The IEEE 802.16e standard adopts a power-saving mode (PSM) with a truncated binary exponent (TBE) algorithm for determining sleep intervals. Although the TBE algorithm allows more flexibility in determining sleep intervals, it does not consider the network delay of a response packet. In this letter, we suggest PSM that is based on the conditional probability density function (c-pdf) for the response packet's arrival time at the base station (BS). The proposed algorithm determines sleep interval placement so that the response packet may arrive at the BS during each sleep interval with the same conditional probability. The results show that the proposed algorithm outperforms the TBE algorithm with respect to the packet-buffering delay in the BS and the energy consumption of a mobile station (MS).