Antonyms for "lightened up" can mean to make something heavy or severe. Examples of antonyms for "lightened up" are to bear down, to burden, to darken, to depress, to gloom, to oppress, to sadden, and to weigh down. These antonyms carry the opposite tone of "lightening up," which means to become less tense, to relax, to soften, to brighten, and to lift the mood. Using antonyms of "lightened up" instead will lead to a more serious or negative atmosphere. Therefore, it is important to choose the appropriate words based on the message being conveyed.