In this trial, patients undergoing elective colonoscopy under sedation will be randomised to BIS-guided sedation targeting either "light" (bispectral index [BIS] 70-80) or "deep" (BIS<60) sedation. Sedation will be achieved with a standardised regimen of target-controlled infusion of propofol and fentanyl bolus. The primary end point will be the incidence of procedure recall in each group - this will be assessed at the conclusion of the procedure. The procedural conditions, cardio-respiratory complications and recovery including cognitive function will be assessed during and immediately after the procedure. The incidence of dreaming and patient satisfaction with anaesthesia care will be recorded immediately after the procedure. The study will be complete when the patient leaves the hospital on the day of the procedure.