An adaptive sleeping and awakening protocol (ASAP) is proposed for nodes in a synchronous adhoc sensor network. In order to increase energy efficiency, nodes within the network enter into a sleep mode and awaken at pre-determined time slot(s) to listen for transmissions from its immediate neighbors. Implicit knowledge of awakening slots for neighboring nodes is used to schedule transmissions within the neighborhood. Finally, nodes adapt their sleeping cycles based on neighbor topology and remaining battery life in order to maximize the network lifetime while satisfying the latency requirements of the underlying sensor application. Simulation results show that with delay constrained routing, ASAP can achieve twice the energy efficiency (or battery life) of a synchronous awakening approach.