The invention relates to a method for monitoring a passive optical network having a tree-like structure with a main line and a plurality of branches. The method includes transmitting a wake-up signal from an optical line termination (OLT) arranged in the main line to a plurality of monitoring units arranged in the branches. The method also includes detecting the wake-up signal and listening to information requests from the OLT in the monitoring units during a listening time interval after the detection of the wake-up signal. The method further includes transmitting an information request to be received in the listening time interval from the OLT to the monitoring units. The method additionally includes receiving the information request in the monitoring units during the listening time interval, one of the monitoring units which is addressed by the information request transmitting a response message back to the OLT.