Circadian rhythms control the sleep-wake cycle, mainly via exposure of the body to light. The circadian clock resets itself once a day, triggered by daylight. In some people, the clock does not reset properly, yielding winter depression, or seasonal affective disorder (SAD).
Our society now has a ready supply of light at any hour of the day. Working in the evening has shifted the clocks of some people by 4-5 hours, which means that they must incur a sleep debt the next day when they are forced to wake up early.