No one knows precisely why we sleep. But we do need sleep. Without it we will die. Yet many aspects of American life get in the way of a nice, restful sleep. Modern electronics seduce us into staying up late. The stress of daily life can cause us to toss and turn as we try to go to sleep, or after we wake up in the night. The expanding obesity rate means more Americans suffer from sleep apnea and other nighttime disturbances.