General anesthesia allows people to have surgery without experiencing the procedure or pain. To remain unconscious, the depth of sleep must be monitored. Various monitors have been developed, one of which is BIS (short for bispectral index). BIS monitors the level of sleep during anesthesia and improves patient recovery because the amount of sleep drugs can be fine-tuned to the individual. Patients who need a lot to stay asleep get more, and those who need less get less. As a result, patients tend to wake up faster with BIS monitoring as compared to standard practice not using BIS. Little is known about the long-term effects of BIS monitoring. This study investigates whether BIS monitoring during anesthesia improves long-term outcome, well after surgery is over. The hypothesis is that it does. Two groups of patients are compared: one in which BIS monitoring was used, and one in which it was not. Groups are compared on tests of memory, concentration and mental well-being, to see if one does and feels better than the other. The investigators also take blood samples to see how well patients' bodies deal with the surgery. The investigators expect the BIS monitoring group to do better.