Whenever I would see one of my happy employees looking like they have had better days or were not quite themselves, I would typically give them some space, just enough. If after some time I still saw them in this state, I would always make sure to check in with them. Engaging with them here is a must. As a leader who cares about her employees, I want to know purely out of concern. Plus, if they are feeling down, having their manager not give a damn will definitely add fuel to their sad-fire. Something that you could easily put to rest by showing proper (yes, proper) concern.