Sleeping Bag
Either place the backpack on the ground to manually unfold the sleeping bag and sleep, or equip it to use the quick-sleep function. In either case, you need to click the sleeping bag button in the backpack's interface.
Note: Sleeping bag does not reset your spawn point, so it works great as a portable utility to skip the night. To make the sleeping bag set the spawn point simply change enableSleepingBagSpawnPoint to true in the mod's config.
Hold the desired sleeping bag in your hand and Shift + Right-click on a placed backpack on the ground to attach/change it
Open the backpack inventory while equipped or on the ground, find the sleeping bag button, and click it while holding Shift
Disclaimer:
Attach/Detach functionality available only on 1.20.1, 1.21.1, 26.1.2+