A method of controlling a screen brightness in a user device, the method comprising: detecting a first illumination surrounding the user device when a display unit of the user device is turned on; determining a first optimal screen brightness corresponding to the first illumination with reference to a first profile of a plurality of predefined brightness profiles for respective illuminations, wherein each of the plurality of predefined brightness profiles expresses a predefined mapping of each of a plurality of illumination values to a respective screen brightness, and wherein the first optimal screen brightness is the screen brightness value of the first profile which maps to the value of the first illumination; controlling the display unit according to the first optimal screen brightness; detecting a change in illumination from the first illumination and determining whether the changed illumination reaches a second illumination; maintaining the display unit at the first optimal brightness until the changed illumination reaches the second illumination, wherein the second illumination is an illumination value which maps to the first optimal brightness with reference to a second profile of the plurality of predefined brightness profiles; determining (511, 513), in response to the changed illumination reaching the second illumination, a second optimal screen brightness corresponding to the second illumination with reference to the first profile of the plurality of predefined brightness profiles, wherein the second optimal screen brightness is the screen brightness value of the first profile which maps to the value of the second illumination; and controlling the display unit according to the second optimal brightness, wherein the plurality of predefined brightness profiles for the respective illuminations include a minimum brightness profile, an optimal brightness profile, and a maximum brightness profile, wherein, for each illumination value, the screen brightness value which maps to said each illumination value according to the maximum brightness profile is greater than the screen brightness value which maps to said each illumination value according to the optimal brightness profile, and the screen brightness which maps to said each illumination value according the optimal brightness profile is greater than the screen brightness value which maps to said each illumination value according to the minimum brightness profile, wherein the first profile is the optimal brightness profile, and wherein the second profile is the minimum brightness profile when the change in illumination is an increase in illumination, and the second profile is the maximum brightness profile when the change in illumination is a decrease in illumination. detecting a first illumination surrounding the user device when a display unit of the user device is turned on; determining a first optimal screen brightness corresponding to the first illumination with reference to a first profile of a plurality of predefined brightness profiles for respective illuminations, wherein each of the plurality of predefined brightness profiles expresses a predefined mapping of each of a plurality of illumination values to a respective screen brightness, and wherein the first optimal screen brightness is the screen brightness value of the first profile which maps to the value of the first illumination; controlling the display unit according to the first optimal screen brightness; detecting a change in illumination from the first illumination and determining whether the changed illumination reaches a second illumination; maintaining the display unit at the first optimal brightness until the changed illumination reaches the second illumination, wherein the second illumination is an illumination value which maps to the first optimal brightness with reference to a second profile of the plurality of predefined brightness profiles; determining (511, 513), in response to the changed illumination reaching the second illumination, a second optimal screen brightness corresponding to the second illumination with reference to the first profile of the plurality of predefined brightness profiles, wherein the second optimal screen brightness is the screen brightness value of the first profile which maps to the value of the second illumination; and controlling the display unit according to the second optimal brightness, wherein the plurality of predefined brightness profiles for the respective illuminations include a minimum brightness profile, an optimal brightness profile, and a maximum brightness profile, wherein, for each illumination value, the screen brightness value which maps to said each illumination value according to the maximum brightness profile is greater than the screen brightness value which maps to said each illumination value according to the optimal brightness profile, and the screen brightness which maps to said each illumination value according the optimal brightness profile is greater than the screen brightness value which maps to said each illumination value according to the minimum brightness profile, wherein the first profile is the optimal brightness profile, and wherein the second profile is the minimum brightness profile when the change in illumination is an increase in illumination, and the second profile is the maximum brightness profile when the change in illumination is a decrease in illumination.