Provided are a method by which a first apparatus performs wireless communication and an apparatus supporting same. The method comprises the steps of: obtaining a plurality of sidelink (SL) DRX inactivity timers that are mapped to a plurality of SL quality of service (QoS) profiles associated with a destination layer-2 (L2) identifier (ID); receiving, through a physical sidelink control channel (PSCCH), first sidelink control information (SCI) for scheduling a physical sidelink shared channel (PSSCH) and second SCI; receiving, through the PSSCH, the second SCI comprising a destination layer-1 (L1) ID and cast type information; selecting one SL DRX inactivity timer from among the plurality of SL DRX inactivity timers associated with the destination L 1 ID, on the basis of failure to decode a medium access control (MAC) protocol data unit (PDU); and starting the one SL DRX inactivity timer, wherein the one SL DRX inactivity timer may be an SL DRX inactivity timer that has the longest duration among the plurality of SL DRX inactivity timers.