Being in Season

The body's biological clock is parallel to that of the climate one is in. 
Unfortunately, supermarkets today give the illusion that there are no Seasons.  As an example, you can for purchase bananas or pineapple (both tropical fruits) in Winter even if you live in a temperate zone where there is a harsh Winter (e.g. Chicago).

To expand on this example, people living in the tropical climates, where for instance bananas or pineapples grow, have evolved eating certain foods over thousands of years.   Eating tropical fruits such as pineapples actually opens up the pores of the skin, a natural cooling mechanism for people of these regions.  In the same manner, the Eskimo people who evolved in a very cold climate, have for centuries eaten more rich animal-based foods because these foods are contracting, and allow the body to shield against the harsh cold. 

Eating foods out-of-season and those not meant for your region (such as tropical fruits and juices in Winter) will throw the body off, and make it more susceptible to contracting colds and different strains of flu.  It will also cause discomfort because the body will be less able to adapt to weather changes and usually will cause a person to rely overly on artificial heating and cooling, which for some people can throw off their sleep cycle, further weakening the immune system.  It is important and essential to let the body experience the weather changes because this allows the body's internal clock to 'know what time it is'.  

If you are traveling, and need to adjust your body's clock, periodically expose yourself to a light and moderate amount of sunlight in the new area you find yourself in.  This will help the body adjust your internal clock and will lessen the effect of jet-lag.