An apparatus (1; 2) for stimulating neurons, comprising - a stimulation unit implantable into a patient's body, said stimulation unit comprising a plurality of stimulation elements for stimulating with stimuli neurons in a target area of the patient's brain and/or spinal cord, and - a control unit controlling the stimulation unit such that each of a plurality of groups of stimulation elements generates the stimuli, wherein - each of the plurality of groups comprises a plurality of the stimulation elements of the stimulation unit, - each of a first group and a second group of the plurality of groups repetitively generates sequences of stimuli within a respective time pattern consisting of successive cycles, - the sequences of stimuli generated by the first group differ from the sequences of stimuli generated by the second group in the number of consecutively generated sequences after which the order in which the stimulation elements generate the stimuli within a sequence is being varied, and/or in the length of the respective cycles, and - the control unit operates the stimulation elements of the first group in different stimulation modes during a first time interval and a second time interval following the first time interval, wherein - the control unit controls the stimulation elements in a first stimulation mode during the first time interval such that the order in which the stimulation elements generate the stimuli within a sequence remains constant for up to five consecutively generated sequences and is then being varied, and - the control unit controls the stimulation elements in a second stimulation mode during the second time interval such that the order in which the stimulation elements generate the stimuli within a sequence remains constant for at least 25 consecutively generated sequences and is then being varied, - wherein the intensity of the stimuli in the first stimulation mode is smaller than or equal to a predetermined intensity of stimuli, and the intensity of the stimuli in the second stimulation mode is at least 1.3 times the predetermined intensity of stimuli. - a stimulation unit implantable into a patient's body, said stimulation unit comprising a plurality of stimulation elements for stimulating with stimuli neurons in a target area of the patient's brain and/or spinal cord, and - a control unit controlling the stimulation unit such that each of a plurality of groups of stimulation elements generates the stimuli, wherein - each of the plurality of groups comprises a plurality of the stimulation elements of the stimulation unit, - each of a first group and a second group of the plurality of groups repetitively generates sequences of stimuli within a respective time pattern consisting of successive cycles, - the sequences of stimuli generated by the first group differ from the sequences of stimuli generated by the second group in the number of consecutively generated sequences after which the order in which the stimulation elements generate the stimuli within a sequence is being varied, and/or in the length of the respective cycles, and - the control unit operates the stimulation elements of the first group in different stimulation modes during a first time interval and a second time interval following the first time interval, wherein - the control unit controls the stimulation elements in a first stimulation mode during the first time interval such that the order in which the stimulation elements generate the stimuli within a sequence remains constant for up to five consecutively generated sequences and is then being varied, and - the control unit controls the stimulation elements in a second stimulation mode during the second time interval such that the order in which the stimulation elements generate the stimuli within a sequence remains constant for at least 25 consecutively generated sequences and is then being varied, - wherein the intensity of the stimuli in the first stimulation mode is smaller than or equal to a predetermined intensity of stimuli, and the intensity of the stimuli in the second stimulation mode is at least 1.3 times the predetermined intensity of stimuli.