A method of operating a first apparatus 100 in a wireless communication system is proposed. The method may comprise: obtaining an SL DRX configuration including information related to an SL DRX HARQ RTT timer; receiving SCI for scheduling a PSSCH from a second apparatus through a PSCCH based on an active time of the SL DRX configuration; receiving a MAC PDU through the PSSCH from the second apparatus; and starting the SL DRX HARQ RTT timer, based on that a PSFCH resource is configured in an SL grant related to the SCI and that a HARQ feedback related to the MAC PDU is disabled.