Abstract. Sensor network devices have limited battery resources primarily consumed by radio communication. Network nodes play different communication roles and consequently consume different amounts of energy. Nodes with heavier communication burdens prematurely deplete their batteries and potentially partition the network such that other nodes are unable to communicate despite having energy remaining. We have developed Seesaw, an asynchronous and asymmetric MAC protocol that balances the energy consumption among nodes with differing loads, and thus prolongs network lifetime. Balancing is possible through Seesaw mechanisms that allow heavily burdened nodes to shift some of the effort of maintaining communication to more lightly loaded neighboring nodes. We show how to exploit the flexibility of asynchrony and asymmetry to balance energy consumption across the network, and develop methods for automatically tuning each node to achieve this.