IEEE 802.15.4 is the Low-Rate Wireless Personal Area Network (LR-WPAN) standard that is suitable for wireless sensor networks and wireless home networks among others. The IEEE 802.15.4 is specifically designed for energy efficiency since many 802.15.4-compliant devices are expected to operate on battery. Because of inadequate design of the downlink frame transmission mechanism in the standard, however, 802.15.4 devices can waste their energy due to collisions even under modest downlink traffic in the network. In order to solve this problem, we propose a novel mechanism which evenly distributes the downlink frame transmissions and decimate collisions. It exploits the information already imbedded in the beacon frame, so it does not require modifications of the IEEE 802.15.4 standard. Our scheme significantly reduces the energy consumption of 802.15.4 WPAN devices under modest to heavy downlink traffic, while not adversely affecting the system performance under low utilization.