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 .