Trial of combination drug therapy to treat sleep apnoea

INTERVENTION: Randomised, placebo‐controlled, two arm crossover study ( 1 week washout between visits): Arm 1: oral capsules of betahistine (96 mg) and oxybutynin (5mg) immediately prior to sleep (single night, in laboratory study) Arm 2: placebo oral capsules immediately prior to sleep (single night, in‐laboratory study) Interventions will be administered by a study investigator on each laboratory visit just prior to sleep for these acute sleep studies to ensure adherence. CONDITION: Obstructive sleep apnoea; ; Obstructive sleep apnoea Respiratory ‐ Sleep apnoea PRIMARY OUTCOME: Sleep apnoea severity using the apnoea/hypopnoea index (AHI) assessed from overnight polysomnogram[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] SECONDARY OUTCOME: Arousal index and other standard polysomnography parameters measured from overnight polysomnogram[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Daytime sleepiness measured by Karolinska Sleepiness Scale questionnaire[Questionnaire administered upon waking after overnight sleep study after placebo and drug nights.] Next day alertness (breaking reaction time) measured by driving simulator performance [After each single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Next day alertness (steering deviation) measured by driving simulator performance[After each single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Oxygen desaturation index from overnight polysomnogram measured using pulse oximetry[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Polysomngraphy‐derived endotype (loop gain)[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Polysomnography‐derived endotype ( upper airway collapsibility)[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Polysomnography‐derived endotype (arousal threshold)[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Polysomnography‐derived endotype (pharyngeal muscle responsiveness)[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] Sleep efficiency from overnight polysomnogram[Single overnight acute sleep studies (placebo and drug nights, approximately 1 week washout between studies)] INCLUSION CRITERIA: Otherwise healthy adults with obstructive sleep apnoea aged 18 ‐ 75 years with BMI < 40.0kg/m^2 at screening.