Inclusion Criteria:  Patients over 19 years who undergo laparoscopic colorectal surgery for colorectal cancer at Gangnam Severance Hospital, Seoul, South Korea.  Exclusion Criteria:  Emergency surgery Patients undergoing colostomy Patients with history of open abdominal surgery Patients with arrhythmia Patients with pacemaker insertion Patients with history of heart transplantation Patients taking medications that may affect ANI (antimuscarinics, alpha-agonists, beta blockers) Patients with chronic opioid medication. Cognitive impairment Unable to read consent form (eg illiterate, foreigner, etc.)