With the fast expansion of Wireless Sensor Networks (WSNs) and the increasing emergence of new scenarios and applications, extending their lifetime is crucial. Usually, WSN developers use generic algorithms and deployment arrangements without considering the specific needs of their network's application. Taking this application into account can result in a significant enhancement of performance, both in terms of increasing the lifetime and improving the quality of service (QoS). Furthermore, most WSN developers do not consider the final behavior of the network when nodes are nearly depleted and resources are scarce. In this paper we introduce the concept of the controlled degradation of the network, to refer to the strategies aimed at managing this deterioration process. The existing definitions of the network lifetime do not normally consider the specific purpose or application for which the WSN is intended. Thus, they are not suited to describe and test controlled degradation strategies. Consequently, we propose a new formal and comprehensive definition for the network lifetime. Finally, this work presents a proof of concept that confirms our statements and reinforces the potential of this research line.