Fatigue is different from drowsiness, where a patient feels that sleep is required.Temporary fatigue is likely to be a minor illness like the common cold as one part of the sickness behavior response that happens when the immune system fights an infection. Prolonged
Prolonged fatigue is a self-reported, persistent (constant) fatigue lasting at least one month. Chronic
Chronic fatigue is a self-reported fatigue lasting at least six consecutive months. Chronic fatigue may be either persistent or relapsing. Chronic fatigue is a symptom of many diseases and conditions. Some major categories of conditions that feature fatigue include:

Autoimmune diseases, such as celiac disease, lupus, multiple sclerosis, myasthenia gravis, Sjögrens syndrome, and spondyloarthropathy
Anxiety disorders such as generalized anxiety disorder
Blood disorders such as anemia and hemochromatosis
Cancer, in which case it is called cancer fatigue
Chronic fatigue syndrome (CFS)
Substance use disorders including alcohol use disorder
Depression and other mental disorders that feature depressed mood
Developmental disorders such as autism spectrum disorder
Eating disorders, which can produce fatigue due to inadequate nutrition
Endocrine diseases or metabolic disorders: diabetes mellitus, hypothyroidism and Addison’s disease
Fibromyalgia
Gulf War syndrome
Heart failure
HIV
Idiopathic chronic fatigue (ICF), which is chronic fatigue with no known cause that does not meet chronic fatigue syndrome criteria
Inborn errors of metabolism such as fructose malabsorption. Infectious diseases such as infectious mononucleosis or tuberculosis
Irritable bowel syndrome
Kidney diseases e.g. acute renal failure, chronic renal failure
Leukemia or lymphoma
Liver failure or liver diseases e.g. Hepatitis
Lyme disease
Neurological disorders such as narcolepsy, Parkinsons disease, Postural Orthostatic Tachycardia Syndrome and post-concussion syndrome
Physical trauma and other pain-causing conditions, such as arthritis
Sleep deprivation or sleep disorders, e.g.
Depression and other psychological conditions can produce fatigue, so people who report fatigue are routinely screened for these conditions, along with substance use disorders, poor diet, and lack of physical exercise, which paradoxically increases fatigue. Basic medical tests may be performed to rule out common causes of fatigue. These include blood tests to check for infection or anemia, a urinalysis to look for signs of liver disease or diabetes mellitus, and other tests to check for kidney and liver function, such as a comprehensive metabolic panel. Other tests may be chosen depending on the patients social history, such as an HIV test or pregnancy test. Comparison with sleepiness
Fatigue is generally considered a more long-term condition than sleepiness (somnolence). Although sleepiness can be a symptom of a medical condition, it usually results from lack of restful sleep, or a lack of stimulation. Chronic fatigue, on the other hand, is a symptom of a greater medical problem in most cases. It manifests in mental or physical weariness and inability to complete tasks at normal performance. Both are often used interchangeably and even categorized under the description of being tired. Fatigue is often described as an uncomfortable tiredness, whereas sleepiness is comfortable and inviting. Measurement
Fatigue can be quantitatively measured. Devices to measure medical fatigue have been developed by Japanese companies, among them Nintendo (cancelled). Nevertheless, such devices are not in common use outside Japan.