Considering the body, when one is exposed to environmental coldness the body contracts. The pores close, the hair stands on end - one gets "goose bumbs". The posture closes to retain heat. And in the absence of activity (yang) the body shivers to produce heat. All these reactions to cold are characterized by contraction. Likewise, the plants and other animals contract their energy for the winter.
In the summertime when it is very hot, the body responds quite differently than described above. The pores open - blood and sweat come to the surface to release heat. The metabolism slows producing lethargy or a loss of appetite. It is good to sprawl out under the shade of a tree. These things are characterized by expansion. Likewise, in the summer the energy of the plants is expansive and abundant.
If you are experiencing disease related to the seasonal conditions, consider the yin and yang properties as expansion and contraction. If you are too yin (cold or expanded), eliminate yin foods from the diet and focus on heartier nutrition and herbs that nourish energy. If you are too yang, it is generally recommended to eat foods and use herbs that are more yin (cooling, cleansing, moistening, expansive in structure). However, in the wintertime care must be taken to avoid too much yin even in those with yang conditions. Even those of a warmer disposition, must protect their vital energy in the winter and provide the body with the substance to generate warmth. Nature provides those foods that are appropriate for the season. In general, storage foods (which have condensed their vital energy) are appropriate for the wintertime.
Nature provides foods for each season. In the spring when it is time to cleanse from winter's excesses and stagnation Nature provides greens and shoots and mushrooms. In the summer, when yang is abundant, nature provides an array of vegetables and fruits. In the autumn the greens become heartier and root vegetables and other storage crops come in to harvest. In the winter the storage crops can be used and meats are hunted from the wild. This is the way of Nature.