Introduction: An excess of postoperative complications have been reported in patients with Obstructive Sleep Apnoea (OSA) following surgical procedures, however, studies reporting outcomes in patients with OSA following cardiac surgery are sparse and of limited quality. The cause of worse surgical outcomes in the OSA population is unknown but deleterious effects of opiates/opioids, common pain relieve medication following surgery have previously been proposed. There is a move towards pre-operative screening for OSA prior to surgery but the best screening methodology has not yet been established and more importantly the effect of treatment, in particular Continuous Positive Airway Pressure (CPAP), on surgical outcomes in patients with OSA is unknown. Aim: This thesis examined the prevalence of sleep apnoea and its association with postoperative outcomes in patients undergoing major cardiac surgery. It also explored the usefulness of the STOP-Bang questionnaire, as a screening tool for OSA prior to cardiac surgery. In addition, current evidence regarding the effects of opiates/opioids and sedatives on patients with OSA was investigated and summarised in the Systematic Cochrane Review. The effect of morphine on severity of sleep apnoea in patients with moderate OSA was examined in a separate study. Methods: The prevalence and association of sleep apnoea with postoperative outcomes in patients undergoing cardiac surgery and the usefulness of the STOP-Bang questionnaire in identifying patients at risk of OSA prior surgery was examined in a prospective, observational cohort study. The Systematic Cochrane review included randomised controlled trials examining the effects of opioids and sedatives, compared to placebo on severity of OSA in patients with established diagnosis of OSA. The effect of intravenous morphine sulphate on the severity of sleep apnoea was examined in a prospective, paired design trial which recruited patients with moderate OSA. Results: A high prevalence of sleep apnoea (47%) and a significant association between its severity and postoperative complications was found in 122 participants undergoing major cardiac surgery. The most significant risk factor for complications was found to be oxygen desaturations during the night reflecting the severity of sleep apnoea (OR=1.1 for each unit increase in oxygen desaturation index (ODI), 95% CI 1.02-1.17; p=0.014). It was found that the STOP-Bang scores between 0-2 would with high confidence exclude patients with at least moderate sleep apnoea prior surgery. The best diagnostic performance for diagnosis of at least moderate sleep apnoea was found at higher STOP-Bang scores of ≥6 which could identify those patients who might benefit from a sleep study before cardiac surgery. A systematic Cochrane review found that opiates/opioids, sedatives and hypnotics have no deleterious effect on the severity of OSA but most of the studies included in the review were of short duration, small size and with indiscernible methodological quality. The results of the Systematic Cochrane Review informed the development of my study, studying the effect of opiate, morphine sulphate, on patients with moderate OSA. This showed no change in Apnoea/Hypopnoea Index (AHI) where median difference (MD) was -12.95, IQR 9.45, p=0.173 but showed significant improvement in sleep apnoea indices including: obstructive apnoea index (MD -2.7, IQR 7.37, p=0.03), central apnoea index (MD – 0.35, IQR 0.83, p=0.04 ).However there was a fall in median nocturnal oxygen saturation. Conclusion: This thesis reports high prevalence of sleep apnoea which was also found to be a risk factor for postoperative complications in patients undergoing major cardiac surgery. In this population, STOP-Bang score ≥6 could identify patients in need of a sleep study to identify those who may be at increased risk of postoperative complications. To date there is no strong evidence supporting deleterious effects of opioids/opiates on patients with OSA but larger studies are needed to clarify its effect.