Unsedated or slightly sedated colonoscopy has fast recovery time, less cost and lower or no incidence of drug-related side effects. May be slightly painful or discomforting for the patient. Based on numerous recent reports, the investigators designed this randomized controlled trial hypothesizing that the use of warm water irrigation versus air insufflation during the insertion phase of colonoscopy might increase the global tolerability of the examination and the proportion of patients undergoing complete colonoscopy without sedation or with a low dose of sedatives.