I'm a bit peeved at the grimoire guy. That's not at all an actual rule when it comes to falling asleep. Lots of people can easily fall asleep anywhere, lots of people can fall asleep very fast. It is also normal to fall asleep quickly when one is feeling quite tired.
That frikking book just confused the princess by telling her something false and presenting it as a fact, thus making her think that she wasn't getting good sleep when she was actually already sleeping well enough to get a good rest.
I feel sad for the princess here. She's so confused because she's looking for a problem that isn't even there in the first place!