CLINICAL REVIEW: How light therapy can help treat patients' depression - Dr Peter Standing examines the delivery of light therapy to treat patients who suffer seasonal affective disorder.

Most GPs are aware that many patients become gloomier and more irritable in winter. Seasonal affective disorder (SAD) represents the clinical end of a spectrum of low mood, where some residents of temperate lands become significantly depressed during winter's shorter daylight hours.

According to the SAD Association's publication The Little SAD Book, about one million people experience significant winter depression in the UK. Another 10 million suffer winter blues, a mild form of SAD. Psychiatric references usually quote an adult incidence of around 3 per cent, with women aged 20 to 50 most commonly affected.

To make a diagnosis of SAD, consider the condition …