Energy efficiency is a fundamental theme in the design of wireless sensor networks protocols, especially for medium access control (MAC) protocols. An energy-efficient MAC protocol can significantly elongate the lifetime of wireless sensor networks by reducing the duty-cycle of sensor nodes to an ultra-low level. Synchronous MAC can be even more efficient in data transfer at the cost of requiring tight time synchronization through periodical beacon dissemination. The length of the beacon interval may greatly affect the energy efficiency of a synchronous MAC. A shorter beacon interval leads to higher synchronization cost due to frequent beacon sending and receiving, while a longer beacon interval will lead to a larger guard time and longer idle listening due to clock drift. Therefore, there is a tradeoff between these two parts of energy consumption. In this paper, we investigate the optimal beacon interval for synchronous MAC in low duty-cycle sensor networks, and then present a strategy that adaptively utilizes the optimal beacon interval in a TDMA-based MAC protocol (called Opt-TDMA). By configuring the beacon interval to its optimal value according to the data packets rate and network size, Opt-TDMA can reduce the overall power consumption of both sending/receiving beacons and data packets. Experimental results demonstrate that Opt-TDMA is more energy-efficient than pure TDMA protocol and SCP-MAC by using optimal beacon interval and contention-free transmission.