An air conditioner may include a detection sensor configured to detect an object, an illuminance sensor configured to detect illuminance, an air conditioning module, a communication module, memory storing at least one instruction, and at least one processor comprising processing circuitry. The at least one processor may execute the at least one instruction to cause the air conditioner to receive, from an external device through the communication module, a sleep mode schedule, obtain, while a current time corresponds to the sleep mode schedule, an absence duration during which a person is not detected in a target space, based on a sensor detection value, and control, based on the absence duration being greater than or equal to a sleep reference value and based on an illuminance value being less than an illuminance reference value, the air conditioning module to operate in a sleep mode.