In wireless ad hoc and sensor networks, the best way for a node to spare energy is to sleep. In this paper, we propose SERENA, a solution to allow router nodes to sleep in such a way that the basic network functionality, i.e. end-to-end communication, is always ensured. SERENA is based on distributed and localized two-hop coloring. The node's color is then mapped in time slots. Each node is ensured to get at least one time slot, it also gets additional time slots proportionally to its traffic rate. Such a solution adapts to varying traffic rates and supports late node arrivals. A performance evaluation allows us to compare our coloring algorithm with existing ones. We show that with SERENA, the network lifetime is maximized as well as the amount of user data delivered. Moreover, energy is used more efficiently: less energy is dissipated in interferences.