The Internet of Things (IoT) is an emerging paradigm that has gained popularity in recent years. The large number of high performance and sophisticated devices connected to the IoT system consumes huge amount of energy. Thus, the issue of energy consumption in IoT based systems is an important research focus. Green IoT represents the issue of reducing energy consumption of IoT devices, which achieves a sustainable environment for IoT systems. This paper presents the current state-of-the-art research on energy optimization in IoT. The researcher investigated the literature, categorized the existing energy efficient techniques and presented the open challenges and research opportunities that can assist the research community. The main contribution of this paper is that it systematically summarizes and analyses the existing energy aware techniques in tabular form on the basis of different layers and components of IoT.