The Internet of Things (IoT) has emerged as a transformational technology capable of improving the quality of life via interconnected gadgets and systems. However, the rapid rise of the Internet of Things has resulted in increased energy consumption and environmental implications. This study investigates the notion of Green IoT, which strives to reduce the energy consumption and environmental impact of IoT devices and systems. The study investigates energy optimization techniques at several layers of the IoT architecture, including sensors and actuators, data centers, and applications. Solar, thermal, mechanical, and radio frequency energy harvesting approaches are also discussed in the Green IoT. While Green IoT has the potential to improve sustainability, significant technological obstacles remain, particularly in building energy-efficient processing units and sensors. This study summarizes existing research on energy efficiency in IoT and recommends critical areas for future work to enable more environmentally friendly IoT systems.