Techniques for delivering electrical stimulation therapy comprising a complex variation to at least one electrical stimulation parameter are described. In one example, processing circuitry of an implantable medical device (IMD) identifies a plurality of electrical stimulation parameters for at least one pulse train of electrical stimulation. The processing circuitry defines a complex variation to at least one electrical stimulation parameter of the plurality of electrical stimulation parameters. The processing circuitry modifies the at least one pulse train of electrical stimulation by introducing the complex variation to the electrical stimulation parameter function and controls a stimulation generator of the IMD to generate, as modified, the at least one pulse train of electrical stimulation.