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Energy consumption continues to be a major concern in multiple application domains including power-hungry data centers, portable and wearable devices, mobile communication devices and wireless sensor networks. While energy-constrained, many such applications must meet timing and QoS constraints for sensing, actuation or multimedia data processing. Many modern power-aware processors and microcontrollers have built-in support for active, idle and sleep operating modes. In sleep mode, substantially more energy savings can be obtained but it requires a significant amount of time to switch into and out of that mode. Hence, a significant amount of energy is lost due to idle gaps between executing tasks that are shorter than the required time for the processor to enter the sleep mode. We present a technique called rate-harmonized scheduling that naturally clusters task execution such that processor idle times are lumped together. We next introduce the energy-saving rate-harmonized scheduler which guarantees that every idle duration on the processor can be used to put the processor into sleep mode. This property can be used to even eliminate the idle power mode in processors but nevertheless it is predictable, analyzable, and saves more energy. We finally evaluate the practical benefits of rate-harmonized scheduling implemented in the nano-RK real-time operating system  for wireless sensor networks.