In many synchronized MAC protocols for wireless sensor networks such as IEEE 802.15.4 in beacon-enabled mode, periods where all the nodes are active alternate with periods where all the nodes are inactive. This approach is used in order to save the energy of nodes as they are powered by small batteries. However, having all nodes active simultaneously can yield to congestion, which increases the packet loss rate and the delay. In this paper, we propose a new MAC scheduling mechanism that distributes the activities of nodes into several periods, thus reducing the number of active nodes during each period. The scheduling is based on the routing information provided by the network layer. We propose an heuristic to compute this schedule, and we derive a protocol with limited overhead. We compare their performance with the performance of IEEE 802.15.4 where all nodes are active simultaneously, as well as with the optimal solution computed using an integer linear program. The simulation results show that our heuristic can greatly improve both packet loss rate and delay in a large variety of scenarios without increasing the energy consumption.