Apparently, I have a reputation for being an exercise nut. I'm not sure why. Maybe it's the sweaty trainers sitting under my desk. Or the occasional appearance in the office dressed in Lycra. So perhaps the Lucozade on intravenous drip was taking things too far, but it's not as if I exercise every day.

If I do the arithmetic, I suppose there'll be a 50-mile bike race once a week; another ride, plus a training session on an exercise bike; at least one lunchtime run; and an occasional swim or game of tennis (which barely counts as exercise in my book). OK, so maybe five times a week - that hardly makes me a freak.

Or does it? Last week, Ulrika Jonsson confessed that her fitness habit had got a bit out of hand. "I've become a bit obsessive about it," she admitted. Commenting on her case, Deanne Jade, principal at the National Centre for Eating Disorders, said: "Exercise produces a lot of endorphins - it is like a self-esteem drug. But the problem is that it is only a prop ... people start imagining all sorts of things will happen if they stop doing it.

"The best way of stopping the addiction is, like any drug, to go cold turkey and stop exercising."

So could I go cold turkey for a week? That was the challenge.

So no bike-racing, and no training? "Obviously."

No lunchtime run? "Definitely not."

No sit-ups or press-ups either? "What, you do sit-ups? You maniac. Out of the question."

Can I run for the bus? "No. And you have to use the lift. If I see you using the stairs, it'll count as cheating."

Jeez. It's a harsh regime, this not-exercising. But I like to think I'm the sort of guy who doesn't quail at a tough challenge. Seven days of inactivity? Huh, I can vegetate with the best of them. A week without a workout? Bring it on.

My wife, Anna, reckons that my exercising is a borderline case of obsessive-compulsive disorder. In fact, she doesn't think it's borderline at all. I added that bit because, of course, I'm in control. It's not like an addiction; I could give up any time. Well, now my bluff has been called.

The thing is, I know the effort involved in getting fit, so just the idea of losing it makes me twitchy. Surreptitiously, I check out just how much damage a week without a workout will do: I Google "detraining", as we obsessive-compulsives like to call it.

The good news is that probably not too much happens in one week. Studies show that just one good session every seven days is enough to maintain aerobic fitness. I won't get this benefit, but even if I did nothing for three whole weeks, there would be only a 7% decline.

Doesn't sound much, perhaps, but the decline is inexorable. According to one cycling website, fitness has a half life, a bit like radioactivity: "After two weeks of inactivity, you'll go halfway from your trained state to the level you'd be at if you hadn't trained at all. Take another two weeks off and you'll lose half of what's left, and so on."

A week in that context is scarcely a blip. Still, I'm taking no chances. It just so happens I manage to do a bike race on the last day before my sloth regime kicks in. So that means that I'm not really worrying about not exercising for the first couple of days. Ha!

But the third day is the crunch time: I rarely go three straight days without doing something. The first thing I notice is that I didn't sleep as well. Without being physically tired, and without having the stress-busting effect, I wake up early and can't get back to sleep.

Am I more irritable? My colleagues are on the lookout. I think not, but I tell them to back off anyway. They're enjoying this, the malicious bastards.

So I'm my usual even-tempered self. But maybe a little mania is creeping in. I feel a bit hyper, more susceptible to anxiety. And another weird thing: I stop craving chocolate, ice cream and sweet snacks. Normally, I try to eat well, with plenty of fruit etc, but I don't have to watch what I eat. I just burn up the calories no matter.

Now, the idea of a Kit Kat or a flapjack has little appeal. This feels like a purely physical symptom, but perhaps I'm kidding myself. Perhaps I'm compensating for not exercising by taking my self-control freakery elsewhere?

It's a mental-health hall of mirrors, this exercise thing. On the one hand, it's recommended by doctors as a preventive cure for depression; on the other hand, over exercise and it's an obsessive-compulsive disorder. You can't win: exercise too little, exercise too much - there's always someone on your case.

Day six sees me skipping a bike race I'd otherwise have done. I try to put it out of my mind by doing household chores. I have to mow the lawn. It's a tiny patch of grass but it's a hot day, so I work up a lather. Mmm, feels good. What, cheating? Talk to the hand. Fist, in fact.

Deadline day, I heave a sigh of relief. I can exercise first thing tomorrow morning. The ordeal is almost over.

Seven days without exercising? No sweat. More than a week? Let me get back to you on that.