lighten (v.2) Look up lighten at
"shed light upon, illuminate, make light or bright," early 14c., from light (n.) -en (1). Intransitive meaning "to become brighter" is late 14c.; of faces, expressions, etc., from 1795. Meaning "to flash lightning" is from mid-15c. Related: Lightened; lightening.
lighten (v.1) Look up lighten at
"to make less heavy, to lighten a load," mid-14c., lighten, lightnen, from light (adj.1) + -en (1). Figuratively "to make cheerful" from c. 1400. Intransitive sense "become less heavy" is from 1720. Related: Lightened; lightening.