Use the Sleepwear Configuration item to open the Sleepwear Menu (found in the Misc section of your inventory or in COBL menu).
From here you can set what items you would like to wear when getting into bed. You must have the item on you when you sleep for it to equip.
Try not to use two items that take up the same slot, eg don't use the "(-) Top" token along with a shirt. It won't do anything bad, but only one will take effect.