Apparatus for use with a subject, comprising: a memory, storing a set of computer instructions and storing an initial form of a multi-phase biorhythmic activity pattern and a desired form of the multi-phase biorhythmic activity pattern, wherein the multi-phase biorhythmic activity is respiration of the subject, and wherein the stimulus unit is adapted to configure the time-varying stimulus to guide the subject to modify the respiration, wherein a ratio of durations of two phases in the desired form is different from a ratio of durations of the respective phases in the initial form, and wherein at least one phase of the multi-phase biorhythmic activity pattern corresponds to a respective phase of a multi-phase biorhythmic activity of the subject; and a stimulus unit, executing the stored instructions and generating responsive thereto a time-varying stimulus that has a multi-phase pattern that is characterized by a series of transitional forms intermediate the initial form and the desired form that guide the subject to modify the biorhythmic activity, characterized in that - the memory stores a plurality of exercise routines each having an initial form of the multi-phase biorhythmic activity pattern and an indication of a desired form of the multi-phase biorhythmic activity pattern, - the stimulus unit comprises a user interface enabling the subject to select one of the stored exercise routines, and - the stimulus unit generates the time-varying stimulus responsive to the selection, the time varying stimulus being not responsive to ongoing measurement of the multi-phase biorhythmic activity during generation of the time-varying stimulus. a memory, storing a set of computer instructions and storing an initial form of a multi-phase biorhythmic activity pattern and a desired form of the multi-phase biorhythmic activity pattern, wherein the multi-phase biorhythmic activity is respiration of the subject, and wherein the stimulus unit is adapted to configure the time-varying stimulus to guide the subject to modify the respiration, wherein a ratio of durations of two phases in the desired form is different from a ratio of durations of the respective phases in the initial form, and wherein at least one phase of the multi-phase biorhythmic activity pattern corresponds to a respective phase of a multi-phase biorhythmic activity of the subject; and wherein a ratio of durations of two phases in the desired form is different from a ratio of durations of the respective phases in the initial form, and wherein at least one phase of the multi-phase biorhythmic activity pattern corresponds to a respective phase of a multi-phase biorhythmic activity of the subject; and a stimulus unit, executing the stored instructions and generating responsive thereto a time-varying stimulus that has a multi-phase pattern that is characterized by a series of transitional forms intermediate the initial form and the desired form that guide the subject to modify the biorhythmic activity, characterized in that - the memory stores a plurality of exercise routines each having an initial form of the multi-phase biorhythmic activity pattern and an indication of a desired form of the multi-phase biorhythmic activity pattern, - the stimulus unit comprises a user interface enabling the subject to select one of the stored exercise routines, and - the stimulus unit generates the time-varying stimulus responsive to the selection, the time varying stimulus being not responsive to ongoing measurement of the multi-phase biorhythmic activity during generation of the time-varying stimulus. - the memory stores a plurality of exercise routines each having an initial form of the multi-phase biorhythmic activity pattern and an indication of a desired form of the multi-phase biorhythmic activity pattern, - the stimulus unit comprises a user interface enabling the subject to select one of the stored exercise routines, and - the stimulus unit generates the time-varying stimulus responsive to the selection, the time varying stimulus being not responsive to ongoing measurement of the multi-phase biorhythmic activity during generation of the time-varying stimulus.