzled, nothing like that bright-eyed lawyer of Springfield,” Von Drehle said. Lincoln continued to sit for a famous series of portraits, and “by the last set of photographs, he looks 75 years old, but he’s 56.”

To the best of the public’s knowledge, recent presidents have not exacerbated their stress through bad behaviors such as drinking. Obama, however, admitted to kicking a cigarette habit of unknown intensity at some midpoint in his first term.

The side effects of smoking might show up as those lines in his face, the doctors said. While sun exposure can also make a face look withered, Obama’s darker skin has melanin to alleviate UV-ray damage. That same coloring, however, can make his white hair look more pronounced.

Like appearance, endurance is highly individual: One person crumbles in circumstances in which another thrives; what one person actively avoids, another embraces. “The one thing I noticed is that presidents have very unusual personalities,” Lee said. “Each is a different person, but for all, there is no easy day.”

“These are tough people from the outset,” Mariano said. “They go looking” for challenges, mental and physical. She could still clearly envision both the first President Bush and Clinton pushing on, with bags under their eyes, to the next campaign event.

“One factor Bill Clinton had in his favor: He could power-nap,” she said. When his helicopter went up, his eyelids went down. Upon landing, he would announce: “Okay, I’m ready! Showtime!”

Despite the extraordinary stress levels, many recent presidents have lived well beyond normal life expectancy. Reagan and Ford died at 93; Carter and George Bush are 88.

Doctors are coming to understand that stress may have an upside. “Human beings need some degree of stress to keep their systems tuned,” Fried said. “Some people enjoy the stimulation of it and the excitement and couldn’t live without it.”

Plus, human minds literally seek reasons to live. “Many people, as they get older, deeply care about future generations and the world’s survival,” Fried said. “If they have a chance to make a difference, that keeps people healthy.”