Due to the massive expansion of the Information and Communication Technology (ICT) sector, the power consumption of this sector is rapidly increasing. Besides greenhouse gas (GHG) emissions, the increase in power consumption also causes these networks to have higher operating costs. Passive Optical networks (PONs) are energyefficient compared to legacy copper-based access networks and Active Optical Networks (AONs), but still the power consumption of PONs is significant and requires further research. The main cause of the higher power consumption of PONs is the optical network unit (ONU). In this study, we performed a survey on the energy conservation schemes for present and next-generation PONs (TWDM and OFDM PONs), with a focus on ONU-based schemes. The focus of this study is on medium access control (MAC) schemes as they are more economical and can be adapted easily. The review shows that if the cyclic sleep mode (CSM) is efficiently configured, this will be the most economical, efficient, and delay-aware scheme for current as well as next-generation PONs (NG-PONs) for energy conservation with delay guarantees.