In wireless sensor networks, energy efficiency is achieved by making nodes sleep. In this paper, we propose SERENA, a node activity scheduling algorithm based on three-hop coloring. We study the performance of SERENA by simulating the conditions of a realistic environment including unidirectional links. The existence of unidirectional links causes color conflicts, which can be solved by exploiting a collaboration with the MAC layer. Such cross layering approach takes into account the real wireless environment and so enables substantial performance improvements (e.g. better reactivity to topology changes). Finally, we evaluate the benefits brought by this solution, namely the increase of network lifetime and spatial reuse.