It started with Stephen Hawking's A Brief History Of Time. Then came Dava Sobell's Longitude. Now there's The Private Life of the Brain by Susan Greenfield. After years of skulking in the cellar of the unfathomable, science is sexy.

Of course it does help if the package is delivered by a slim, blonde, shoot-from-the-hip female, and Susan Greenfield is just that. When we meet in her Oxford laboratory, she is wearing black jeans, platform shoes and a tightly waisted red jacket. It also helps that she wasn't always a scientist, nor even a straight A student who got into Oxford with an eclectic mix of maths, history, Latin and Greek.

"Greek philosophy was something I cut my teeth on as a schoolgirl and that gave me an interest in what is an individual, what is individual consciousness, what is the mind. I came up to Oxford and I wanted to do philosophy but it had to be done with something else, so I did it with psychology.

I had no science background at all, but my training had made me ask these big questions, so gradually I went from psychology to physiology and then neurochemistry for my doctorate. "It meant that I could then get my credentials as a scientist - and I've been a scientist for 27 years - that now enable me to revisit the problems I had as a schoolgirl. So I do have the insight, that extra bit of background, that the average neuroscientist would not have."

Susan Greenfield agrees that the problem with most scientists is their tendency to be reductionist, and thus to lose sight of the big questions. "I'm not saying it's their fault; if you're doing experiments you have to focus on one particular question." Greenfield is no different; her own area of academic research and expertise is Alzheimer's and Parkinson's and the links between them. The question she addresses in The Private Life of the Brain is possibly the biggest of all: how the brain generates consciousness.

"I didn't have a burning zeal to bring it to the general public," she says. "Like some people might like gardening or cooking, I've always been interested in this. It's something I like doing in my spare time."

However, when Greenfield was invited to give the Christmas lectures at the Royal Institution in London in 1994, everything changed. (In 1998 she became the first woman director of the institution in its 200-year history.) The lecture was televised and when Greenfield proved a natural in front of the cameras, a media super nova was born.

Over the past few years she has become the face of high-brow science made easy. (Last year she gave a consultative seminar to the British prime minister, Tony Blair.) Brain Story, a series for the BBC, (plus accompanying book) arrives on our screens in July, prompted by the success of The Human Brain: A Guided Tour, published in 1997 and translated into 14 languages.

The Private Life of The Brain, is different: no case histories, no brightly coloured illustrations of neuron networks or electrical circuitry; just a neurological explanation of consciousness and the problems faced by contemporary Western society related to an understanding of consciousness - from drugs to depression, pleasure and pain - written from Greenfield's unique stance of scientist/philosopher.

"Science is very bad at these big subjective problems. It's just because consciousness is such a difficult subject that it has been the province of philosophers." Only now that scientists are feeling more confident about the rudiments of how the brain works, she explains, have they joined in.

The Private Life of the Brain is not "some kind of self-help book," she insists. "It's not one of these new age or cult books which offers people a new way of happiness by buying into a certain theory or following a certain person or guru. It's a harder route I'm offering, but I think a more genuine one. No quick fixes or glib philosophy. If you have insight into your condition, how your brain is working and what is happening, it does help."

One area that Greenfield feels very strongly about is recreational drugs. In explaining exactly how they work on the brain, she hopes to illuminate their effects and dangers in purely empirical terms, without resort to moral censorship. "I would like to think there is a new way of showing people that if your mind is the connection between your brain cells, blowing your mind is exactly what you're doing. If people know that, it might change their views."

Greenfield speaks regularly at both schools and prisons. "I explain how the brain works, and how the connections work and then how drugs work on these connections. And then I point out, as these connections are your mind, then losing your mind, blowing your mind and so on, is going to be exactly what happens."

Her descriptions of the states of mind certain drugs induce are fascinating and go some way to explaining their appeal. Although Greenfield admits to smoking cannabis in her youth ("like Clinton I didn't inhale, so it didn't do much for me"), these days she limits herself to alcohol. Despite its dangers, also graphically documented, she is, she says, "not that much of a Calvinist".

It is always a calculated risk, she says. "Certainly what we know about alcohol is that in moderation it is not going to be as dangerous as many of the drugs that work very potently on the brain. Moreover, one could argue that the pleasure I get from the taste, and the way it complements the food, offsets the high stress that might also be life-threatening on my heart. In an ideal world, of course, one wouldn't drink either but I'm only human and these things are nice to do."

From nicotine to heroin, people take recreational drugs in a search for happiness that otherwise seems to elude them. In The Private Life of the Brain, Susan Greenfield sheds light on our increasingly inflated, and unrealistic, expectations of happiness. "I am hoping it will make people ask themselves what happiness is, why they're not necessarily happy, and more importantly, come to terms with the fact you don't have to be happy all the time, that you can't be happy all the time."

The Private Life of the Brain by Susan A. Greenfield is published by Penguin today, £18.99 in UK