A display device comprising: a display panel comprising a plurality of pixels coupled to scan lines (S1-Sn) and data lines (D1-Dm); a data driver coupled to the data lines (D1-Dm) and configured to transmit a data signal; a scan driver coupled to the scan lines (S1-Sn) and configured to sequentially transmit a scan signal; a frequency selector configured to select a driving frequency for displaying an image, the frequency selector further configured to select a low frequency as the driving frequency if it is determined by the display device that the image to be displayed on the display panel is a first image and to select a normal frequency as the driving frequency if it is determined by the display device that the image to be displayed on the display panel is not the first image, wherein the normal frequency being higher than the low frequency; and a timing controller configured to control the data driver and the scan driver based on the driving frequency selected by the frequency selector, wherein the timing controller controls the data driver and the scan driver such that luminance of the display panel on which the first image is displayed is periodically changed according to the low frequency selected as the driving frequency if the image displayed on the display panel is the first image, the display device further comprising a bio-signal sensor arranged to obtain a user's heart rate from a heart-rate measuring device; the display device further comprising a biological information receiving unit configured to receive the heart rate of a user obtained by the bio-signal sensor, wherein the frequency selector is configured to select a value of the low frequency to correspond to the received heart rate of the user, wherein the driving frequency is the selected low frequency value, and wherein the first image is an always on display image for displaying the received heart rate of the user on the display panel at all times even though an input signal for activating the display panel is not detected. a display panel comprising a plurality of pixels coupled to scan lines (S1-Sn) and data lines (D1-Dm); a data driver coupled to the data lines (D1-Dm) and configured to transmit a data signal; a scan driver coupled to the scan lines (S1-Sn) and configured to sequentially transmit a scan signal; a frequency selector configured to select a driving frequency for displaying an image, the frequency selector further configured to select a low frequency as the driving frequency if it is determined by the display device that the image to be displayed on the display panel is a first image and to select a normal frequency as the driving frequency if it is determined by the display device that the image to be displayed on the display panel is not the first image, wherein the normal frequency being higher than the low frequency; and a timing controller configured to control the data driver and the scan driver based on the driving frequency selected by the frequency selector, wherein the timing controller controls the data driver and the scan driver such that luminance of the display panel on which the first image is displayed is periodically changed according to the low frequency selected as the driving frequency if the image displayed on the display panel is the first image, the display device further comprising a bio-signal sensor arranged to obtain a user's heart rate from a heart-rate measuring device; the display device further comprising a biological information receiving unit configured to receive the heart rate of a user obtained by the bio-signal sensor, wherein the frequency selector is configured to select a value of the low frequency to correspond to the received heart rate of the user, wherein the driving frequency is the selected low frequency value, and wherein the first image is an always on display image for displaying the received heart rate of the user on the display panel at all times even though an input signal for activating the display panel is not detected.