An air conditioner and a method for controlling the air conditioner are provided. The air conditioner may include an air-conditioning device having a variety of components that provide air-conditioning of an indoor space, an input device that receives signals to manipulate the air-conditioning device and signals to select a sleep mode, and a controller that, when the input device receives a signal to select the sleep mode, controls the air-conditioning device to perform a rapid eye movement sleep operation at least one time to air-condition the indoor space at a temperature higher than a temperature that is set in accordance with the sleep mode.