Sunlight can influence the breakdown of medicines in the body (Science Daily)
"A new study has shown that the body's ability to break down medicines may be closely related to exposure to sunlight, and thus may vary with the seasons. The findings offer a completely new model to explain individual differences in the effects of drugs, and how the surroundings can influence the body's ability to deal with toxins."
Widespread seasonal gene expression reveals annual differences in human immunity and physiology (Nature)
"Seasonal variations are rarely considered a contributing component to human tissue function or health, although many diseases and physiological process display annual periodicities. Here we find more than 4,000 protein-coding mRNAs in white blood cells and adipose tissue to have seasonal expression profiles, with inverted patterns observed between Europe and Oceania."