It can be normal in response to physical activity, emotional stress, boredom, or lack of sleep.
Fatigue can also be caused by many other factors, including:
Medical conditions: Such as the flu, glandular fever, chronic fatigue syndrome, thyroid disorders, heart disease, or diabetes
Lifestyle-related causes: Such as alcohol or drugs, or lack of regular exercise
Workplace-related causes: Such as workplace stress
Other causes: Such as anemia from multiple myeloma), depression or grief, iron deficiency, medicines, persistent pain, sleep disorders, infection, kidney disease, liver disease, or malnutrition