Energy efficiency is a primordial issue in the wireless sensor networks. This is achieved by deactivating nodes when possible. In this paper we describe the MAC protocol MaCARI that synchronizes nodes in order to schedule active and inactive periods. MaCARI divides time into three periods: a synchronization period, a scheduled activities period where communications are constrained by a tree and an unscheduled activities period where nodes can communicate whenever in range. With this synchronization, nodes are able to save energy during specific time intervals. Therefore, we focus on the synchronization period and apply an optimization to reduce its duration. We validate this approach by simulations under different tree topologies.