Wireless sensor networks are mainly designed for environment surveillance, wherein wireless sensor nodes cooperate to get their job done. Generally, wireless sensors are battery powered; therefore, it is crucial for them to efficiently use their battery resources. Most of the existing power-saving protocols achieve power savings by periodically putting sensor nodes to sleep. Such a regular sleep/awake mechanism fails to adjust a sensor node's sleep duration based on its traffic load, thus causing either lower power efficiency or higher latency. Furthermore, sensors may be deployed in hostile environments and may thus unexpectedly fail. Most power-saving protocols do not promptly react to such link breakage, resulting in long transmission delays. In this paper, we propose a quorum-based medium access control (QMAC) protocol that enables sensor nodes to sleep longer under light loads. Since traffic flows toward the sink node in wireless sensor networks, a new concept, i.e., the next-hop group, is also proposed to reduce transmission latency. Simulation results verify that the proposed QMAC saves more energy and keeps the transmission latency low.