Nap time!

Khloe Kardashian dozed off... on big sis Kim's rear end at the Tuesday night launch of Beach Bunny Swimwear's newest Beverly Hills boutique.

"At one point I got really tired, so I had to take a nap on Kim's booty," blogged Khloe, 25, who wore a skintight Herve Leger dress, Givenchy heels and Loren jewels. "It really does make the best pillow."

Added Khloe, "These long days are killing me!!! LOL."

Kim, 29, has said her backside isn't that large.

"My butt is probably not as big as you might think, because I have small legs and a small waist, which makes it appear bigger," she told Harper's Bazaar.