Energy efficiency of cellular networks can be greatly improve if base stations (BSs) can be put into a low power operation mode during low load periods. In this paper, we present a new dynamic scheme to manage the sleep mode of energy-aware BSs that can be autonomously governed. This kind of BSs, such as those operating in heterogeneous or hyper-cellular networks, can be put to sleep without causing undesired coverage holes, thus preserving the cellular service over the whole coverage area. With the proposed mechanism, energy-aware BSs enter the sleep mode as soon as they have no traffic to send and then apply a coalescing algorithm that dynamically adjusts the length of the sleeping periods to maximize the energy savings while maintaining the average service delay around a target value at the same time.