Thanks for the post!
They can be distinctly stubborn about it depending on their personality type and the food they've been eating (some of them provide minor or major bonuses to their energy levels that can keep them awake a long time), but they'll sleep when they reach a house or the door to the castle if they're sufficiently tired.
If they've reached critical tiredness levels try dropping them onto a house or into the door to the castle, if they don't sleep then petting them a few times when they're in position can encourage them to stop sitting around complaining about being tired and actually go and rest.
If they're both extremely tired and extremely hungry, you can also just feed them said energizing fruits to pep them up and skip the snooze.
Let me know if that helps, and feel free to share screenshots of your Lintwians!