Wrath hadn't understood sleep in the beginning. From the early days, before he regained his memory and earned his name, he simply saw sleeping humans and disinterestedly assumed them to be dead, wondering only how to make up for the loss of playmates. After he lost his limbs, and his supply of red stones dwindled, he found himself preferring to nap and rest after long periods of activity, but had never really needed or craved sleep as other people did. Staying with Alphonse, he learned to comprimise his anxious nature, and began conditioning himself in the habit of sleeping at night, just as everyone else did.
It was easy to learn, however he still didn't need it, and thus often spent bored hours awake, as Al slept curled against him, and that was when Wrath made the most trivial and yet rich of tiny discoveries. Alphonse Elric, in spite of how cherubic and serene he appeared in his sleep, would snore. It wasn't especially loud, nor was it grotesquely unappealing, but in his rhythmic sleeping breath, the sound was just so faintly porcine, that Wrath often had to clap a hand over his own mouth until his giggles ceased to tremor in the squeaky mattress springs. Al, with his mouth ever so slightly agape, and serenading the silent room with his sleeping song was a sight so adorable, yet so ridiculously comical, and Wrath found his vicious side tempted to make a joke out of it by grabbing a wisp of blanket fuzz and sneaking it into Al's parted lips. He was able to resist temptation, though, and politely, he moved a finger under Al's chin, pushing his jaw up and closing his mouth. He never told Al about his snoring. He just couldn't bring himself to. It was one of those things that he delighted in knowing in secret, and was just one of those topics that most couples choose to omit from each other by tacit agreement.
By similar unspoken truce, Al would never reveal his secret knowledge that Wrath still sucked his thumb.