The Internet of Things (IoT) is characterized by a myriad of physical deices, together with high heterogeneity in both software and hardware. Middleware platforms have been proposed in order to alleviate such heterogeneity, providing relevant services and easing application development. In IoT systems, energy consumption is a key concern due to the proliferation of devices and their limited battery capacity. IoT middleware platforms can play an important role in providing applications with strategies, and support, for energy awareness and energy efficiency. Although there is a significant existing body of work related to IoT middleware, there is, as yet, no complementary overview of the state of the art on how these platforms can contribute to energy efficiency and energy awareness in IoT systems. This paper provides such an overview in the form of a systematic literature review (SLR). The SLR was carried out by following a systematic, rigorous procedure to search, select, and analyze primary studies available in the literature. Our corpus, as presented in this paper, is made up of twenty-two such studies, each presenting strategies and solutions on middleware support for energy efficiency and energy awareness in IoT systems. These strategies mainly focus on network adaptation, task offloading, and concrete implementations. However, most of these studies do not consider energy-aware/efficiency abstractions, and focus on solutions working at the end-user application side. In conclusion, this paper also raises relevant challenges and potential directions for further research resulting from the main SLR findings.