In S-MAC-based sensor networks, border nodes consume more energy since they follow multiple listen and sleep schedules. Therefore they switch into the listen state frequently and reduce the network lifetime. This paper proposes a simple but powerful algorithm, termed the Schedule Unifying Algorithm (SUA), to minimize energy consumption of border nodes by unifying multiple listen and sleep schedules into a single unified schedule. The simulation results show that SUA incorporated SMAC-based nodes consume less energy, thereby extending the network lifetime approximately 2 times more.