This invention provides an image forming apparatus which performs sleep control for a shift to the sleep mode for each function and provides a user-friendly operation system, and a method of controlling the same. To accomplish this, the image forming apparatus sets a shift time until a power state of the image forming apparatus shifts to the low-power state after execution of image formation; and performs control to shift the power state of the image forming apparatus to the low-power state, when the shift time is not more than the predetermined time, before an elapse of the shift time after execution of image formation by the printing function, while shifting the power state of the image forming apparatus to the low-power state, in response to an elapse of the shift time after execution of image formation by the copying function.