Wireless sensor networks (WSNs) are increasingly used in order to monitor the environment. It is important to ensure that protocols for WSNs are scalable (as WSNs can be composed of hundreds of devices) and energy-efficient (as they are designed to operate for years). In this paper, we show that in synchronized MAC protocols, synchronization requires a lot of energy, and in unsynchronized MAC protocols, it is difficult for neighbor nodes to communicate together in an energy-efficient manner (as they are not synchronized). We use a mathematical model to quantify the average communication delay for unsynchronized MAC protocols, and to compute the average probability that this delay is infinite. Then, we propose a distributed MAC mechanism, based on the beacon-enabled mode of IEEE 802.15.4, without the synchronization mechanism. Our mechanism greatly reduces the probability that the communication delay is infinite, allows nodes to communicate with their neighbors periodically (but not systematically), and ensures that the energy consumption is constant. Finally, we evaluate the performance of our mechanism by simulation, and conclude that it can be integrated into a scalable MAC protocol.