Detailed Instructions: In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.
Q: Article: Mark Twain, the famous American writer, was traveling inprefix = st1 /France. Once he was very tired and wanted to sleep. He therefore asked the conductor to wake him up when they came toDijon. But first he explained that he was a very heavy sleeper. "I'll strongly protest when you try to wake me up," he said to the conductor. "But do not take any notice, just put me off the train anyway." Then Mark Twain went to sleep. Later, when he woke up it was night time and the train was inParisalready. He realized at once that the conductor had forgotten to wake him up atDijon. He ran up to the conductor and began to shout at him." I have never been so angry in all my life," Mark Twain said. The conductor looked at him calmly. "You are not half so angry as the American whom I put off the train atDijon," he said loudly.
Answer: he was so heavy a sleeper
A:
Mark Twain couldn't wake up himself because _ .