Apparatus is provided, including a sensor, adapted to generate a sensor signal indicative of biorhythmic activity of a user of the apparatus, the sensor signal having a first characteristic, indicative of a voluntary action of the user, and a second characteristic, indicative of a benefit-related variable of the user. The apparatus also includes a control unit, adapted to receive the sensor signal, and, responsive to the second characteristics generate an output signal which directs the user to modify a parameter of the voluntary action indicated by the first characteristic.