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This paper presents an optimization of the power saving mechanism in the Distributed Coordination Function (DCF) in IEEE 802.11 standard. In the 802.11 power saving mode specified for DCF, time is divided into so-called {\em beacon intervals}. At the start of each beacon interval, each node in the power saving mode periodically wakes up for a duration called the {\em ATIM Window}. The nodes are required to be synchronized to ensure that all nodes wake up at the same time. During the ATIM window, the nodes exchange control packets to determine whether they need to stay awake for the rest of the beacon interval. The size of the ATIM window has a significant impact on energy saving and throughput achieved by the nodes. This paper proposes an adaptive mechanism to dynamically choose a suitable ATIM window size. We also allow the nodes to stay awake for only a fraction of the beacon interval following the ATIM window. On the other hand, 802.11 DCF mode requires the nodes to stay awake either for the entire beacon interval following the ATIM window or none at all. Simulation results show that the proposed approach outperforms the IEEE 802.11 power saving mechanism in terms of throughput and the amount of energy consumed.