Assigning the transmission range for wireless networks is a critical aspect that has been well studied. But, there has been no work on assigning optimal transmission ranges in energy-constrained wireless networks, where nodes periodically sleep in order to prolong the life-time. In this paper, we focus on building an analytical model for networks with mostly sleeping nodes. We study the impact of varying transmission ranges for different networks subjected to varied network loads on both per hop delay and end-to-end delay. Our model is general enough to be applied to any type sleep scheduling mechanism. Extensive simulation results in ns-2 validate our model.