At Friday's White House news briefing, President Obama joined press secretary Jay Carney to announce that Carney would be leaving his post, to be replaced by deputy press secretary Josh Earnest. (Associated Press)

It may be Washington’s ultimate burnout job.

Presidential press secretaries get to consult regularly with the most powerful man on the planet, travel to exotic locales on Air Force One, and become the most visible representative of the White House after the president himself.

But the job takes a toll. Jay Carney, President Obama’s press secretary, seemed to acknowledge as much Friday when he announced that he would be stepping down after 40 months.

As press secretaries go, the 49-year-old Carney was practically an iron man. President George W. Bush had four press secretaries during his two terms; President Clinton had five. Obama is on his third, after Carney’s deputy, Josh Earnest, 39, was named his successor. The last person to last two full terms as press secretary was President Eisenhower’s spokesman, James Hagerty.

“The grueling part isn’t just the hours, which are bad, it’s that your mind never gets a rest,” says Ari Fleischer, George W. Bush’s first press secretary. “You’re always war gaming. It’s constant intellectual chess. You’re thinking of the next question that the press is going to ask, and that leads to the next question and the next question et cetera, et cetera.”

Fleischer, who served 31 months during an intense period spanning the Sept. 11, 2001, terrorist attacks and the start of the Iraq war, grew weary of being in the middle of the tug of war between the news media and the administration. The latter, he says, is cautious about saying too much; the former are “insatiable,” demanding more and more disclosure. “Even if you think you’re succeeding, someone will tell you it’s not enough,” he says. “It’s never enough.”

Now a communications consultant and occasional TV talking head, Fleischer says one of the reasons he left the White House is because he sensed the press was biased against Bush and his administration. “You can’t do your job if you think that way,” he says. “It will blind you when you stand at that podium. I tried not to bring that into the briefing room, but it started to eat at me.”

Joe Lockhart, who was President Clinton’s spokesman from 1998 to 2000, says press secretaries aren’t really off even when they’re off.

“You don’t have the luxury of getting behind because you’ll never catch up,” says Lockhart, who also runs a communications firm. “It’s hard to go off the grid. You don’t get pure time off.”

And this was before smartphones turned everyone into a 24/7 info zombie.

Lockhart says he knew he was doing his job well when everyone was mad at him. “You walk into the briefing room and the reporters yell at you because you haven’t given them enough. And you walk into the next room and [White House officials] are screaming at you for telling the press too much. That’s when you know you’ve hit the sweet spot.”

Despite its downsides, Lockhart says being press secretary was “the best job I ever had, and the best job that the people who are there now will ever have. If you like politics and policy and the news media and how they interact, you’ll have the most impact you’ll ever have in your career. You get this incredible view of history being made. And when the leader of the free world turns to you in a meeting and asks you, ‘What do you think?’ that’s pretty exciting.”

Carney, who hasn’t announced his plans, received praise from his boss (“I’m going to continue to rely on him as a friend and adviser,” Obama said), but more mixed assessments from reporters.

“He was intensely proud of having been a journalist and never forgot the days when he sat in one of the seats in the briefing room,” says National Journal’s George Condon, a longtime White House reporter. “I believe that he fought behind the scenes for more openness.”

Steve Thomma, a veteran White House reporter for McClatchy newspapers and the president of the White House Correspondents Association, had a more impassioned assessment.

“This White House has not only failed to become more transparent, it has in many ways become less transparent,” Thomma says. “Notably, it’s kept journalists out of events that later are publicized by White House photos or videos. That trend started before Carney. We have made some small progress with him, but not enough.”

During a January TV interview with Al Jazeera America, former New York Times executive editor Jill Abramson, without singling out Carney, called the White House “the most secretive” she had ever dealt with. Abramson, a former Washington bureau chief for the Times, was critical of the administration’s pursuit of classified leaks to the news media through such extraordinary tactics as secretly seizing reporters’ phone and e-mail records.

Yahoo News reported last June that Carney — a former White House correspondent for Time magazine — had responded to questions at the daily briefings with some variation of “I don’t know” nearly 2,000 times since his first briefing in 2011. It also reported that Carney had somehow dodged reporters’ questions approximately 9,486 times.

Peter Baker, a New York Times White House correspondent, says Carney was “a zealous advocate for his boss, and he didn’t hesitate to push back against his former colleagues just because he once sat where we sit.”