We propose a novel adaptive MAC protocol for wireless sensor networks. In existing protocols such as SMAC, the sensor nodes are put to sleep periodically to save energy. As the duty cycle is fixed in such protocols, the network throughput can degrade under heavy traffic, while under light loads, unwanted energy consumption can occur. In the proposed Pattern-MAC (PMAC) protocol, instead of having fixed sleepwakeups, the sleep-wakeup schedules of the sensor nodes are adaptively determined. The schedules are decided based on a node's own traffic and that of its neighbors. Our analytical and experimental results show that in comparison to SMAC, PMAC achieves more power savings under light loads, and higher throughput under heavier traffic loads. Furthermore, unlike SMAC, only the sensor nodes involved in communication wake up frequently in PMAC and hence energy is conserved in other sensor nodes.