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This paper studies the sleep scheduling problem for large scale wireless sensor networks (WSNs), which have hundreds to thousands of sensors. Sensors are extremely sensitive to energy consumption because they are powered by batteries. In this paper, we propose stochastic sleep scheduling, a generic duty- cycling scheduling method based on stochastic theory. It lets each sensor fall asleep based on certain stochastic processes. This strategy requires no clock synchronization, and a little coordination amongst sensors. In order to characterize this design, we analyze its end-to-end communication delay and energy consumption using stochastic methods. According to the analysis, this method actually has a reasonably small delay due to the redundancy in WSNs. Simulation studies are used to verify the analytical results. We also apply stochastic sleep scheduling to S-MAC, a state-of- the-art duty-cycled sensor MAC protocol. Simulation results verify the energy consumption of the new MAC protocol.