1. A method for assessing efficacy of an administered therapy for brain state changes of a subject via an implantable medical device, the method comprising: (a) receiving at least one cardiac signal of the subject via the implantable medical device; (b) detecting a change in a brain state of the subject based on a detected first cardiac signal feature value of the cardiac signal, the change in the brain state indicative of a seizure; (c) determining at least one characteristic of the detected brain state change based on the cardiac signal, the characteristic including at least one of an onset time of a detected brain state change, an end time of the detected brain state change, a duration of the detected brain state change, an intensity of the detected brain state change, and a severity of the detected brain state change; (d) logging the at least one characteristic of the detected brain state change at a memory; (e) administering a therapy for treating the detected brain state change of the subject; (f) logging data associated with the administered therapy, wherein the data includes at least one of a dose, an administration site, and a timing of an administration; (g) assessing the efficacy of the administered therapy based on the logged data and the at least one logged characteristic; and (h) generating therapy efficacy information based on the logged data; wherein the intensity of the detected brain state change is determined by at least one of: (a) a maximal deviation of a second cardiac signal feature value occurring after an indication of the detected brain state change event, the maximal deviation being measured from a baseline for the first signal characteristic prior to the indication; (b) a first time from the indication to the maximal deviation; (c) a second time from the maximal deviation to a recovery to the baseline; (d) a third time from the indication to the recovery to the baseline; (e) conformance of a third cardiac signal feature value to a stereotypical pattern for the third cardiac signal feature value during the third time between the indication and the recovery to the baseline; and (f) a change in morphology of a cardiac signal.