An air conditioner and a method for controlling an air conditioner are provided. The air conditioner includes an air-conditioning device that air-conditions an indoor space, a lighting device that illuminates the indoor space, an input device that receives signals to operate the air-conditioning device and for selecting a sleep mode, and a controller that controls the lighting device and the air-conditioning device. When the input device receives the signal for selecting the sleep mode, the lighting device is controlled to sequentially perform a sleep entry operation in which the indoor space is illuminated to have a predetermined luminance and color temperature for a predetermined time, a sleep operation in which the indoor space is illuminated to have a predetermined luminance and color temperature until a wakeup time, and a wakeup operation in which the indoor space is illuminated to have a predetermined luminance and color temperature.