The paper proposes an energy-aware contention-based protocol for the medium access control (MAC) sub-layer for wireless sensor networks. The innovative approach with frequent sleeps proactively and aggressively reduces the wasted energy while increasing the throughput and decreasing the latency. The proposed MAC protocol solves the energy inefficiency caused by idle listening, control-packet overhead, and overhearing. Simulation experiments have been performed to demonstrate the effectiveness of the proposed approach, especially compared to the S-MAC and T-MAC protocols.