Probably not what you're looking for, as this one's rather a long fic, but I can't *not* suggest it, as this is the only fic I've read that has Jim crawling into bed with Spock for the purpose of sleeping. Awesome fic, you should read it, even if it's not the one you're looking for. :)

City on the Edge of Forever (The Standing on the Precipice Remix)
by waketosleep
http://waketosleep.dreamwidth.org/26383.html