Make sure you have set up a separate Camp Underground configuration for Night mode.
Open the Camp Underground. Then use the toggle to switch to Night mode. Then pull in the camp equipment that you want in your overnight configuration (higher capacity) from your Inventory. When done, use the toggle switch to go back into Day mode.
If needed, adjust your Day camp configuration for faster regeneration.
Note: The game doesn't automatically switch to either mode. You must manually switch it to the mode you want.