Fog computing has emerged as a promising paradigm to meet the increasing demands of latency-sensitive and bandwidth-intensive applications in the era of the Internet of Things (IoT) and edge computing. However, the proliferation of fog nodes introduces significant energy consumption challenges, particularly in resource-constrained environments. This paper presents a comprehensive review of energy-efficient techniques in fog computing, focusing on optimization strategies to minimize energy consumption while meeting application requirements. We categorize existing research efforts based on their approaches, including task scheduling, resource allocation, communication protocols, and energy harvesting techniques. Additionally, we identify key open challenges and propose future research directions to further enhance energy efficiency in fog computing environments.