One thing that fascinates me about the subway is that it reflects New York City's reputation as the city that never sleeps. It might be one o'clock in the morning but there will almost certainly still be a decent amount of people on the train with you. But here's where the "city that never sleeps" catch comes in: New Yorkers DO sleep...they sleep on the train.
Case in point: the above photo I took the other week (hopefully not too creepily) with my iPhone. Almost everyone in my car was asleep, and if they were not asleep, they were playing games on their cell phone just to try to stay awake.
To conclude: NYC is a city full of over-stimulation. People work themselves to the bone but are united in their constant movements to and fro, like sailors on a ship navigating the ocean. That does not mean they don't get tired. It just means they efficiently use their transit time, often for a little shut-eye.