This paper presents the Self Adapting Receiver Initiated MAC protocol (SARI-MAC), a novel asynchronous MAC protocol for energy constrained Wireless Sensor Networks. SARI-MAC self-adapts to the traffic load to meet specified Quality of Service requirements at the lowest energy cost possible. To do so, SARI-MAC relies on traffic estimation, duty-cycle adaptation and acknowledgement mechanisms. Our performance evaluation assesses that SARI-MAC meets given QoS require- ments in a energy efficient manner and outperforms the state of the art protocol RI-MAC in a broad range of traffic scenarios.