The present invention relates to a display device and a method of operating the display device that reduces power consumption, the method including: operating the display device in a normal driving mode to display an image on a display panel according to an image data signal; operating the display device in a sleep mode, by adjusting a clock signal and an image data signal transmitted from a set controller to of the display device to a first voltage; measuring a sleep mode time period during which the display device operates in the sleep mode; and adjusting a register value of the set controller when the measured sleep mode time period exceeds a first reference time period and maintaining the clock signal at a second voltage according to the adjusted register value as a clock-off mode.