There has been a recent awakening of interest in breathing during sleep, due largely to two factors. Firstly, the recognition that some patients become apnoeic intermittently during sleep and that this can produce the sleep apnoea syndrome in adults [ 11 and may be related to the sudden infant death syndrome . Secondly, the development of accurate ear oximeters  which allow the continual recording of oxygenation during sleep, and have led to the recognition of intermittent sleeprelated severe hypoxaemia in patients with preexisting lung disease [4, 51. These observations have led to interest in the control of breathing during sleep. As both the apnoeic and hypoxaemic episodes are specific to sleep and occur most frequently during certain electroencephalographic stages of sleep, there follows a brief outline of the physiology of sleep. Wakefulness is a state of cortical activation recognized by the occurrence of alpha waves on the electroencephalogram (EEG) and of high muscle tone. The cortical activation is maintained by tonic activity of the reticular activating system, which both directly stimulates the cortex  and also facilitates thalamic relay neurons, allowing onward passage of sensory information to the cortex . Sleep is a state in which alpha activity is lost and muscle tone reduced. It can be subclassified into different stages  according to the frequency and voltage of the EEG and electromyogram (EMG) and the occurrence of eye movements on an electro-oculogram. Sleep is a cyclical process which in normal adults always starts with non-REM sleep, with rapid eye movement (REM) sleep cycles interspersed approximately every 90 min throughout non-REM sleep, each REM period lasting 15-30 min and tending to be longer later in the night.