Energy efficiency is a critical issue for sensor network. In this paper we propose an energy efficient context adaptive MAC protocol for wireless sensor networks. The existing approaches improve energy efficiency by controlling the duty cycle of transmission period. The proposed protocol achieves that by letting each node stay in the sleep mode if the number of packets in the buffer is smaller than the threshold, while the threshold value is decided according to the distance of the node to the sink node. The variable threshold for each switch node, however, may cause increased latency. This problem is solved by immediately transmitting the packets of high priority context data. Computer simulation using OMNeT++ reveals that the proposed protocol significantly reduces the energy consumption compared with the existing S-MAC and T-MAC protocol.