We propose a medium access control protocol for wireless sensor networks (WSN) called Adaptive sensor Medium Access Control (AMAC), which is based on the Sensor Medium Access Control (S-MAC) protocol [1]. Since WSNs are energy constrained, the lifetime of the network must be increased by making it as energy efficient as possible. Whereas S-MAC uses a fixed duty cycle for sleeping, AMAC adapts to traffic conditions by incorporating multiple duty cycles. Under a high traffic load, AMAC has a short duty cycle and awakes more often. Under a low traffic load, AMAC has a longer duty cycle and awakes infrequently. The AMAC protocol is simulated in OPNET Modeler. Analysis indicates that AMAC uses 15% less power and 22% less energy cost per byte than S-MAC with a tradeoff in twice the latency. For an application insensitive to latency, the AMAC protocol offers an extended lifetime.