 Intrathecal morphine is commonly used as an analgesic adjunct for hip arthroplasty performed under spinal anaesthesia. Sleep apnea syndrome is a condition that affects up to 10 of the patients. Effect of intrathecal hydrophilic opioids on the frequency and duration of apneic episodes is unknown. The objective of this randomised controlled trial is to investigate the effect of intrathecal morphine and surgery on apneic episodes. 