What is Lighten ?
Lighten is (verb) 1. to make or become brighter, not so dark You can lighten the room by painting it white. The sky lightened as dawn broke. 2. to make or become lighter, not so heavy I’ll have to lighten my backpack – it’s much too heavy. 3. to make someone happier, less gloomy The news has lightened the gloom in the office considerably.
source: Easier English, Student Dictionary Upper Intermediate Level