This thesis examines the relationships between a wide range of nocturnal sleep and breathing variables and daytime function. Additionally this thesis examines the use of subjective and objective measures of daytime sleepiness, to determine which tests provide the most useful information for SAHS patients. A pilot study found that neither the 103 patients' nor their partners' Epworth rating of sleepiness were strong predictors of SAHS severity. In 150 patients with a wide range of SAHS severity, relationships between nocturnal events and daytime function were examined using newer definitions of arousal and measures of sleep continuity. A broad battery of daytime tests were used including the Maintenance of Wakefulness Test (MWT) and the Short Form (SF)-36. Unlike previous studies, all correlations were controlled for age and awake oxygen saturation, known to influence the variables measured. The current study also examined these correlations in an unselected patient sample with a range of disease severity. The study showed a lack of strong relationships between conventional nocturnal sleep and breathing variables and daytime function. Few baseline variables significantly predicted CPAP use. Daytime function measures were compared within the 150 patients. The Multiple Sleep Latency Test (MSLT) and the MWT displayed a weak, discordant relationship. Measures of cognitive function, psychological well-being and subjective sleepiness better related to the MWT than MSLT, suggesting that the MWT may be a more useful tool in assessing functional impairment in sleep apnoea. The studies presented in this thesis demonstrate a lack of identified factors affecting daytime function in a group of unselected SAHS patients. This may be due to inter-individual patient variability. Also, more sophisticated nocturnal SAHS measures should be examined, as should more 'real-life' daytime assessments, such as ambulatory EEG recorded during a patient's normal daily routine.