beauty sleep

noun2

1. •
   Originally, sleep taken before midnight, on the belief that early sleep hours conduce to beauty and health; now (chiefly humorous), sleep at any time needed by one to stay beautiful; (countable) an instance of such sleep.

   • A medical man, who may be called up at any moment, must make sure of his ‘beauty-sleep.’ — 1857
2. •
   (humorous) Extra sleep; also (generally), any sleep; (countable) an instance of this; an extra nap.

   • "I know I ought to be taking a beauty sleep," she thought, "so I'll be all fresh and fine for the evening, but I must find it [a piece of turquoise], for I promised Phil I'd wear it." — 1906