By reviewing the literature, this paper explores the nature and function of sleep Most of the evidence for the functional theories of sleep has been obtained as a result of examining the effects of sleep deprivation, the physiological, emotional and behavioural effects of which are discussed There is a discussion on how an awareness of the theoretical knowledge may help in the nursing care of patients with advanced cancer and other chronic diseases, as well as their carers The physiological effects of stress, and the possible relationship to patients and their carers, leads the author to highlight the need for further research, and possible benefit of proachve intervention for the bereaved The effects of poor nutrition and common symptoms such as pain and dyspnoea on sleep, and the Iatrogenic causes of sleep disturbances, are discussed The importance of individualized patient care is stressed The conclusion is drawn that although researchers do not seem to have been able to prove conclusively any essential function of sleep, the nurse is in a unique position to facilitate and enable patients and their carers to cope during the waking hours, without the added stress that sleep disruption and deprivation bring