This paper presents the FTA-MAC (Fast Traffic Adaptive MAC) protocol , a novel energy-efficient MAC protocol based on asynchronous duty cycling for Wireless Sensor Networks (WSNs). In FTA-MAC protocol, the communication between the sensor nodes is initiated by sending a wake-up beacon from the receiving node. The latter fast adapts its wake-up interval according to the traffic rate in order to reduce the idle listening of the transmitting nodes, which results in reducing the energy consumption. FTA-MAC protocol is implemented and evaluated in OMNeT++/Mixim network simulator. Simulation results show that FTA-MAC outperforms state-of-the-art protocols under fixed and variable traffic rates and also with multiple concurrent nodes. Thanks to this better wake-up schedule, the lifetime of a sensor node is increased 1.5 to 2 times.