Damn Interesting's post Shortness of Dark puts forth the interesting theory that man's invention of the light bulb may serve to counteract some of the benefits of the circadian rhythm:

Before the incandescent bulb began to proliferate in the early twentieth century, human sleep schedules were largely governed by the Earth's day and night cycle. But once humans possessed the technology to ward off an appreciable chunk of nighttime, we soon extended our usable waking hours by an average of 13%. Some researchers believe that this modern convenience, credited with bringing the human race in from the dark, may also be responsible for numerous ills.