In the "Radio Times", Steven Moffat says the following about "Night Terrors":
Horror in "Doctor Who" comes in many forms and many places, and a cry of distress reaches the Tardis from the darkest and scariest place in the universe. But where is that exactly? This story began with Mark Gatiss and me chatting on the set of "Sherlock". "I'd like to do scary," he said. "Do you know where the scariest place in the Universe is?" I've known Mark for a while. If he's decided to tell me where the scariest place in the universe is, I'd better listen. "Where?" I quavered from behind my rapid-deployment "Doctor Who" fear sofa. "A child's bedroom," he answered. Parents of Britain - sorry, and good luck!