When the days get longer, the body increases the production of the happiness chemical serotonin. At the same time, levels of hormones more active during the winter are still elevated. This hormonal change is thought to contribute to tiredness. In addition, as a reaction to raising temperatures, the blood vessels in the body can dilate, resulting in lower blood pressure and possibly fatigue. Many people also suffer from a vitamin deficiency due to decreased consumption of fresh fruits and vegetables in the winter.
By Margaret Powell