Different variants of synchronous duty-cycle MAC protocols have been designed for wireless sensor networks to reduce energy consumption. However, the synchronization process of these protocols remains a significant contributor to the energy consumption. In this paper, a new energy-efficient synchronization algorithm referred to as C-Sync is proposed. C-Sync reduces energy consumption by adaptively regulating the synchronization traffic and synchronization wakeup period based on the changing network neighborhood conditions through counter-based and exponential-smoothing algorithms. Extensive simulations of multi-hop multi-neighborhood network scenarios are performed using ns-2. We compare C-Sync with the fixed periodic synchronization (F-Sync) algorithm and the 1-Sync algorithm and show that C-Sync outperforms F-Sync and 1-Sync in energy-efficiency over a wide range of node densities, drift rates and duty cycles.