An air dryer for a locomotive air supply system that includes a controller programmed to initiate a sleep mode that inhibits the air dryer and de-energizes all valves. The controller determines when it is appropriate to enter into sleep mode based on various conditions of the locomotive air supply system. More specifically, the controller is programmed to calculate whether the air dryer should be placed into sleep mode based on operational characteristics of the locomotive air supply system that are indicative that the system has been idled. The initiation of an air dryer sleep mode by the controller prevents undesirable venting of air from the air supply system and unnecessary usage of locomotive battery power.