every time Bale lights a cigarrete in his car with the car-lighter u notice that the direction in which the lighter is facing is changed. this is another sign that there are scenes in the film in which Bale is not in the "real-world"
The locker numbers in the locker room scene are his weights. Throughout the movie, Trevor was losing weight. His weight loss was recorded by post-it notes on his bathroom wall. They were also the numbers of the lockers in the locker room scene. They are 119, (his lowest weight), 120,(his weight before 119), 122, etc. up until 124, one of his higher weights.
The Machinist is a film about guilt. Guilt eats away at the soul, corrupting and torturing its inhabitant. The protagonist feels guilt over a crime he committed, which we find out at the end. He has blocked out that crime from his mind. He starts to become corrupted and gets eaten away that is why he becomes so skinny but yet he eats so much, he is a mess. He cannot sleep becomes his nightmares of the incident haunt him. He has shut himself off from the world and the loneliness has him trying to re-instate himself into society but the guilt is always there. He was reckless and now suffers for it until he finally redeems himself and admits his guilt then he will be rewarded a good nights sleep.