For example, one pre-registered study of 686 participants found that after exerting mental effort, people are likely to disengage and become less interested in exerting further effort. Decreased attention can also be described as a more or less decreased level of consciousness. In any case, this can be dangerous when performing tasks that require constant concentration, such as operating large vehicles. For instance, a person who is sufficiently somnolent may experience microsleep. However, objective cognitive testing can be used to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.The perception of mental fatigue is believed to be modulated by the brains reticular activating system (RAS).Fatigue impacts a drivers reaction time, awareness of hazards around them and their attention. Drowsy drivers are three times more likely to be involved in a car crash and if they are awake over 20 hours, is the equivalent of driving with a blood-alcohol concentration level of 0.08%. Primary vs secondary
In some areas it has been proposed that fatigue be separated into primary fatigue, caused directly by a disease process, and secondary fatigue, caused by more general impacts on the person of having the disease. Causes
Fatigue is a normal result of work, mental stress, anxiety, overstimulation and understimulation, jet lag, active recreation, boredom, lack of sleep or disease. Acute
Causes of acute fatigue include depression; chemical causes, such as dehydration, poisoning, low blood sugar, or mineral or vitamin deficiencies.
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.