Why did the writer wake up in the middle of his sleep? What did he see in his dream ? ( Chapter 10 )

J. woke up because something was digging into his back. He dreamt that he'd swallowed a coin, and that someone was digging it out.

I did get to sleep for a few hours, and then some part of the boat which seemed to have grown up in the night—for it certainly was not there when we started, and it had disappeared by the morning—kept digging into my spine. I slept through it for a while, dreaming that I had swallowed a sovereign, and that they were cutting a hole in my back with a gimlet, so as to try and get it out. I thought it very unkind of them, and I told them I would owe them the money, and they should have it at the end of the month. But they would not hear of that, and said it would be much better if they had it then, because otherwise the interest would accumulate so. I got quite cross with them after a bit, and told them what I thought of them, and then they gave the gimlet such an excruciating wrench that I woke up.