Geographic routing is assumed to be the most potential routing scheme in wireless sensor networks (WSNs) due to its scalability and efficiency. Recently more and more research work about geographic routing pay attention to its application scenarios in duty-cycled WSNs because of the natural advantage of saving energy consumption with duty-cycling. However, it may cause significant latency issue when applying geographic routing in duty-cycled WSNs and almost all current researches try to handle the latency problem from the point of changing the geographic forwarding mechanism, apart from the connected-k neighborhood (CKN) algorithm which focuses on sleep scheduling. In this paper, we discuss and analyze the first transmission path's performance of the two-phase geographic forwarding (TPGF) in a CKN based WSN and further propose a geographic routing oriented sleep scheduling (GSS) algorithm to shorten the first transmission path of TPGF in duty-cycled WSNs. Further theoretical and simulation results show that GSS can achieve a good tradeoff between the length of the first transmission path explored by TPGF and the total energy consumption to transmit data with the explored first transmission path, compared with the CKN sleep scheduling algorithm.