The power-saving class of type II (PSC II), a type of sleep mode operations in the IEEE 802.16e standard, is designed to reduce power consumption for a mobile station with unsolicited grant service and real-time variable-rate connections. However, the configuration of fixed-length listening windows in the PSC II incurs unnecessary energy consumption or packet loss for these types of connections. In order to improve the energy efficiency of sleep mode operation, a cycle-based flexible window adjustment (FWA) approach for IEEE 802.16m systems is proposed in this paper. The FWA scheme dynamically adjusts the ratio of listening window to sleep window for each sleep cycle based on the number of both arrival and retransmission packets. Numerical analysis and simulation results show that the proposed FWA approach can achieve better energy conservation with reduced packet loss rate.