"to make less heavy," figuratively "to make cheerful," mid-14c., from light (adj.1) + -en (1). Related: Lightened; lightening.
"shed light upon, illuminate, brighten," early 14c., from light (n.) -en (1). Meaning "to grow brighter" is late 14c. Of faces, expressions, etc., from 1795. Related: Lightened; lightening.