Sleep by Eric Whitacre. This song is a choral arrangement by the BYU singers.
My favorite composer of all time. Eric Whitacre is a wonderful composer who has made quite a few breath taking choral and instrumental arrangements. My choir tries to sing at least one of his pieces a year, and so far this is still my favorite of them all. If you like this song then you should go to www.ericwhitacre.com and check out some of his other albums. If you decide to buy one I would go with Cloudburst and Other Choral Arrangements.