When the dark chill of winter gives way to the sunny warmth of spring, many people find themselves in the throes of spring fever: restless, energetic, romantic. Other symptoms, says Michael Terman—an expert on biorhythms at New York–Presbyterian Hospital—include increased heart rate, appetite loss and mood swings. Clearly, the condition is real, even if it is not, as Terman notes, “a definitive diagnostic category.”
Researchers may lack an explanation of its biological underpinnings, but they do have a number of clues. Some evidence that the changing seasons affect human mood and behavior comes from Matthew Keller of the University of Colorado at Boulder. In a study of 500 individuals in the U.S. and Canada, he found that the more time people spend outside on a sunny spring day, the better their mood. Such good moods decrease during the hotter summer months, and there is an optimal temperature for them: 72 degrees Fahrenheit, otherwise known as room temperature.
Of course, spring does not just lighten our mood; as Alfred, Lord Tennyson famously observed, “In the Spring a young man's fancy lightly turns to thoughts of love.”
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Why the Sap Rises
Mammals track seasons by measuring the length of days—which grow longer in spring—through an internal clock in the brain, and this length is what controls breeding. The clock, called the suprachiasmatic nucleus (SCN), monitors light through a pathway from the retina and conveys information about day length to the pineal gland, at the base of the brain. In response to these signals, the pineal gland modulates its secretion of melatonin, dubbed the sleep hormone because it is released only in dim light or the dark. As nights shorten in spring, the brain's release of melatonin goes down. Some investigators suspect that this decline contributes to the increase in sexual activity, as well as in overall energy, in spring.
Studies of humans also show seasonal cycles in rates of conception. David Lam of the University of Michigan at Ann Arbor's Population Studies Center reports, for example, that birth rates in northern Europe typically rise to 10 percent above the average in March—meaning the babies were conceived in June. Cultural and social factors certainly influence the timing of conception in humans, but analyses of hormones add further support to the notion that biology plays a strong role. One study showed that levels of testosterone in men and luteinizing hormone (which triggers ovulation) in women peak at 20 percent above average during June.
Thomas Wehr of the National Institute of Mental Health says it is plausible that humans, like other mammals, respond to increases in day length by making less melatonin and that the reduction in melatonin, in turn, affects the levels of reproductive hormones, sexual behavior and energy.
Not everyone agrees, however, including Terman. “Melatonin,” he suspects, “is more like the hands of the clock; it's not the essential variable.”
Terman is more concerned with understanding a disorder that might be considered the opposite of spring fever: winter depression, or seasonal affective disorder (SAD). No one knows the exact cause of SAD either, he says, but it distinctly lifts in spring. A key to that rise in mood, he argues, is the earlier onset of morning light. He has shown that the incidence of winter depression is greater on the western edges of time zones in the U.S., where the sun rises later.
The correlations between mood, behavior and the lengthening of days in spring are real. The precise spur for our renewed energy, however, remains an enigma.
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