A characteristic of a washout period following the delivery of therapy to a patient according to a therapy program may be determined based on a physiological parameter of the patient. A washout period includes the period of time during which a carryover effect from the therapy dissipates. The washout period characteristic may include, for example, a duration of the washout period, an amplitude or a trend in a physiological signal during the washout period or a power level or a ratio of power levels in frequency bands of the physiological signal. In some embodiments, washout period characteristics associated with a plurality of therapy programs may be used to compare the programs. In other embodiments, a washout period characteristic may be used to determine a mood state of the patient and, in some cases, modify a therapy program. Monitoring a washout period may also be useful for timing therapy program trials.