Provided are a method for performing wireless communication by a first device, and a device for supporting same. The method may comprise: obtaining a sidelink (SL) discontinuous reception (DRX) configuration including a value of a SL DRX retransmission timer; receiving, from a second device through a physical sidelink control channel (PSCCH), first sidelink control information (SCI) for scheduling of second SCI and a physical sidelink shared channel (PSSCH); receiving, from the second device through the PSSCH, the second SCI and data; determining a value of a SL DRX hybrid automatic repeat request (HARD) round trip time (RTT) timer as zero, based on that information related to a next retransmission resource is not included in the first SCI; and starting the SL DRX retransmission timer.