Late fall and winter are known for two health problems: seasonal affective disorder (SAD) and infectious respiratory disease. Flu season is now upon us, and we can expect more colds, coughs, and depression as well as flu. Many researchers suspect a link between our immune function and mood state, and both may be seasonally variable.
“Wash your hands, wash your hands, wash your hands” is the usual health advice for flu season. This is quite reasonable, but I'm pretty sure that most of us wash our hands the same amount year 'round. So there's more to flu season than soap deficiency. Other factor that have been suggested include less sunlight, more time indoors with airborne pathogens, and stress. Stress and SAD may be closely linked.
Summer sunlight offers many benefits. One, it catalyzes the production of vitamin D in our skin. Researcher Dr. Michael Holick told Science News that the current daily recommended intake of 400 international units of vitamin D is far too low. Adults need 1,500 to 2,000 IU, he recommends in his new book. Vitamin D deficiency has been linked to increased risk of infectious diseases, autoimmune diseases, mood disorders, and more. Americans have low vitamin D levels even in the summer, according to Holick, and can easily slip into a deficiency state in the winter.
The visible part of the sunlight spectrum is believed to affect our mood directly. You can easily find bright lights for sale online as a treatment for SAD. These lights do not produce the ultraviolet light that is responsible for vitamin D production and, in excessive quantities, skin cancer. Reference 1 reports a study of SAD patients with increased levels of inflammatory cytokines that were not changed by successful light therapy. That is, the visible light made people feel less depressed, but did not improve their immune function.
Research with animal models demonstrates that the ultraviolet part of the sunlight spectrum has a beneficial effect on the immune system beyond what can be explained by vitamin D production. Very low doses of UV light were effective. One hypothesis is that the immune Th1/Th2 balance is shifted by sunlight.