The recent ratification of IEEE 802.15.4 PHY-MAC specifications for low-rate wireless personal area networks represents a significant milestone in promoting deployment of wireless sensor networks (WSNs) for a variety of commercial uses. The 15.4 specifications specifically target wireless networking among low rate, low power and low cost} devices that is expected to be a key market segment for a large number of WSN applications. In this paper, we first analyze the performance of the contention access period specified in IEEE 802.15.4 standard, in terms of throughput and energy consumption. This analysis is facilitated by a modeling of the contention access period as non-persistent CSMA with backoff. We show that in certain applications, in which having an inactive period in the superframe may not be desirable due to delay constraints, shutting down the radio between transmissions provides significant savings in power without significantly compromising the throughput. We also propose and analyze the performance of a modification to the specification which could be used for applications in which MAC-level acknowledgements are not used. Extensive ns-2 simulations are used to verify the analysis.