This paper presents a top-down survey of energy efficiency in wireless sensor networks (WSNs), focusing on the trade-offs between application requirements and network lifetime extension. The authors identify the main categories of WSN applications and their specific requirements, and propose a new classification of energy-conservation schemes found in the recent literature. They then discuss how these schemes conflict with the specific requirements of the applications. Finally, they survey the techniques applied in WSNs to achieve a trade-off between multiple requirements, such as multi-objective optimisation. The paper discusses various WSN applications, including healthcare, industry, transportation, public safety, environment and agriculture, underground and underwater sensor networks. For each application, the authors identify the specific requirements and the challenges associated with meeting them. For example, in healthcare applications, the requirements include hard real-time data delivery, confidentiality and access control, and support for mobility. In industry applications, the requirements include bounded delay, robustness and security. In transportation systems, the requirements include hard real-time delays, security and QoS while supporting mobility. In public safety and military systems, the requirements include short delays, service differentiation and data integrity provisioning. In environment and agriculture applications, the requirements include scalability, coverage and lifetime prolongation. In underground and underwater sensor networks, the requirements include robustness and coverage. The paper also discusses existing standards for low-power WSNs, including IEEE 802.15.4, ZigBee, WirelessHART, ISA100.11a, Bluetooth low energy, IEEE 802.15.6, 6LoWPAN, RPL and MQTT. The authors compare these standards with other well-known wireless standards (Wi-Fi, WiMax, WiMedia, Bluetooth) regarding data rate, transmission range, scalability and applications. The paper then reviews the major existing approaches proposed to tackle the energy consumption problem of battery-powered motes. These approaches include radio optimisation, data reduction, sleep/wakeup schemes and energy-efficient routing. The authors discuss the trade-offs between these approaches and their effectiveness in meeting the specific requirements of different WSN applications. The paper concludes that the design of WSNs requires the development of energy-efficient solutions that meet a specific set of requirements, and that the integration of different technologies and standards is necessary to respond to the needs of emerging and challenging applications such as Smart grids, Intelligent Transportation Systems and Healthcare Information Systems.This paper presents a top-down survey of energy efficiency in wireless sensor networks (WSNs), focusing on the trade-offs between application requirements and network lifetime extension. The authors identify the main categories of WSN applications and their specific requirements, and propose a new classification of energy-conservation schemes found in the recent literature. They then discuss how these schemes conflict with the specific requirements of the applications. Finally, they survey the techniques applied in WSNs to achieve a trade-off between multiple requirements, such as multi-objective optimisation. The paper discusses various WSN applications, including healthcare, industry, transportation, public safety, environment and agriculture, underground and underwater sensor networks. For each application, the authors identify the specific requirements and the challenges associated with meeting them. For example, in healthcare applications, the requirements include hard real-time data delivery, confidentiality and access control, and support for mobility. In industry applications, the requirements include bounded delay, robustness and security. In transportation systems, the requirements include hard real-time delays, security and QoS while supporting mobility. In public safety and military systems, the requirements include short delays, service differentiation and data integrity provisioning. In environment and agriculture applications, the requirements include scalability, coverage and lifetime prolongation. In underground and underwater sensor networks, the requirements include robustness and coverage. The paper also discusses existing standards for low-power WSNs, including IEEE 802.15.4, ZigBee, WirelessHART, ISA100.11a, Bluetooth low energy, IEEE 802.15.6, 6LoWPAN, RPL and MQTT. The authors compare these standards with other well-known wireless standards (Wi-Fi, WiMax, WiMedia, Bluetooth) regarding data rate, transmission range, scalability and applications. The paper then reviews the major existing approaches proposed to tackle the energy consumption problem of battery-powered motes. These approaches include radio optimisation, data reduction, sleep/wakeup schemes and energy-efficient routing. The authors discuss the trade-offs between these approaches and their effectiveness in meeting the specific requirements of different WSN applications. The paper concludes that the design of WSNs requires the development of energy-efficient solutions that meet a specific set of requirements, and that the integration of different technologies and standards is necessary to respond to the needs of emerging and challenging applications such as Smart grids, Intelligent Transportation Systems and Healthcare Information Systems.