For those of us in the Northern hemisphere, this is a highly relevant topic at this time of year. Seasonal Affective Disorder is is a type of depression that’s related to changes in seasons and the accompanying decrease in natural daylight. It is a very good example of an interaction between the environment and a physiological process.
This blog by Dayna Evans carries an excellent description of her suffering from SAD.
Rosenthal et al (1986) summarised a series of studies and concluded that the hormone melatonin may play a key role in SAD. Melatonin is secreted by the pineal gland in the brain in response to darkness. An excess of melatonin seems to be related to SAD, though much more research is needed, as treatment with phototherapy (natural light lamps) and oral melatonin have been inconclusive.
SAD has also been associated with low levels of the neurotransmitter serotonin. This is why tryptophan, which is an amino acid used by the body to help make serotonin, is sometimes recommended as a dietary supplement. Most of the commentators seem agreed that getting outside on those bright cold days is also important.
Finally, this link to Psychology Today will give you a lot more information on Seasonal Affective Disorder. Questions regarding environmental effects on physiology can safely be answered using SAD and its relation to melatonin increase and serotonin decrease.