Chronic pain is an undesirable condition that impacts predominantly quality of life at all levels. Chronic pain might occur in healthy young patients when acute postoperative pain is undertreated and persists in time. There are some indexes in the market to evaluate pain, but they assess mainly parasympathetic activity. Hence, it´s a measure of the physiological response to pain, which is still a not well-defined concept. Patients under General Anesthesia might be experimenting unnoticed pain as there is no direct standard method to measure it in clinical practice. This study aims to detect brain oscillatory activity in the intraoperative setting in four situations; awake-no pain, awake-pain, sleep-no pain, and sleep-pain. Pain can be assessed by studying the local and global dynamics of brain activity. A promising upcoming measure of pain could be implemented in clinical practice to detect and treat pain.