“SO,” says Stelios Haji-Ioannou, the founder of easyJet, easyCar, easyGroup and a host of other easyThings equally untroublesome and anti-grammatical. “What is it you think I am?”

A hypomanic, I say. Sir. A sort of milder version of a manic depressive who never gets the depressive bit. Somebody who is on a constant up, indefatigable and tireless, who loves taking risks, hardly sleeps, brims with ideas and feels an almost religious zeal about what they do. It’s a benign form of madness, basically. Do you think you’ve got it?

“Hmm,” he says. “Well. It’s certainly a theory."

It is that. The hypomanic theory is currently causing a stir Stateside, off the back of a new book by a psychologist, John D. Gartner, The Hypomanic Edge: The Link Between (A Little) Craziness and (A Lot of) Success in America (Simon & Schuster). Gartner’s theory is bipartite. First, he thinks that entrepreneurs are nuts. Secondly, and by extension, he thinks that America is such an economically successful nation because it is full of nuts. Poverty-stricken, whingeing Brit that I am, I like this theory a lot.

“During the 1990s,” writes Gartner, “I was planning to write a book about religious movements started by manic prophets. But I began to be distracted by messianic movements happening around me in real time . . . I was a member of one.”

Gartner is talking about the dotcom boom. As an investor in tech stocks, he immersed himself in this world thoroughly. With theoretical millionaires exploding all around him, Gartner began to realise that your religious loon who comes staggering out of the desert and your entrepreneurial demon who comes staggering out of the bank might have an awful lot in common. Almost all entrepreneurs, in his view, are hypomanic. In this sense, hypomania is not a mental illness. Illness suggests something broken, or undesirable. In Gartner's view, “hypomanics” are very much to be envied. As individuals, invariably, they are economically successful. They are tireless. Rather than being afraid of risk, they revel in it. They’re too nuts to quit, so they don’t. They thrive.

Here is a snapshot of a few of what Gartner thinks are the key signs of a hypomanic: “filled with energy; flooded with ideas; driven, restless, and unable to keep still; channels his energy into the achievement of wildly grand ambitions; often works on little sleep; feels brilliant, special, chosen, perhaps even destined to change the world; a risk-taker; charismatic and persuasive; prone to making enemies”.

It goes without saying that these are all also the signs of an entrepreneur. And also, it goes without saying, that somebody needs to tell Sir Richard Branson.

I tried. He didn’t return my calls. In fact, it’s amazing how many hugely successful businessmen don’t get back to you when you approach them, out of the blue, to ask whether they are nuts. Gartner reckons that a true hypomanic will always be thrilled to be identified but, in my experience, this doesn’t seem to be the case. I must have tried eight or so, all semi-household names. Barely a whisper. Thank heaven, then, for Stelios.

He’s not convinced, though. “I do sleep,” says Stelios. “I need a good night’s sleep a couple of times a week. I can only survive on three or four hours for a few nights in a row." Stelios describes himself as a “serial entrepreneur” and is famed for checking his BlackBerry e-mail the very moment that he wakes up, when still in bed. “I work maybe ten to 12 hours a day, and a couple of hours at the weekend,” he says. “Although, thanks to that bloody BlackBerry, a day off is a relative term.”

Stelios wouldn’t quite say that he channels all his energy into work, at the expense of other things. “There aren’t really other things,” he says, a little apologetically. “I have this terrible habit of turning other things into work as well.”

Does he grow irritated at minor obstacles? Would he panic, for example, if his business empire had to survive without him for a whole day? “It doesn’t happen,” he says. “There’s the BlackBerry, you see.”

Yes, Stelios. But imagine that the BlackBerry was broken. Imagine that the phone lines were down, and you couldn’t get to a PC. What then? The millionaire entrepreneur is silent for a moment. “I can’t imagine it,” he says, finally. “I just can’t imagine a day without having anything to do. I suppose that’s your answer, isn’t it?” I’m not so sure. I suspect that Stelios called me back not because he’s a hypomanic, but just because he’s a nice, helpful guy. Professionally, and over the telephone, he seems altogether too calm, and grounded, to fit the bill.