An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
lose sleep over (someone or something)
to worry about someone or something so that you cannot sleep
I have been losing sleep over my recent problems at work.
List of common idioms, their meanings and examples of their usage in sentences
Idioms can be a powerful linguistic tool only when they are used in the right way in the right places.