Obstructive sleep apnoea (OSA) is a highly prevalent sleep-related breathing disorder. The most effective treatment for OSA is continuous positive airway pressure (CPAP). CPAP therapy has been shown to significantly reduce subjective sleepiness and blood pressure in patients with symptomatic OSA. Its effectiveness tends to depend on its nightly usage and a commonly held view is that CPAP should be used for at least 4h/night. However, previous studies have estimated that a considerable proportion of CPAP users fail to achieve this. In addition, there is inadequate evidence to support this apparent threshold effect and so it is unclear whether such patients actually benefit from treatment or whether they could be withdrawn from CPAP, thus substantially reducing health care costs, or encouraged to increase their nightly usage of CPAP.