My style usually tends toward the relaxed end of the spectrum. When I'm wearing a simple loungewear outfit (which is most of the time lol), I love mixing in pieces with prints or patterns, because it adds some fun detail and helps make the outfit look slightly more planned and purposeful.
Remember, in the end it's all about how you wear it. Depending on how you wear them, so many pieces can double as both clothes and pajamas. ; )