Anyone who has spent any time on a psych ward knows that the mad are usually far less scary in real life than they are in the popular imagination. Part of the reason for this is that, depending on category and stage of treatment, most psychiatric patients are heavily medicated, but there are other factors too: the profoundly depressed tend to be fairly quiet anyway while, much of the time, the schizophrenics are glued to the television screen, conjuring messages from dead relatives and extraterrestrials out of the ether. I once knew a woman who spent much of her time receiving (or just waiting for) instructions on what to do next from a lost cosmonaut who'd drifted into space years before: to begin with, I found it disappointing that she called him "Major Tom", after David Bowie's hit single, "Space Oddity", but I discovered later that she knew this wasn't his real name. It was, she said, a codename he had been obliged to adopt "for security reasons".

The night before Dennis O'Donnell began work as a nursing assistant in the intensive psychiatric care unit (IPCU) of a modern mental hospital, he dreamed that he was an orderly in a Victorian asylum, a place "gloomy and cold as a dungeon. Naked madmen, with their heads shaved against lice, were chained to the stone walls howling lunatic bans In a padded room a straitjacketed maniac bounced off the walls. Fellow orderlies turned a hose on a violent fight between two inmates. In the last room a man was being restrained on an operating table as a surgeon prepared to trepan him". In short, he shared the same scary fantasies about madness that most of us entertain, dark visions of animalistic chaos in which it is hard to distinguish between the terrors of insanity and the horrific approaches to incarceration and treatment.

Yet, in spite of his nightmares  and some fairly frightening events during his first few shifts  O'Donnell, an English graduate and former school teacher, worked on the locked ward for another seven years and, during that time, overcame his initial fears and acquired respect, occasional fond feelings and even admiration for the people who, in the darkest weeks or months of their lives, were entrusted to his care. A short man who had initially resisted the move from a geriatric ward into IPCU because he wasn't "a fighter", O'Donnell became adept in the art of control and restraint, which usually has much more to do with empathy and quick thinking than physical prowess. In his attempts to understand the various triggers for mental illness (religion, sex, money, family and drug use among others), he studied the ancient texts and superstitious beliefs that, if they fester long enough in an unquiet mind, can drive a man to dash his face to pieces on a bathroom wall in his attempts to drive out "the devil". Most importantly, though, he learned to pay attention  and in psychiatric wards, attention is everything.

The Locked Ward is crammed full of vivid anecdotes and careful observations that run from terrifying to laugh-out-loud to exquisitely painful. But O'Donnell's main purpose, his true and clearly heartfelt mission, is to get us to cast off the prejudices that we bring to the madhouse and see the patients as people very much like ourselves. He quotes Salvador Dalí's remark that "the only difference between me and a madman is that I am not mad" and summarises his seven years on the ward as follows: "One of the most important things I learned was that there are as many illnesses as there are people. All different. All unique. All requiring to be treated seriously and with delicacy." At one point, he quotes the Qur'an: "Do not give the property with which God has entrusted you to the insane; but feed and clothe them with this property and speak kindly to them," adding: "There's hardly a better recommendation for treatment."

The Locked Ward can be a difficult read, yet it is also strangely exhilarating to watch this keenly compassionate and intelligent man learning to care for those who, at the start of the book, populated his most frightening nightmares. For obvious reasons, O'Donnell is careful to maintain a direct, no-nonsense and doggedly unsentimental approach throughout. (At one point, after the staff of the locked ward have worked something of a miracle to bring a patient named Wayne back from catatonic depression  back, in short, to his "real self"  O'Donnell concludes: "Unfortunately, his real self turned out to be a spoiled selfish fuckwit and nobody liked him. But life's like that sometimes.") At times, he strives too hard for the common touch and occasionally, in his zeal, he can sound like one of those free information booklets on mental illness you pick up at the doctor's surgery. Overall, though, this is a valuable contribution to the literature of madness and will increase understanding of how intensive psychiatric care can save lives. "I hope this book is full of humanity as well as humour," O'Donnell says, with touching directness, in his introduction. He can rest assured that it is, in spades  and, as he himself says, the two are always inseparable.

John Burnside is the winner of this year's TS Eliot prize with his poetry collection Black Cat Bone (Jonathan Cape)