Apparatus (20) is provided, including a sensor (32), adapted to generate asensor signal indicative of biorhythmic activity of a user (22) of theapparatus, the sensor signal having a first characteristic, indicative of avoluntary action of the user, and a second characteristic, indicative of abenefit-related variable of the user. The apparatus also includes a controlunit (30), adapted to receive the sensor signal, and, responsive to the secondcharacteristic, generate an output signal which directs the user to modify aparameter of the voluntary action indicated by the first characteristic.
