Peter Howson is dreaming again. After years of drink and drug abuse that left him in a deadened stupor or with head-spinning insomnia, he has turned his back on excess, found God, escaped a brush with the occult, sought knowledge in Islam, discovered the benefits of energy drinks and recaptured the artistic vigour which once threatened to leave him. Of course that was in the days when standing in front of a canvas meant drinking half a bottle of spirits first, when the drugs came up from London hidden in record sleeves and he had a £700 a week habit funded by painting. Its now a year since Howson checked himself into Castlecraig detox centre near Peebles for a month of medical boot camp, and, he says, he's still clean.

And so the dreams have returned. "I'm dreaming all the time now, I'm having amazing dreams," he says. "I keep a diary every night and I write down what I dream about. A lot of the time I don't remember it but I know I do dream."

"I had a dream a few weeks ago that I went into a bar and had two large gin and tonics and suddenly realised what I was doing and panicked and then woke up". He laughs. "I was so relieved. I thought I'd ruined everything."

We're sitting in Howson's glassy eerie, an enormous top-floor studio just off Glasgow's Great Western Road. In one corner is a table filled with tubes of oil paint; at right-angles to that is another on which paint residue has hardened into multi-coloured sculptural whirls. In the opposite corner are a family of filing cabinets which contain virtually every newspaper article ever written about him. Some praise him, some damn him, but he keeps them all. Most of the rest of the room that isn't window is obscured by neatly stacked canvases in various sizes. It's an ordered environment, arty without being bohemian. Under the window is a stack of Shark energy drinks; Howson gets them from a member of the group he meets with every Thursday night. As well as the soft drink seller it includes a publican and a couple of other artists. They meet, talk politics, morals, ethics. Howson dips regularly into his stash as we talk. Perhaps it's a way of reconnecting with the sense of security he gets from the weekly meetings; or perhaps he just likes the taste. He offers me one but I decline.

The sky over the city is as bruised and threatening as the subjects of Howson's famous Saracen Heads series and as my tape unspools it captures their creator's measured speech and at least a flavour of his measure-less personality.

He's friendly, seems happy, is open and frank about everything from his Asperger's Syndrome (a form of autism) to the shortcomings of Tracey Emin and the upcoming Turner Prize. But then he'll tell me other things, scraps of information and outlines of stories which he refuses to flesh out when pressed for names and dates. Standard practise for plumbers, it's more challenging in an artist whose work is lauded for its brutal honesty. He intigues me, but he makes me wary too. So: a measure-less personality.

Dominating the room is a painting which looks finished but which has, I'm sure, a long way to go, Howson gestures to it. "It's called The Enemy Within. It's loosely based on the American paranoia and fear of attack from within their own country. There's a kind of Britney Spears thing with the snake."

There kind of is. Naked figures draped in the Stars and Stripes tangle with serpents on a canvas big enough to ceilidh on if you laid it flat. Whether the critics will oblige when its unveiled remains to be seen: it's certainly powerful in this confined space.

"I'm 43 and last year I couldn't stand up and do a big one like that without having to rest every five minutes. Now I can do this in two or three weeks of heavy work and I'm at it every day. I feel elated. I don't feel tired in the slightest...It's getting back to my old way of how I should be painting which is quite energetic and full of vitality, which I was losing before. The colours have become more vivid." Now his only problem is how he reaches the corners. The mouth drops into a sly smile. He has, he says, a wobbly ladder. "But that adds a bit of adventure to it." He shoots me an impish look.

Stuck up under the window is a cutting from a newspaper showing an Afghani civilian. Howson, like everyone else, is fascinated and appalled by the war. What he has that few others have, however, is first hand experience of combat zones, having served as a war artist in the Balkans.

"For artists like me it's like manna from heaven. It's an irony that some of the greatest paintings that have been done in our history have been done about terrible things. Every image that's powerful is a tragedy in a way.

My best work came out of Bosnia, Kosova wasn't as good because I was drinking too much at the time and my mind wasn't quite as focused. The work that I'm doing on Afghanistan and the World Trade Center is going to be good work."

In April next year, Howson unveils this new work in a major exhibition at the McLaurin Gallery in Ayr. The Enemy Within may be among the works on show, so too may a portrait of Madonna that's planned but not yet executed. An avid collector of British artists, the singer owns several of his paintings though Howson is dismissive of the false kudos the "collected by Madonna" tag brings him.

"It gives me more of a thrill to sell to a plummer in Dennistoun than it does to sell to Madonna," he says. "People make such a hoo-hah about her. When I first met Madonna I was star-struck and my knees were knocking together because I was so nervous. Doesn't mean a bloody thing to me now, this idea of having known her."

