A method of performing a ranging process between a base station and a mobile station in sleep mode in a wireless access system, wherein the base station provides the mobile station with an initial notification of a periodic ranging time that occurs during a sleep time interval and during which the mobile station is to perform the ranging process, the initial notification included in a first message, the first message indicating whether the mobile station should terminate sleep mode to receive downlink data, and wherein the base station provides the mobile station with subsequent notifications of periodic ranging times that occur during the sleep time interval, the subsequent notifications indicated in a second message, the second message transmitted to the mobile station as part of the ranging process such that the mobile station performs a plurality of ranging processes within the sleep time interval.