So she told me that if I still felt the same in a year's time, she would help me commit suicide." When I ask if he thought she meant it, he insists, as tears well up in his eyes, "yes, absolutely"."Jill never understood my depression, it was a total mystery to her." But her breaking-point with Wolpert was the catalyst for him to seek professional help, in the psycho-geriatric ward at the Royal Free Hospital in north London There he began his journey back to mental health. "I was suicidal, I thought I was going mad."His waking moments were consumed with planning his exit, as he hoarded his heart pills and sleeping tablets. "My wife was very angry and thought that my behaviour was totally unfair to her and to the children. "It took only a few weeks before I had descended into depression," he says, the furrows increasing in his usually animated face. He stopped functioning, shedding all the activities that gave him intense pleasure, such as working, writing and cycling. His wife was convinced the trip to South Africa (where his father was murdered 20 years before), along with his impending retirement, had fuelled the depression.Then suddenly, he was unable to sleep and began to take tranquilisers. I can describe it no other way."Wolpert realised he was ill when he became obsessed with thoughts about suicide, which grew more insistent over several weeks.

But when he developed atrial fibrilation (a common and non-threatening arrythmia of the heart), he began to fantasise about falling ill and dying in a remote place "I began to feel very weird. It was before Jill became ill: they had a happy marriage, he had a great job and was looking forward to travelling to his former home in South Africa. The February sun streaks through grimy windows; all is right with the world.Then Wolpert delves back into the time when doubt, anxiety and despair enfolded him. We squeeze into his closet-sized office, piled high with books, papers and coffee cups, to sit on dilapidated chairs. Within seconds of meeting we have launched into a conversation about cycling, since I have appeared with my helmet tucked under my arm, and he is a committed urban cyclist. Deemed the "lord high contradictory" by Jill Neville, he rocks with an intellectual passion, is unafraid to admit his frailties and appears to have an unquenchable appetite for life. I was in a state that bears no resemblance to anything I had experienced before."There is a delicious irony in meeting Professor Wolpert to discuss the darkest moments of his depression.

"I am ashamed to admit that my depression felt worse than her death," he writes, "but it is true. In the hierarchy of pain, he believes his illness was worse than witnessing in 1997 the death from breast cancer of his beloved second wife, Jill Neville, the Australian writer. The book is striking in its clarity about the mind's capacity to swing out of control and in our growing understanding that such events may be triggered by neurochemicals rather than simple human tragedy.Wolpert argues that unless you have suffered depression, you cannot begin to imagine its torture. Malignant Sadness: the anatomy of depression (Faber, pounds 9.99), which accompanies a BBC2 series, chronicles the intense despair that lead him to an obsession with suicide and finally to find redemption in a psychiatric ward.

The World Health Organisation predicts that, by 2020, depression will top the global chart as the most pervasive serious illness, more pervasive than either heart disease or cancer.Lewis Wolpert, biology professor at University College, London, has stepped out of the closet to describe his own slide into depression and to provide sufferers with a deeper understanding of the illness. In the UK, more than 5,000 people commit suicide every year; more than 100,000 make a serious attempt. The charity SANE fields 1,000 calls per week, the majority from people with clinical depression. One person in five suffers from depression at some point in their lives. Despite its prevalence, sufferers from mental illness still bear stigmata of weakness and shame.The facts, however, are indisputable: depression remains a hidden seam of misery within our society. When he saw the doctor coming, he turned and melted into the crowd.

But although the psychiatrist had seen his patient through a terrifying bout of mental illness, the man felt unable to acknowledge him. Since the man had ended his treatment, my friend made his way through the crush to enquire how he was. His second wife, the novelist Jill Neville, died in 1997. shire with his wife and daughter.A psychiatrist friend of mine described meeting a former patient in a lobby of the National Theatre. He is chairman of the Royal Society's Committee for the Public Understanding of Science and was awarded the CBE in 1990. His books include A Passion for Science, The Triumph of the Embryo and, with Alison Richards, The Unnatural Nature of Science.