In wireless sensor networks, the simplified and energy efficient protocol should be designed in order to maximize the network lifetime because of its stringent resource constraints, ultra power limitation, and tiny embedded devices. In this paper, we propose an enhanced cross-layer protocol for energy efficiency in wireless sensor networks by integrating medium access control and routing protocol. Our proposed protocol utilizes a synchronous medium access control scheme by using the adaptive duty cycling technique to improve energy efficiency and solve long end-to-end delay problem. We also design a tree-based energy aware routing algorithm to prolong the network lifetime in our protocol. Simulation results show that the proposed protocol outperforms other existing algorithms in terms of energy efficiency and latency.