The correct answer is circadian rhythm.
The term circadian comes from Latin where it means 'around the day'.
Circadian rhythm disorders are disruptions in a person's internal body clock, very often associated with working shifts, stress or prolonged trauma, medication, jet leg etc.
Every person develops his own circadian rhythm, times and period of activity and rest. This occurs in accordance with the light-dark circle, day or night. The body gets used to the allocated periods of rest/activity and responds to it with different brain wave activities, hormone productions, organ regeneration etc.
Working shifts disrupts a person's body clock and causes the body to disorientate itself, which can lead to serious physical and mental problems.