I finally figured out why Japanese people sleep on the train. They have jet lag. It's not jet lag from jets, but jet lag from trains, caused by crossing time zones on the train.
Although Japan has only one time zone for the entire country, it has many mini-time zones. Riding the train, one passes through several mini-time zones. I call this time-zoning.
The time in the countryside, for example, is slower than time in the faster-paced cities. So with each train stop closer to the city, time gets a little faster. It's this time difference between the house and the office that throws the Japanese for a loop.