Internal body clock
Fatigue is linked to the circadian rhythm. The body’s circadian rhythm is an internal biological clock. It coordinates the physiological priorities for daily activities, including sleep, body temperature, digestion, performance, and other variables. Therefore, it has a direct affect on alertness, mood, motivation, and performance.
The body’s natural cycle, or circadian rhythm, plays an important role in how fatigue affects people. The brain and the body are so accustomed to the normal body cycle that they resist changes (such as caused by work-schedules). The human body has a greater need for sleep at certain times in the 24-hour cycle than at other times (approximately between midnight and 4 a.m.; and, to a lesser extent, 2 p.m.- 4 p.m.). At these moments, there is a natural tendency to sleep and, if this cannot be given way to, a sleepy feeling occurs.
Shift work for instance interferes strongly with normal sleep patterns. Pronounced sleepiness is therefore a typical characteristic amongst most shift workers .