Sleep disorders are common in all sections of the population and are either the main clinical complaint or a frequent complication of many conditions for which patients are seen in primary care or specialist services. However, the subject is poorly covered in medical education. A major consequence is that the manifestations of the many sleep disorders now identified are likely to be misinterpreted as other clinical conditions of a physical or psychological nature, especially neurological or psychiatric disorders. To illustrate this problem, examples are provided of the various possible causes of sleep loss, poor quality sleep, excessive daytime sleepiness and episodes of disturbed behaviour at night (parasomnias). All of these sleep disorders can adversely affect mental state and behaviour, daytime performance or physical health, the true cause of which needs to be recognised by clinicians to ensure that appropriate treatment is provided. As conventional history taking in neurology and psychiatry pays little attention to sleep and its possible disorders, suggestions are made concerning the enquiries that could be included in history taking schedules to increase the likelihood that sleep disorders will be correctly identified.