To ensure sustainable operations of wireless sensor networks, environmental energy harvesting has been well recognized as one promising solution for long-term applications. Unlike in battery-powered sensor networks, the authors are targeting a duty-cycle adjustment to optimize the network performance, e.g., delay minimization, with full harvested energy utilization. In this paper, they introduce a set of duty-cycle adjustment schemes that will minimize Cross Traffic Delay (CTD) in energy-harvesting sensor networks. They first present an offline solution by assuming that the link reliability and traffic distribution are known a priori. Based on the sub-modular property of the CTD function, they theoretically prove that a simple greedy algorithm can achieve constant approximation.