Power efficiency is one of the most important aspects in mobile networks, and is a critical issue for almost all kinds of portable devices. Much research has been done in saving power for mobile devices. IEEE802.11 features a power saving mechanism (PSM) in Distributed Coordination Function (DCF). In PSM for DCF, time is divided into so-called beacon intervals. At the start of each beacon interval, each node must stay awake for a fixed time, called an ATIM window. During the ATIM window, if the nodes do not have data to send or receive, they will enter the doze state in the data transmission interval. The size of the ATIM window has a significant impact on energy saving and throughput achieved by the nodes. However, ad hoc networks with PSM have larger end-to-end delays in delivering packets and suffer lower throughput than the standard IEEE802.11. This paper introduces a new MAC protocol, NMAC, to dynamically adjust the length of the beacon interval and the ATM window. We also allow the nodes to stay awake for only a fraction of the beacon interval following the ATIM window. On the other hand, IEEE 802.11 PSM only allows for the nodes to stay either awake or sleep for the entire beacon interval. Simulation results show that the proposed protocol reduces the delays and achieves high throughput and energy efficiency on various traffic loads.