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With advances in energy harvesting techniques, it is now feasible to build sustainable sensor networks (SSN) to support long-term applications. Unlike battery-powered sensor networks, the objective of sustainable sensor networks is to effectively utilize a continuous stream of ambient energy. Instead of pushing the limits of energy conservation, we are aiming at energy-synchronized designs1 to keep energy supplies and demands in balance. Specifically, this work presents the Energy Synchronized Communication (ESC) as a transparent middle-ware between the network layer and data link layer that controls the amount and timing of RF activity at receiving nodes. In this work, we first derive a delay model for cross-traffic at individual nodes, which reveals an interesting stair effect in low-duty-cycle networks. This effect allows us to design a localized energy synchronization control with O(1) time complexity that shuffles or adjusts the working schedule of a node to optimize cross-traffic delays in the presence of changing duty-cycle budgets. Under different rates of energy fluctuations, shuffle-based and adjustment-based methods have different influences on logical connectivity and cross-traffic delay, due to the inconsistent views of working schedules among neighboring nodes before schedule updates. We study the trade-off between them and propose methods to update working schedules efficiently. To evaluate our work, ESC is implemented on MicaZ nodes with two state-of-the-art routing protocols. Both test-bed experiment and large scale simulation results show significant performance improvements over randomized synchronization controls.