The antonyms for the word "lightens up" denote various opposite meanings. "Darkens down" refers to the action of making something darker, duller, or gloomier. "Worsens" indicates the deterioration or aggravation of a situation or problem. "Heavies" or "weighs down" imparts the quality of being burdensome, tiresome, or oppressive. "Intensifies" or "deepens" portrays the notion of increasing the intensity, seriousness, or depth of something. Lastly, "sterns" denotes a strict, unyielding, or severe attitude that does not allow for any levity or humor.