beauty sleep

Short definition from Wiktionary

Noun

1. 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.
2. 2
   Extra sleep; also (generally), any sleep; (countable) an instance of this; an extra nap.
3. 3
   Alternative form of beauty sleep.