A characteristic of a stimulation period, which occurs during the delivery of stimulation therapy to a patient according to a therapy program, may be determined based on a physiological parameter of the patient. The stimulation period characteristic may include, for example, an amplitude or a trend in a physiological signal during the stimulation period or a power level or a ratio of power levels in frequency bands of the physiological signal. In some embodiments, stimulation period characteristics associated with a plurality of therapy programs may be used to compare the programs. In other embodiments, a stimulation period characteristic may be used to determine a mood state of the patient and, in some cases, modify a therapy program.