The pervasiveness of wireless sensor networks (WSNs) across different applications keeps increasing due to their versatility, although energy consumption is a huge constraint for these types of networks. Therefore, it is essential to incorporate consistent and reliable energy sources. Mobile nodes require independent sources, often composed of a battery with one or more energy harvesting devices (EHDs). The exigency for energy-efficiency improvements is a collateral effect of the growth of WSNs. Different methods have been proposed to improve the energy efficiency of these systems. This paper focuses on the techniques of energy-efficiency developed in the medium access control (MAC) layer. The proposed solution incorporates EHDs and analytic information, such as the estimation of the network lifetime, to achieve an energy efficient system, potentially self-sustainable. A widely used MAC-protocol technique for energy conservation is duty cycling, because it facilitates the control over the transitions of the active and sleep modes of the node, conserving energy if used efficiently. Due to the importance of MAC protocols in energy conservation, it is necessary to compile a review of the MAC protocols that address this issue, covering the traditional way of classifying them, the characteristics presented by each group and how they have been adapted to the appearance of new tools such as EHDs and the information obtained from the battery to estimate the lifetime of the sensor node.