Method for operating a motor vehicle, with the method having the following steps: - Detection, by a sensor device of the motor vehicle, of at least one breathing movement parameter which describes an upper body movement of a user of the motor vehicle, wherein a sensor of the sensor device is positioned on a seatbelt of the vehicle to detect the breathing movement parameter and to generate a breathing movement signal which describes the detected breathing movement parameter, and - Detection of a driving movement parameter which describes a movement of a motor vehicle seat of the motor vehicle and generating of a driving movement signal which describes the detected driving movement parameter, and - Using the breathing movement signal and the driving movement signal, identification of a vitality parameter by normalising a value described by the breathing movement signal using a value of the driving movement signal, wherein the vitality parameter describes a breathing movement of the user, by an evaluation and control device,, characterised by the steps carried out by the evaluation and control device: - Ascertaining of a reference value range, in which an ascertained value of the vitality parameter lies, wherein the reference value range comprises user-specific reference values of the vitality parameter stored in a storage device, - Using the ascertained reference value range, ascertaining of a driver state index which describes a current physiological state of the user, - Depending on the basis of the identified driver state index, generating of a control signal for operating a driver assistance device of the motor vehicle (10, S8), and - Transmission of the control signal to the driver assistance device (26, S9). - Detection, by a sensor device of the motor vehicle, of at least one breathing movement parameter which describes an upper body movement of a user of the motor vehicle, wherein a sensor of the sensor device is positioned on a seatbelt of the vehicle to detect the breathing movement parameter and to generate a breathing movement signal which describes the detected breathing movement parameter, and - Detection of a driving movement parameter which describes a movement of a motor vehicle seat of the motor vehicle and generating of a driving movement signal which describes the detected driving movement parameter, and - Using the breathing movement signal and the driving movement signal, identification of a vitality parameter by normalising a value described by the breathing movement signal using a value of the driving movement signal, wherein the vitality parameter describes a breathing movement of the user, by an evaluation and control device,, - Ascertaining of a reference value range, in which an ascertained value of the vitality parameter lies, wherein the reference value range comprises user-specific reference values of the vitality parameter stored in a storage device, - Using the ascertained reference value range, ascertaining of a driver state index which describes a current physiological state of the user, - Depending on the basis of the identified driver state index, generating of a control signal for operating a driver assistance device of the motor vehicle (10, S8), and - Transmission of the control signal to the driver assistance device (26, S9).