Energy efficiency has become a critical priority across various sectors, including communication networks. With the physical layer responsible for the majority of energy consumption in mobile networks, achieving significant energy savings in this domain is imperative. This need is especially pressing in light of the exponential growth in data traffic, which necessitates a substantial reduction in energy consumption per bit for the sixth-generation (6G) mobile networks. This chapter explores promising strategies to enhance energy efficiency within the physical layer of communication networks. We present the concept of “Gearbox-Physical Layer”, which dynamically adapts to different modulation schemes and front-end configurations to optimize energy efficiency. In this regard, we investigate innovative modulation techniques, such as zero-crossing modulation and impulse radio, promising a significantly enhanced energy efficiency if the user's demand can be satisfied with low to medium spectral efficiencies. We also showcase an implementation of impulse radio in the context of wake-up receivers with extremely low power consumption. To address energy efficiency from a channel coding perspective, we analyze the potential of short block-length coding and broadcast coding techniques. Furthermore, we also explore advancements in multi-antenna systems, focusing on sparse antenna arrays and hybrid beamforming approaches to achieve energy-efficient operation. By integrating these solutions, this chapter outlines a holistic approach to advance energy efficiency in next-generation mobile communications networks, contributing to a sustainable and scalable future for 6G systems.