There exists a small, significant but often neglected sub-genre of literature comprising novels with plots lasting the course of less than one day - Herman Melville's The Confidence Man, James Joyce's Ulysses and Saul Bellow's Seize the Day are examples of what might be called the 24-hour novel. To these, Graham Swift adds Tomorrow, an intense and gripping story about family that spans only six hours.

The novel takes place inside the mind of Paula Hook, a successful art dealer and mother of twins who lies sleeplessly next to her husband Mike, agonising over the day to come. The cause of Paula's insomnia is soon revealed: tomorrow she and her husband will tell their 16-year-old twins the truth about their past - something she worries will irrevocably change their family life.

The book's ending comes at sunrise, before the impact of the secret is known by the reader.

The task Swift sets himself in writing an entire novel in such a limited time frame without even allowing a final resolution is no easy one, but he pulls it off. In the time-restricted world of Tomorrow the only potential for plot development comes through the memory of its narrator - an arena that proves fertile ground for Swift, who endows his protagonist with a well-developed sense of history and a penchant for observation and analysis.

The book takes the form of a confessional memoir that traces the history of the Hook family, beginning during the second world war and coming up to the present. But the journey is far from direct. Swift is an excellent commentator on subjectivity; his narrator's account is pieced together from her selective memories, constantly shifting between time and place and only reaching its conclusion after a long struggle with her own actions in her story.

The subjectivity of personal history is a hallmark of Swift's writing. In Waterland, for example, his protagonist, a history teacher, mingles the story of his own life with an account of wider British history.

Tomorrow confirms Swift as both artful observer and employer of the cliche. Where contemporaries have increasingly looked to the outer reaches of language and absurd plot digressions, Swift's settings, characters and, above all, his language have remained understated by comparison. He excels in his ability to capture the essence of the everyday and so it is with Tomorrow. Swift's narrator relates her story in a stream of consciousness that perfectly renders the tone of a worried, middle-aged mother.

This is a perfectly measured commentary on marriage, family life and identity.