Chronic Fatigue Syndrome
The cause of chronic fatigue syndrome (CFS) is not clearly understood, but it may result from multiple unrelated causes.
Although similar to various acute or chronic infections, it seems clear that CFS is not due to any single infectious disease. Another hypothesis is that the immune system, which is used to fight infection, may play a role in developing CFS. Findings that would typically occur in this situation have not been observed, however. Some research has investigated the role of the hypothalamic-pituitary-adrenal (HPA) axis, which can alter cortisol and other hormones that affect the immune system and other body functions. This research has been largely inconclusive. Finally, there is no evidence that vitamin deficiencies cause CFS.
Jet Lag
The cause of jet lag is a lack of synchrony between body rhythms including sleep cycles and hormone patterns and the environment. This often is caused by rapid travel across time zones, with increased symptoms corresponding to a greater number of time zones crossed.
Seasonal Affective Disorder
The cause of seasonal affective disorder (SAD) is not well understood. It may result when decreased light is available in the fall and winter, triggering depression in those predisposed to it. This may relate to abnormal melatonin metabolism or changes in the circadian rhythm, but the evidence for these causes is inconsistent. SAD may also be caused by abnormally low levels of the neurotransmitter called serotonin.
Sources:
Clemens, S. et al.. “Restless legs syndrome: revisiting the dopamine hypothesis from the spinal cord perspective.” Neurology. 2006 67:125.
Allen, R.P. et al. “Increased CSF hypocretin-1 (orexin-A) in restless legs syndrome.” Neurology. 2002 59:639.
Earley, C.J. et al. “Abnormalities in CSF concentrations of ferritin and transferrin in restless legs syndrome.” Neurology. 2000 54:1698.
Merlino, G. et al. “Association of restless legs syndrome in type 2 diabetes: a case-control study.” Sleep. 2007 30:866.
American Academy of Sleep Medicine. “International classification of sleep disorders, 2nd ed: Diagnostic and coding manual.” American Academy of Sleep Medicine, Westchester, IL 2005.
Scammell, T.E. “The neurobiology, diagnosis, and treatment of narcolepsy.” Annals of Neurology. 2003 53:154.
Chronic Fatigue Syndrome. Centers for Disease Control and Prevention, Department of Health and Human Services.
Checkley, S.A. et al. “Melatonin rhythms in seasonal affective disorder.” Br J Psychiatry. 1993 163:332.
