1. A method for administering an hypnotic drug to a patient, said method comprising the steps of: (a) establishing a desired hypnotic level to be provided in the patient; (b) administering the hypnotic drug to the patient; (c) obtaining EEG signal data from the patient; (d) deriving at least one measure of the complexity of the EEG signal data; (e) determining the hypnotic level existing in the patient from the complexity of the EEG signal data; (f) comparing the hypnotic level existing in the patient to the desired hypnotic level; and (g) controlling the administration of the hypnotic drug to the patient in accordance with the comparison of step (f).