Interactive: the ageing of John Key

Any leader of a country visibly ages before your eyes, says John Key — it’s a high-stress environment.

The 100-hour weeks, high stress and five hours' sleep a night have taken their toll on the Prime Minister, as crow's feet firm up their grip around his eyes and more grey appears at his temples.

John Key, 53, admits he has aged rapidly over the past six years as Prime Minister - but he says he is oozing energy, and if he wasn't excited about the job, he would walk away.

Scroll down for video of John Key in the Herald Hot Seat

"Look at any leader of a country, and they do visibly age before your eyes. There's no question I've got a bit greyer and have a few more [wrinkle] lines. If you're working 100 hours a week in a high-stress environment, it's unrealistic to believe you wouldn't age rapidly.

Swipe to see the effect high office has had on the PM

"You look at someone like President Obama and how much he has aged in the time he's been in office, so it's not unique to me. Obviously he has a much bigger job and a much higher stress load, but it's not completely dissimilar."

With the hustle and bustle of the campaign, the added stress and odd eating habits, he has actually lost 2kg to 3kg in the past few weeks (down to 84.5kg). Going teetotal for six weeks has also deprived him of additional calories.

A day in the life of John Key starts at 5.40am. If the House is sitting, he is often found at the parliamentary gym two or three mornings a week on the treadmill - doing sprint bursts - or unleashing aggression on the punchbag.

Afterwards, he prepares for meetings, question time and media events, and often gives speeches or attends openings.

"Very often it's 10pm and you still have a whole briefcase load of work to get through. I'm not a masochist. If I can go to bed earlier, I will.

"In the weekends, I try for a bit of normality, like going to [son] Max's rugby game, or nine holes [of golf] on Sunday afternoon."

VIDEO: John Key in the Herald Hot Seat

He also runs on the treadmill at home at weekends.

He says he watches what he eats without "fixating" on it. Coffee with trim milk. Diet Coke. He might eat a pie twice a year and he can't remember going to McDonald's in the past two years.

When Mr Key was a backbencher, the workload was much lighter, but it exponentially increased as he climbed the ladder to Opposition finance spokesman, Leader of the Opposition and then Prime Minister.

He typically offers a rugby analogy: "Playing rugby for the 1st XV will be tough and hard, but nothing compared to playing for the All Blacks. Every level is exponentially more work."

Psychologist Sara Chatwin said sleep deprivation and excessive amounts of stress could have a negative - sometimes dangerous - effect on health.

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"When stress exceeds the resources - internal and external - that you have and becomes unmanageable, then it has an effect on how you feel, how you operate.

"You've got the weight of the nation resting on your shoulders, and are in the spotlight 24/7 as well as your family members. It's a heavy toll."

But she thought Mr Key was "ageing very gracefully".

"It hasn't affected his appearance that radically. There are other politicians who show the signs far more than he has," Ms Chatwin said, declining to name names.

Labour MP Phil Goff said the workload as Leader of the Opposition or a minister was "phenomenal".

"I guess it was about 110 hours a week as Leader of the Opposition, and as a minister, probably in excess of 100. I'm not grizzling about that. You're a volunteer and you're doing it because you want to do it. At times it can be stressful, but if I didn't have a stressful environment, I think I'd feel underwhelmed."

Mr Goff said he felt 27 years old when he wasn't in front of a mirror, but when he was, "I think, 'Who's that ol' bastard looking back at me?'"

Mr Key joked that the constant demand for selfies had seen the crow's feet around his eyes take a deeper hold.

"Everyone wants a photo."