Achieving the desired data rate from a sensor network while simultaneously maximizing the lifetime of individual sensors are somewhat conflicting goals, further complicated by the fact that collisions at the MAC layer will affect both the throughput and power consumption. We address this problem in the context of a beacon enabled 802.15.4 sensor network by analytically modeling the network reliability as the function of the activity management policy and MAC layer parameters. The network coordinator monitors the throughput and adjusts the probability with which individual sensors go to sleep after sending their data. Results show that this activity management policy is able to maintain the desired data rate in a wide range of sensor network sizes and packet arrival rates at the nodes.