Final answer:
Jet lag and working at night cause disruptions in circadian rhythms due to mismatches between internal biological clocks and external light-dark cycles, resulting in symptoms like fatigue and insomnia.
Step-by-step explanation:
Jet lag and working at night disrupt one's circadian rhythms because these activities alter the normal light-dark cycles that our bodies are used to. Circadian rhythms are daily cycles of behavior and physiology in humans, governed by a biological clock in the brain sensitive to light changes. When we travel across time zones or work night shifts, we experience mismatches between our internal clock and the external environment. This leads to jet lag symptoms such as fatigue and insomnia, and can cause problems with sleep regulation for those doing rotating shift work. Exposure to bright light when working and darkness when resting can help realign our biological clocks and alleviate symptoms.