Many Media Access Control (MAC) protocols for wireless sensor networks (WSNs) such as Sensor MAC (S-MAC) and Carrier Sense Multiple Access with Minimum Preamble Sampling (CSMA-MPS) exploit listen/sleep cycles to conserve energy. The cycles of nodes influence the network performances such as energy efficiency, packet latency and throughput in WSNs. In this paper, an Adaptive Schedule Medium Access Control (AS-MAC) for WSNs is presented. AS-MAC introduces the adaptation of the cycles in MAC layer to the data traffic. The effect of the protocol is that it can decrease the latency, improve the throughput and energy efficiency when traffic is dynamic. We also compare AS-MAC with S-MAC and CSMA-MPS through simulations and the results show it has better performances than the former protocols.