Low energy consumption and load balancing are required for enhancing lifetime at Wireless Sensor Networks (WSN). In addition, network dynamics and different delay, throughput, and reliability requirements demand costaware traffic adaptation. This paper presents a novel capacity optimization algorithm targeted at locally synchronized, low-duty cycle WSN MACs. The algorithm balances the traffic load between contention and contention free channel access. The energy-inefficient contention access is avoided, whereas the more reliable contention free access is preferred. The algorithm allows making cost-aware trade-off between delay, energy-efficiency, and throughput guided by routing layer. Analysis results show that the algorithm has 10% to 100% better energy-efficiency than IEEE 802.15.4 LR-WPAN in a typical sensing application, while providing comparable goodput and delay.