In the story Christmas Day in the Morning by Pearl S. Buck there is this sentence
He woke suddenly and completely. It was four o'clock, the hour at which his father had always called him to get up and help with the milking. Strange how the habits of his youth clung to him still! Fifty years ago, and his father had been dead for thirty years, and yet he waked at four o'clock in the morning. He had trained himself to turn over and go to sleep, but this morning it was Christmas, he did not try to sleep
What does it mean exactly?