Cäsar Flaischlen (12 May 1864 â 16 October 1920) was a German poet. He is best known as the author of "Hab' Sonne im Herzen" ("Have Sunshine in Your Heart"), which has been translated into various languages. Composers such as Pauline Volkstein and Jórunn Viðar have set FlaischlenâÂÂs text to music.