I would have called it "The Nightmare" but either way.. the piece is well done hence it matters not what it is called. I love the use of alliteration. The harsh 'H' sounds really add to the impact of the first line and the 'Shatter' that cuts into the piece at the beginning of the second.. in contrast to the softer 'S's which really capture what has been interrupted by the dream. Silken silence is a beautiful expression.
I disagree with the other commenter; in my opinion it is clear what you are meaning by 'once-dreamless sleep'. I wouldn't change it, personally. Tis far from simple and vague when someone is able to capture the impact, the feeling, the scene itself!.. with just three lines.
I'm a little skeptical of your use of the word "dreamless", if this is a poem about waking up after a nightmare. But other than that, it's a very nice Haiku. Simple and still vague, I guess, is a nice change from many of the other poems on this website