The goal of this clinical trial (Balanced-2 study) is to compare light to deep general anesthesia using widely available brain monitors, to see if 'light' anesthesia could reduce rates of delirium, cognitive decline, and disability in older adults undergoing major surgery. Delirium is the most common serious surgical complication, occurring in an estimated one in four older adults undergoing major surgery. Delirium causes significant distress to patients and family, and is associated with prolonged hospital stay, physical disability, progression to dementia-like illnesses, and discharge to long-term care. Between 10 - 30% of adults aged 70 years and above have surgery every year, and preserving brain health and wellbeing is an important priority during this time. Older adults (aged ≥65 years, or Indigenous, Pacific patients aged ≥55 years) undergoing major surgery with general anesthesia (excluding heart and brain surgery) and able to provide consent will be able to participate. Participants will be randomized to two groups - a lighter general anesthesia group and a deeper general anesthesia group using processed electroencephalography (a brain monitor that provides information on depth of anesthesia using brain waves). The anesthesiologist will titrate anesthetic drugs according to the brain monitor. Participants will be followed up to determine if they experience delirium after surgery, and longer term impact of delirium such as cognitive and physical decline will also be measured. If found to be effect, this simple, cheap, and widely available treatment could reduce disability, preserve brain health and wellbeing of many older adults undergoing surgery worldwide, and save millions in healthcare dollars.