A method for performing wireless communication by a first device and a device supporting same are proposed. The method comprises the steps of: obtaining a sidelink (SL) discontinuous reception (DRX) configuration including information related to an active time of a second device; initiating resource selection in a first slot; and determining a selection window on the basis of the first slot. For example, the first device may transmit, to the second device, information for extension of the active time of the second device through a physical sidelink control channel (PSCCH) or a physical sidelink shared channel (PSSCH) related to the PSCCH on the basis that the number of SL resources in the time domain of the selection window is smaller than a threshold.