A display device (110, 410) for use in a computer system comprising the display device (110, 410) and a computing device (170, 470) external to the display device (110, 410), the display device (110, 410) comprising: - interface circuitry configured for receiving a display signal from the external computing device (170, 470) via at least one cable connecting the display device (110, 410) and the external computing device (170, 470), - a display screen configured for displaying the received display signal; - a sensor configured for providing at least one control signal indicative of the presence of a user in a predefined area in front of the display screen; and - a controller configured to control an operating state of the display device (110, 410) and to select an energy saving mode from a plurality of energy saving modes based at least on the control signal provided by the sensor; wherein the controller is further configured such that in operation of the display device (110, 410), the controller immediately selects a first energy saving mode without any waiting time when an absence of the user is detected by the sensor and selects a second energy saving mode if a presence of the user is not detected within a predetermined time period after entering the first energy saving mode, and controls the operating state based on the selected energy saving mode, such that the brightness of the display screen is reduced in the first energy saving mode, and a backlight illumination unit of the display screen is completely deactivated in the second energy saving mode; and - wherein the interface circuitry is further configured such that in operation of the display device (110, 410) the interface circuitry transmits control information regarding a determination of an operating state of the external computing device (170, 470) via the at least one cable back to an energy management function of the external computing device (170, 470) when the second energy saving mode is entered, wherein the control information indicates the absence of a user to the energy management function of the external computing device (170, 470), and the control information is transmitted to the energy management function by means of a manufacturer-specific virtual control panel, 'VCP', in the monitor command control set, 'MCCS', language according to the Display Data Channel, 'DDC', standard. - interface circuitry configured for receiving a display signal from the external computing device (170, 470) via at least one cable connecting the display device (110, 410) and the external computing device (170, 470), - a display screen configured for displaying the received display signal; - a sensor configured for providing at least one control signal indicative of the presence of a user in a predefined area in front of the display screen; and - a controller configured to control an operating state of the display device (110, 410) and to select an energy saving mode from a plurality of energy saving modes based at least on the control signal provided by the sensor; wherein the controller is further configured such that in operation of the display device (110, 410), the controller immediately selects a first energy saving mode without any waiting time when an absence of the user is detected by the sensor and selects a second energy saving mode if a presence of the user is not detected within a predetermined time period after entering the first energy saving mode, and controls the operating state based on the selected energy saving mode, such that the brightness of the display screen is reduced in the first energy saving mode, and a backlight illumination unit of the display screen is completely deactivated in the second energy saving mode; and - wherein the interface circuitry is further configured such that in operation of the display device (110, 410) the interface circuitry transmits control information regarding a determination of an operating state of the external computing device (170, 470) via the at least one cable back to an energy management function of the external computing device (170, 470) when the second energy saving mode is entered, wherein the control information indicates the absence of a user to the energy management function of the external computing device (170, 470), and the control information is transmitted to the energy management function by means of a manufacturer-specific virtual control panel, 'VCP', in the monitor command control set, 'MCCS', language according to the Display Data Channel, 'DDC', standard.