The paper proposes a throughput and energy-aware medium-access-control protocol for wireless sensor networks. The proposed approach provides a sleeping-time adjustment scheme to flexibly adjust the sleep time to reduce the wasted energy in a low-traffic network and increase the throughput in a high-traffic network. The sleeping and waking durations are determined using time counters rather than time slots. A waking-time sharing scheme is responsible for establishing the common waking time among a group of neighbors for group communication. The proposed protocol solves the energy inefficiency caused by idle listening. Simulation experiments have been performed to demonstrate the effectiveness of the proposed approach compared to some other energy-aware medium-access-control protocols.