One theory maintains that spring fever is caused not by warm weather but by increased daylight. The light is measured by the eyes and relayed to the brain's pea-size pineal gland, which responds by reducing its secretion of melatonin, a hormone that regulates the daily biological clock and controls mood and energy levels. As spring days get longer, the chemical starts to disappear and people become more energized. Sex drive, enthusiasm and confidence all increase. Conversely, as days get shorter in the winter, melatonin builds up.
People feel sluggish and lethargic and start to slow down.
Rosenthal also attributes spring fever to the additional light transmitted to the hypothalamus, the part of the brain that regulates behavior such as eating, sleeping and sex drive. Those who have seasonal affective disorder exhibit altered circadian rhythms in the body. When we see light, it helps to reorganize biological clocks that became misaligned during short, overcast time periods of winter months. Light, however, is just one factor. The fragrant aroma and soft winds of spring can trigger tremendous emotional responses, including euphoria and sadness, Fisher said.