Happy daze
… The Attic warbler pours her throat,

Responsive to the cuckoo's note,

The untaught harmony of spring:

While, whisp'ring pleasure as they fly,

Cool Zephyrs thro' the clear blue sky

Their gathered fragrance fling….

Thomas Gray, "Ode on the Spring"


Forget about odes to spring. Neuroscience has taken the magic, not to mention the mystery, out of the poetry.

That surge of optimism? Merely the serotonergic response to increased daylight. The distraction and dreaminess? The neurotransmitter dopamine is responding to light and warmth. And what about the "gathered fragrance" of romance in the air? Hardly poetic, since the sensitivity of the olfactory system has been proven to directly relate to pheromones, the essential chemical ingredient of sexual attraction.

And here the poets thought the "untaught harmony of spring" was inexplicable.

While a formal scientific study of the delightful disorder known as spring fever is yet to be undertaken, the serious work of scientists looking at other maladies and other biological phenomena goes a long way in explaining why this particular season makes us feel the way we do.

For example, the study of seasonal affective disorder, or SAD, whose sufferers are morbidly depressed during the winter months and less so during the spring and summer, has shown how responsive our mood-defining neurotransmitters are to light. The intricacies of olfaction are intimately tied to human social, sexual and emotional responses. And the various chemicals involved in coupling, or what biologists call pair bonding — in the aching world of singles ads, the grimly unromantic "committed relationship" — seem to have special intensity in the spring.

In other words, the season's temperature, light and orgy of scents seemingly conspire to create a trifecta of feel-good stimuli. Interacting with those environmental stimuli, our hormones and neurotransmitters mix a heady cocktail for nearly everyone as the rain, chill and dark of winter give way to the warmth, sunshine and fecundity of spring.

"Anything that is novel and exciting drives up dopamine and norepinephrine in the brain," says Helen Fisher, author of "Why We Love: The Nature and Chemistry of Romantic Love." "In the spring we are more impulsive; we leap up and go into the park rather than drive. The novelty of spring, the warmth and the light all drive up our creativity, our impulsivity, our sex drive. And that sounds a lot like spring fever. "

The most obvious impetus for that sense of health and vitality can be found in two basic characteristics of spring: more daylight and warmer temperatures. Flowers need light to grow, and research has shown that humans need light to regulate the concentration of a number of chemicals, including those related to mood and emotion, among them serotonin, melatonin, dopamine and norepinephrine.

Although the precise causes of SAD are unclear, scientists have found that one explanation for the blank depression that afflicts those with the disorder is that these brain chemicals get out of balance in the winter months. Those with SAD often crave carbohydrates, which create a temporary but not very therapeutic surge of serotonin. At the same time, the brain's basic biological clock is governed by circadian rhythms, and when there is insufficient light, too much melatonin is released, also causing depression.

For those with seasonal depression, a dose of light therapy is often recommended by physicians to lift some of the melancholy created by the darkness of winter. While some doctors have been highly skeptical of light therapy, a comprehensive analysis published in the April issue of the American Journal of Psychiatry found not only that the treatment was effective for seasonal depression but that it also eased moderate depression nearly as effectively as the class of antidepressant drugs known as SSRIs, which focus on serotonin.

And if light affects those who are depressed, it also affects those who aren't. A study of 101 healthy men that was published in the British journal Lancet revealed that the turnover of serotonin in the brain was at its lowest during the dark winter months. The production of serotonin was "directly related to the prevailing duration of bright sunlight and rose rapidly with increased luminosity." Dazzling spring sunshine during daylight savings time can make even the healthiest people feel as if they have experienced the ideal response to Prozac.