There's an illness that has been documented by poets for centuries. Its symptoms include a flushed face, increased heart rate, appetite loss, restlessness and daydreaming. It's spring fever, that wonderfully amorphous disease we all recognize come April and May.

"Spring fever is not a definitive diagnostic category," says Michael Terman, director of the Center for Light Treatment and Biological Rhythms at Columbia University Medical Center. "But I would say it begins as a rapid and yet unpredictable fluctuating mood and energy state that contrasts with the relative low [of the] winter months that precede it."

Such spring fever remains a fuzzy medical category, but there has been a great deal of research on how seasonal changes affect our mood and behavior. Matthew Keller, postdoctoral fellow at the Virginia Institute for Psychiatric and Behavioral Genetics in Richmond, studied 500 people in the U.S. and Canada and found that the more time people spent outside on a sunny spring day the better their mood. Such good moods decreased during the hotter summer months and there is an optimal temperature for them, Keller claims: 72 degrees Fahrenheit, otherwise known as room temperature.

Of course, spring doesn't just lighten our mood; as Alfred Lord Tennyson described, "In the spring a young man's fancy lightly turns to thoughts of love." Studies show that sexual behavior in mammals follows a seasonal pattern, one that promotes survival. In fact, researchers discovered that birth spikes in field mice are more significant the farther the mice are from the equator, as seasons become more pronounced. The same trend was also seen in hares and deer, according to Mammalian Reproductive Biology by biologist Frank Bronson of the University of Texas. It is well documented that animals and humans track seasons by measuring the length of days through an internal biological clock, and this is what controls their breeding.

The biological clock, called the suprachiasmatic nucleus (SCN), sits in mammals' hypothalamus. It monitors light through a pathway to the retina and conveys information about day length to the pineal gland. This pea-size gland, tucked at the base of the cerebrum, controls the secretion of melatonin, dubbed the sleep hormone because it is only released in the dark or in dim light. The duration of melatonin release changes with nocturnal length, which is longest during winter. And it has been thought that our increased energy in the spring months is somehow linked to the decreased duration of melatonin production, due to shorter nights.

"From a biological perspective, most types of animals, and maybe even plants, have seasonal variation in behavior and physiology; there are seasonal cycles in human rates of conception," says Thomas Wehr of the National Institute of Mental Health, who reviewed the effect of biological rhythms on reproduction in 2001 for the Journal of Biological Rhythms. Historically there have been more births in the spring. In the late 16th century birth rates typically spiked to 20 percent above the average in March—meaning the babies were conceived in June—but over the past 400 years rates have flattened to about 10 percent above the average, according to research done by David Lam at the University of Michigan's Population Studies Center in Ann Arbor.

Cultural and social factors influence conception patterns but biology plays a strong role, as shown by peaks that are 20 percent above average during June—technically the tail end of spring—in the production of reproductive fuel: luteinizing hormone, which produces testosterone in men and triggers ovulation in women. Research also shows that successful in vitro fertilization follows the same seasonal peaks as natural birth. "In humans we don't know for sure what the causal connection is," Wehr says, "but if most other mammals are using changes in day length, then the melatonin signal and conception rates is a pretty plausible relationship, but more research is needed."

The idea that melatonin triggers our mood change in the spring is "too convenient an explanation," Terman counters. "Melatonin is more like the hands of the clock, it's not the essential variable." Since the mid-1980s researchers have focused on the seasonal effect on moods, with the emergence of a diagnostic label for winter depression, seasonal affective disorder (SAD). No one knows the exact cause of SAD, Terman says, but there are distinct patterns of winter depression lifting in the spring. And the key for that rise in mood, he argues, is the earlier onset of morning light. He has shown that there is more depression on the western edges of time zones in the U.S., where the sun rises later.

Clearly, there are marked correlations between moods, behavior and the lengthening days of spring, but the precise cause for our renewed energy remains elusive. The evidence for spring fever remains largely anecdotal. But, just as SAD has proved sadly real, spring fever edges away from science fiction, even if it is not quite science fact.