duo needs to add the contractions to the list of possible answers. Don't go to sleep to late is exactly the same as do not go to sleep to late. Thanks for continuing to add to the list of possible correct answers. This is why you need to have contributors who have experience with both languages in the group. Just having a PHD in linguistics does not always allow you to be open and accepting of the many possible correct answers. And the English language is built on the principle of many varieties of possibilities. The more variety used in speech often is more appreciated or more valued it is.