Many people have previously assumed that it was the cold weather that brings on an increase in infectious diseases in the winter. However, winter disease may be more closely linked to reduced sunlight, than to cold weather.
Pneumococcal infections are far more common in the winter than in the summer, with the peak incidence occurring during the darkest days of the year.
The CDC collected population-based data, including dates of pneumococcal culture, collected from January 1996 through December 1998 in seven U.S. states.
They found that the sharp spike in the incidence occurred the same time every year, regardless of the temperature: between December 24 and January 7.
Incidence went up sharply to levels of 150-200 cases per week from about 25 cases per week in summer.
The authors hypothesize that changes in daylight drive changes in susceptibility to infectious diseases.
The authors state that many mammals exhibit seasonal changes in things like reproductive function. For example, rodents, sheep, cattle, etc., are sexually active only at certain times of year.
They state that the sunlight drives melatonin secretion, which in turn changes other things such as the protein and fat content in milk. It may also cause changes in immune function or epithelial cell-receptor expression or have some other effect.
Therefore, the seasonality of these disease outbreaks may be the result of changes in human susceptibility, caused by a lack of sunlight.
International Conference on Emerging Infectious Diseases July, 2000 Atlanta, Georgia
One of the ways that you can compensate for the lack of sunshine in the winter is through the use of full spectrum lights. It is VERY important to know that the only valid full spectrum lights are fluorescent lights. Incandescent neodymium bulbs frequently advertised for under $10 are NOT full spectrum. They are color corrected and will NOT provide the same benefits. The newer full spectrum bulbs are compact fluorescents and can screw in a normal fixture. They have an electronic ballast and have much less EMF radiation. They are costly though and typically run about $30-40 a bulb. Over the years though they do return on the investment as they consume about 80% less electricity for the same amount of light. If one factors in the health benefits it is a non-issue; the bulbs win hands down. I have used them over the past five years and they have provided an enormous benefit in my ability to tolerate the miserable cold and dark winter months.
The lack of sunshine also mandates an earlier sleeping time. Ideally we should be in bed shortly after sunset. In the winter that is 4:00 PM. Most of us go to bed six or more hours later which tends to impair the adrenal glands and secondarily the immune system.
Other issues which can stress the immune system include the major increases in sugar consumption around the holidays and for most people an increase in emotional stress.
For example, people with few family and/or friends to share the holidays with, may feel even more alone and depressed. Also, some people may feel stressed out about having to cook and entertain large family get-togethers. This theory would explain why children's increase in sickness was not as dramatic, since they wouldn't have the same emotional stresses.
In addition to the melatonin mentioned in the article, sunlight also provides vitamin D, which serves mant vital functions as well.