Both network lifetime and reliability are of major concern in wireless sensor networks. In this paper, we propose to optimize the wireless sensor network lifetime under a reliability constraint. First, we define the reliability with regards to the accuracy of observations made through sensor nodes. Then, we introduce a function that links reliability to the average amount of energy consumed by the network when reporting an event to the data collector. Based on this function, we bring out the required number of successive readings to be performed in order to optimize both network lifetime and reliability. Subsequently, we give a different definition to the reliability in order to maximize the network lifetime by relaxing the reliability constraint. In this case, the reliability is defined with regards to the number of non-reported events. We show that the network lifetime does not increase indefinitely when relaxing the reliability constraint. Finally, we conclude that new node placement strategies should be designed for wireless sensor applications that tolerate losing events (i.e. events that are not reported to the data collector), in order to maximize their network lifetime.