This paper focuses on reducing energy consumptions of optical network units (ONUs) in Ethernet passive optical network (EPON). In EPON, optical line terminal (OLT) located at the central office broadcasts the downstream traffic to all ONUs, each of which checks all arrival downstream packets so as to obtain the downstream packets destined to itself. Thus, receivers at ONUs have to always stay in the awake status and consume a large amount of energy. To address the downstream challenge, we propose a novel sleep control scheme which can efficiently put ONU receivers into sleep without extending the standardized EPON MAC protocol. We also theoretically analyze the impacts of different parameters in the sleep control scheme on the delay and energy saving performances by using semi-Markov chains. It is shown that, with proper settings of sleep control parameters, the proposed scheme can save as high as 70% of the ONU receiver energy.