Part of me thinks that it's none of their business. If I do my work, then I shouldn't have to mention it any more than I should have to mention a physical health problem. But, telling your boss can allow them to help you to do your job more successfully. In my past job, my manager would ask me how I was doing on a certain week if she was going to give me more than my normal six shifts. She knew that having too many shifts in a week could make me go downhill, and her knowing that took the pressure off. It gave me the ability to say no to more than I was capable of, and meant that someone had my back when it came to making sure that my needs were met.