A novel energy efficient MAC protocol for Wireless Body Area Network Wireless Body Area Network (WBAN) is the most promising technology in e-health applications. Energy efficiency stands out as the paramount issue for WBAN. In this paper, an energy efficient MAC protocol named Quasi-Sleep-Preempt-Supported (QS-PS) is proposed. The protocol is mainly TDMA-based: nodes transmit packets in the allocated slots, while entering the Q-Sleep mode in other slots. Moreover, for a node with emergency packet, it can broadcast a special designed Awakening Message to wake up the whole network and preempts the right to use the current slot to transmit that emergency packet, thus decreasing delay. Compared with relevant protocols, QS-PS can achieve high energy efficiency and decrease the delay of both normal packets and emergency packets.