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This paper presents an efficient mechanism to achieve significant energy savings in heterogeneous wireless sensor networks such as HPWREN. Environmental sensors are typically located in remote and hard-to-reach locations far from the main high-bandwidth data links. Therefore the sensed data needs to be routed through multiple hops before reaching the backbone. The routing is done by battery-powered nodes using license free radios such as 802.11. In this context, minimizing energy consumption is critical to maintaining operational data links. We propose a distributed low-power scheduling algorithm that limits the number of active nodes and allows a large portion of nodes to sleep. The algorithm works in a completely distributed manner, requiring handshaking only when a node enters and exits the network. Routing of packets relies on a dynamically changing backbone of nodes. The nodes of the backbone provide connectivity and are responsible for delivering the packets to the proper destinations. The proposed solution sits on top of the unmodified MAC layer so that legacy network devices can be used, and expensive hardware/software modifications are avoided. Thus our solution is cheap yet easily deployable. Our results show up to 60% energy savings per battery operated node while maintaining efficient data delivery.