In this paper, we propose a new energy saving mechanism in IEEE 802.16e/m. By choosing dynamically sleep interval T for various traffic types and sending by Base Station (BS) via mobile-traffic-indication (MOB-TRF-IND) message, a MSS only use constant sleep interval T in whole sleep mode duration until next transaction from awake mode to sleep mode. The new mechanism can be applied for PSC I and PSC II by changing T corresponding delay requirement of service. This mechanism also does not use MOB-SLP-REQ/MOB-SLP-RSP that causes more energy consumption because of staying in waiting stage. The analytical results and simulation show that the proposed mechanism gains more than 36% of energy saving in maximized saving mode or 23% in optimized saving mode if it keeps response time change a little in compared with the standard sleep mode mechanism.