General anaesthesia is a reversible drug-induced coma. Too little can result in patients being partially conscious during surgery; too much can slow recovery after surgery. There is a range of drug doses that can be used and some anesthesiologists use more than others. There is no convincing evidence that any particular dose within the usual range is better. Consequently, there are no guidelines on the best depth of anaesthesia. This study will determine whether general anesthetic concentrations at the low end of the usual range are better than those at the high end.

Participants will randomly be assigned to lighter or deeper general anesthesia. The first day after surgery, two short questionnaires about recovery and memories of the surgery will be completed. During the rest of the participant's hospital stay, a survey about how one feels will be completed.

There will be two telephone contacts after discharge from the hospital. One month after surgery three questionnaires will be completed about performing daily tasks, how you feel and memories of the surgery. Then one year after surgery pain will be assessed.

Inclusion Criteria:

• Age ≥60 years
• ASA physical status 3 or 4
• surgery expected to last ≥2 hours
• post-operative hospital stay expected to be ≥2 nights
• general anaesthesia with or without major regional block
• able to monitor BIS throughout anaesthesia

Exclusion Criteria:

• Unable to monitor BIS (e.g. cranial or intracranial surgery)
• unable to consent
• surgery with 'wake-up' test
• propofol infusion for part or all of maintenance of anaesthesia ('total intravenous anaesthesia')
• previous enrolment in Balanced study