However, you can earn two bonus points for entering a “reflection” at the end of each 24 hours, which can be spent treating yourself to something you shouldn’t have. Bonus points are also awarded for getting your full quota of sleep for six nights and, if you tot up ten straight days of exercise, you can skip a day with no points docked. Compliance is a word that’s used a lot by participants — but it doesn’t seem to matter too much if you are not entirely glued to the rules. One woman in my team is drinking prosecco on holiday, but maintains that she is otherwise “compliant”. There’s even a Facebook guide to surviving it while drinking wine every day if you want to.

Petranek says the idea is that the WLC will stretch you just beyond your comfort zone, not leave you ripped. Aside from the string of daily motivational emails and lists of do’s and don’ts, I do wonder what you get for your £38. “My favourite answer to that is nothing,” says Petranek. “Actually, that has always been a particularly funny question for me. Funny, because when people sign up for a marathon, people don’t ask, ‘What do you get?’ Why not? I mean, you can go out any day of the week, any time you’d like and run 26.2 miles — for nothing. OK, sure, you might get the race T-shirt — so you do get something. But in as much as an athletic event provides the date, time, location and course, and shouts ‘three, two, one . . . go’ at the start for thousands of people, we do the same online.”

He then gets so evangelical, I fear I’ve touched a raw nerve. “What if I told you that you would add five years to your life, get a new relationship with your kids and have your parents around a few years longer because of the WLC?” he asks. “We’ve actually been told that by participants. If those are what you ‘get’, how much should we charge? $500? $1,000? $5,000?”

There’s evidence that people do like to pay to get in shape — and proof that it can make a difference. It’s the premise on which all slimming clubs are based, of course. In a 2011 study published in the British Medical Journal researchers from the University of Birmingham compared NHS initiatives with commercial weight-loss programmes such as WeightWatchers and Slimming World. After 12 weeks the weight-loss range among the 740 patients in the trial was from 4.4kg (9.7lb), achieved with WeightWatchers, to 1.4kg (3.1lb), achieved with programmes provided by a GP.

“That physical transaction of paying for something gives you a deeper level of commitment — you feel you want to get your money’s worth and not waste it, so you may well stick with the challenge for longer,” says the performance psychologist Dr Josephine Perry. “In a participant’s mind the challenge has more value if you’ve invested financially in it.”

For some, it’s a route to bragging and narcissistic behaviour

There are other reasons this sort of approach might work. Dr Tara Marshall, a psychology researcher from Brunel University in London, says that by making a public commitment to something — as you do online — you are more likely to stick to it. “Once you join a challenge like this, other people are holding you accountable,” says Marshall. “If they are people you know already, you will want to live up to their expectations, to seek their approval via likes and comments. That level of social support can be very beneficial if you are trying to get fit or lose weight.”

Yet she says that her research has shown that online platforms can be a breeding ground for negativity and narcissism. “I found that some people join these groups when they are already in good shape and following an intense fitness plan,” she says. “When that’s the case, it can be a route to bragging and narcissistic behaviour, to belittling others who are seeking approval for trying to take small steps towards better health.”

Among my group, at least, there’s no evidence of that. Stacey Cullen from London joined just to lose weight. “I need to lose 17kg,” she says. “I’m loving it so far. I do find squeezing in exercise a challenge, but the food is a breeze — I am 2.4kg down in the first two weeks and have not felt hungry once.”

For Renee Harris, a mother of four children under 12, from Berkshire, this is her fifth WLC. She says she is hooked on the team element of it and the fact that there’s no pressure to stick to impossible calorie counts and workout plans. “I’m competitive, and this challenge brings out your competitive side in regards to the point system, even though you are really competing against yourself,” she says. “It also holds you accountable for the choices you have made throughout the day. And if you have a bad day, you just make up for it the next day.”

And what about me? I’ve found that, rather than bringing order and calm to my life, the point tallying and attention to dietary detail can be a source of stress. You have until midnight the next night to enter your daily results — miss the deadline and you get zero points that day. I’ve stuck to the diet plan, not rigidly, but definitely avoided most of my banned list. Sleep has never been a problem — I get eight to nine hours a night anyway — and I have always exercised daily, even if it is only a walk. I’m far from perfect (which is why I signed up), but I can’t see it’s helping me much.

I suggest this to Petranek. “Most people do not find it easy, and if you are, perhaps you need to look at where it is you see the need for improvement,” he says. “If there really isn’t anywhere you want to improve, then no, it’s probably not for you, but you are in the tiny minority. I’ve done every challenge since our inception — this is my 16th — and I am still finding things each time to learn, grow and develop.”
The next Whole Life Challenge begins on September 16;

Peta’s life challenges
1 Exercise — for at least ten minutes a day
2 Mobilise — stretch for at least ten minutes a day
3 Sleep for an allotted time — in my case eight hours
4 Hydrate — set a drink target (mine is 500ml of water) or calculate by drinking fluid ounces of water equal to your body weight (in pounds) divided by three
5 Lifestyle — follow the weekly lifestyle practices distributed each Friday
6 Reflect — at the end of each day, reflect briefly about your achievements
7 Nutrition — on the “performance” plan, I am not allowed: bacon and deli meats, white potatoes, corn, beans and legumes, fermented soy, soy, jam, peanuts and peanut butter, vegetable and seed oils, rice, quinoa, oatmeal, buckwheat, flour, bread, bagels, muffins, cereals, tortillas, diet drinks and any fizzy drinks, fruit juice, milk, wine, spirits and beer, yoghurt, kefit, cream, milk, cheese, coconut sugar, sweets and chocolate, honey, agave, chips, sweet potatoes (fried), hummus, popcorn