beauty sleep 
noun [mass noun] humorous
Sleep considered to be sufficient to keep one looking young and beautiful.We both require eight hours' beauty sleep a night, and sleep together in my bed, back to back - just her head peeps out of the top of the covers....
- At the door he insisted on shaking my hand again, and said to Mum and Patty, ‘I mustn't keep you two girls from your beauty sleep, although Heaven knows you certainly don't need it.’
- So, needing some beauty sleep, and thinking whilst counting sheep, I went to bed, and a magical mantra came to me in a dream (which is where my snazziest artsy notions usually pop up).