One of the main design considerations for wireless sensor networks (WSNs) is the energy consumption of individual sensor nodes. Currently, most WSNs use radio frequency (RF) channels for communications, and this accounts for a significant portion of the energy expended. Free space optical (FSO) communications have been proposed for WSNs, due to the lower communications energy required. However, for FSO communications, line of sight (LOS) is required between the transmitter and the receiver. The hybrid Radio Frequency/Free Space Optical (RF/FSO) WSN has been proposed to lower the overall energy consumption of the traditional RF-only WSN. In this paper, lifetime simulations of hybrid RF/FSO WSNs are described and compared against that of RF-only based WSNs. This paper shows that for the range of scenarios considered, the RF/FSO WSN outlasts its traditional RF-only counterpart.