Much research has been devoted to maximizing the life time of mobile ad-hoc networks. Life time has often been defined as the time elapsed until the first node is out of battery power. In the context of static networks, this could lead to disconnectivity. In contrast, Delay Tolerant Networks (DTNs) leverage the mobility of relay nodes to compensate for lack of permanent connectivity and thus enable communication even after some nodes have depleted their stored energy. One can thus consider the life times of nodes as some additional parameters that can be controlled to optimize the performances of a DTN. In this paper, we consider two ways in which the energy state of a mobile can be controlled. Since both listening and transmission require energy, and since each of these has a different type of effect on the network performance, we study both the activation problem, which determines when a mobile will turn on in order to receive packets, and the transmission control problem, which can be done using transmission scheduling. The optimal solutions are shown to be of the threshold type. The paper introduces new methods to derive the threshold structure of optimal policies as previously used methods are not useful in this context due to the multidimensional nature of the problem. The findings are validated through extensive simulations.