These cycles are called circadian rhythms. Sleeping, waking, digestion, secretion of adrenalin, body temperature, blood pressure, pulse and many other important aspects of body functions and human behaviour are regulated by this 24-hour cycle. These rhythmical processes are coordinated to allow for high activity during the day and low activity at night. However, if the person is working at night, the body temperature does not have as much variation during a 24-hour period as it would normally. This disorientation can lead to feelings of fatigue and disorientation.
Some rhythms adapt in two to three days while others change only after longer periods.
People adapt to new schedules at different rates as do the different rhythms. Frequent changes in schedule and disruption to circadian rhythms can lead to chronic fatigue and other health problems. Disruption of both the quality and quantity of the normal sleep is inevitable in shiftwork particularly where night work is involved. The daytime sleep is seldom as deep or as refreshing as sleep at night. The problem is greater if there is not a quiet, dark, comfortable place to sleep.