The purpose of this study is to determine whether the left lateral tilt-down position, which includes procedures that involve placing patients on their left lateral side at an angle to make their heads lower than their feet, is more beneficial than the left lateral horizontal body position (patient horizontally laying on their left side, traditionally used for colonoscopies) to decrease descending colon insertion time, the difficulty of colonoscopy procedure and patients' pain.

Supine with tilt-down position (trendelenburg position) is used in abdominal and gynecological surgery to allow better access to the pelvic organs, as gravity pulls the bowel out of the pelvic cavity and the rectosigmoid angle straightens, which may also be favorable for colonoscopy insertion. However, few published studies have explored the value of tilt-down position in the colonoscopy (left lateral horizontal body position are usually used) and there is consequently little evidence to support left lateral tilt-down position or left lateral horizontal body position. So the investigators perform a randomized controlled trial of left lateral tilt-down position versus left lateral horizontal position for colonoscopy to investigate the safety and efficacy of lateral tilt-down position.

Inclusion Criteria:

1. Patients whose age is between 18-75
2. Patients who have indications for diagnostic or therapeutic colonoscopy .
3. Patients who have signed inform consent form

Exclusion Criteria:

1. Patients who have undergone colonic resection
2. Patients with glaucoma
3. Patients with intracranial and/or central nervous system disease, including cerebral infarction and cerebral hemorrhage.
4. Patients with severe chronic cardiopulmonary and renal disease.
5. Patients with uncontrolled acid reflux disease or active nausea/vomiting.
6. Patients who are unwilling or unable to consent.
7. Patients who are not suitable for colonoscopy