To conserve energy, periodic active/sleep dynamics is adopted in wireless sensor networks. At the same time, the QoS guarantees, such as packet delay, packet loss ratio and network throughput need to be satisfied. We develop a finite queuing model for sensor nodes and derive network performance for contention-based wireless sensor networks with synchronous wakeup patterns. Furthermore, the impact of active/sleep duty cycle, time scale and node buffer size on the tradeoff between energy efficiency and QoS guarantees is studied based on the model. Simulation results well match our analytical results and validate the accuracy of our model and approach.