Original: Humans, like most animals and plants, have biological rhythms, known as circadian rhythms, which are controlled by a biological clock and work on a daily time scale. These affect body temperature, alertness, appetite, hormone secretion etc. as well as sleep timing.
Simplified: Many people have problems with their sleep, appetite, alertness etc. when they work a shift during the times when their circadian rhythms indicate that they should be asleep.