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.