The continuously increasing bandwidth demand of information and communication technology (ICT) is leading a rapid evolution towards next generation passive optical networks (NG-PON). However, the power consumed at the access network is experiencing a tremendous growth due to the employment of the Optical Network Units (ONU). For energy conservation in this PON, watchful sleep mode is the newest standardized mechanism that simplifies the features of both cyclic sleep and doze modes to provide higher energy savings. To exploits its functionality, this sleep mode with a dynamic bandwidth assignment (DBA) scheme is a promising solution. Therefore, this study presents the performance of watchful sleep mode in a 10 GB-capable PON (XG-PON) network employing the Immediate Allocation with Colorless Grant (IACG) DBA. This study firstly review the current progress of the power saving techniques in XG-PON. Our simulation results show a better performances compared to the standard in term of upstream (US) and downstream (DS) delays. We also achieve 41 % of energy savings at 0.47 network load.