So how many times has she sat for him? "Can't tell you. I've been told not to say a thing about anything to do with Madonna. All I know is that she's not happy about posing for me because she thinks I'll make her look ugly, but I'm doing it anyway. But she has (sat) in the past." He laughs. "I'm thinking about doing her as the Whore of Babylon."

Robbie Coltrane is another would-be sitter. "I haven't done it yet," he says of the proposed portrait. "I'm having it done probably at the end of the year. I don't know whether to do him as Hagrid or not. I'm thinking of doing two Robbies - the real Robbie and Hagrid because my commercial instinct says I'll be able to sell the one of Hagrid pretty easily."

In a fortnight's time, Madonna will leave her Howson collection to present the Turner Prize live on the television. Howson himself has been vocal in the past about what he calls the "art mafia" and believes that one of the world's most prestigious art prizes should be reserved for painting alone. "I'm an old-fashioned bastard," he admits cheerfully, though I suspect he might tune in to see who wins.

As it is, it's open to all artists under the age of 50. If he is ever to win it, he has only seven years left to do it, but given the current love affair between that "art mafia" and all things conceptual - Hirst, Emin, Whiteread and friends - it seems unlikely. He is by his own admission, massively intolerant of conceptual art. But when he talks about it, its more with incomprehension than snobbish disdain.

"If I go into the National Gallery and I see one small painting by Durer that would capture my attention for two hours, just looking at that tiny canvas. I get more out of seeing a small painting that doesn't do anything, that just glows at me and moves me and makes me think and which has a spiritual power about it. If you put that beside Tracey Emin's bed, it turns it into a travesty. It makes a nonsense of art, people actually paying money and going round this bed."

Didn't he even cheer when Chris Offili - a painter - won the Turner Prize? "I thought it was bad because the drawing was useless. Anyone who has to use elephant dung must be crap. For me it's one one of the basic rules of painting - you do not stick things on them."

Howson, of course, has a reputation for courting controversy. It is whispered in some quarters that he also craves publicity. I ask him about it. "If you are an artist and people forget you, then you've had it. You just gradually disappear," he says. "For an artist to survive you have to be in the limelight the whole time. And one way to do that is to be in the newspapers, or on the television or on the radio. So you have to keep on going. The time for becoming a recluse is in later life." But, he adds, "I've never courted publicity just to get in the newspapers."

But his frankness has made for an uneasy relationship with the media: he'll open up about everything under the sun - "Am I raving and rambling?" he asks me at one point; "This sounds shite doesn't it, all this stuff about religion?" at another - and consequently he'll get turned over (at worst) or leave interviewers with a deep sense of scepticism.

If it looks like he does court publicity, perhaps the answer lies in his Asperger's Syndrome, a form of autism which can affect social behaviour and non-verbal communication but whose sufferers often display extreme talents and obsessions. He was diagnosed three years ago and his 15-year-old daughter, Lucie, is also a sufferer.

"The obsessions are wild, wild obsessions that are very destructive sometimes. They can be put to creative use and I was very lucky that one of my obsessions was art which was my creative obsession. But my less creative obsessions were not good at all. Drink, drugs, sex, lots of other things that I'm not going to go into, which could have ended up with me going completely insane." Concepts of personal body space and friendship look different to Asperger's sufferers, something Howson touches on when he tells me: "Journalists aren't what most people think of them as being, lower forms of life. I feel journalists are the same as anybody else and I generally talk to them [as friends]."

By the time he tells me about his Asperger's, the tape has already collected his thoughts on his recent return to Christianity. He was a zealot when he was younger, but since he came out of Castlecraig he has become increasingly drawn to religion in all its guises, slipping off his shoes to enter Glasgow mosques and dipping into the Koran. Islam's asceticism and rigour appeals to him, he tells me; perhaps an echo of the renewed vigour with which he approaches his art?.

A darker development was his becoming "almost involved in Satanism". He doesn't elucidate but it led him to his Damascene moment - or rather Damascene month. "Suddenly I realised that was the wrong path so I decided to change direction," he says calmly. "I had this amazing experience. It wasn't an instant experience that happened in one minute, it happened throughout three or four weeks. And I get a lot of people saying 'That's a physical thing, you're off the drink and it's affecting your mind'. But as a Christian I don't believe that. I believe that it's Jesus Christ."

Whether it's Jesus Christ, or just energy drinks, Howson is convincing as a man in thrall to his new credo - taking one day at a time and ignoring the horizon in favour of what's immediately in front of him. "I feel completely amazing," he says. "I never actually enjoyed the moment (before) because I was always worried about the future. But now I take one day at a time and make sure I don't drink or take drugs. I'm trying," he adds, "to lead a responsible life." Just painting and dreaming.