I have personally been beset on both sides by slumbering riders at least once. This phenomenon is not limited to seats either, as I've had people standing next to me nod off and start wavering in my direction as well.
My theory as to the reasoning for this is a combination of overwork and/or under sleep. I know a few people that count the 1-2 hours of sleep they regularly get on the train in their daily sleep totals... they depend on this time to get the sleep necessary to be (sometimes barely) functioning members of society. Only problem is that this only works for working people, and you see plenty of youngsters nodding off too... my theory is still in need of some tweaking.
Another interesting corollary of the Japanese sleep phenomenon is that you will regularly find scenes such as those to the right on Saturday and Sunday mornings all over downtown Tokyo if you happen to be out early enough. If you yourself are on the way back from a late night, you are guaranteed to spot a few of them, especially in the notorious party districts like Roppongi or Shibuya.
Now granted, this is a situation where the majority of the populace normally are faced with two less than optimal choices - go home on the last train of the night (12:30ish... night just beginning), or the first train (5-5:30ish... past most people's bed times). Those who choose the latter don't always make it the whole way or don't pace themselves and wind up passed out on the sidelines, like our friends here.
By the way, the idea for this post was inspired by a very amusing community on Facebook, which if you are signed up for I highly suggest you check out, like now. All these pics are borrowed from said community.