May is America's favorite month, according to Gallup. College students get out of school, summer starts, and many of us take our first three-day weekend in months for Memorial Day. We buy sandals and sunblock, and get tipsy on patios with our friends during Sunday brunch.
Researchers have begun to quantify this feeling—it's when the winter blues, and associated clinical conditions like seasonal affective disorder, finally lift. We spend more money. Our brains are more focused and efficient. We perceive spring to be eternal—more than twice as long as winter. We become more positive—at least on Twitter—as the days grow longer. Even our babies are more likely to have an "excessively positive outlook on life" if they're born in the spring or summer.
But why? There are a couple overlapping explanations. One is psychological: holiday placement, events, and work and school schedules classically condition us to enjoy some seasons and months more than others. Cinco de Mayo, Mother's Day, Memorial Day and graduations, for example, can trigger culturally-acquired feelings and associations, which cue certain moods.
The second explanation is biological. In countries further from the equator, days are significantly shorter in the winter and longer in the summer. Our hormones respond acutely to the increasing daylight. First, our melatonin levels, which regulate sleep, drop. Kathryn Roecklein, who studies circadian biology at the University of Pittsburgh, explains that our brains release more melatonin during the winter, when nights are longer.
When nights become shorter in the spring and summer, however, they release less. "Melatonin secretion at night signals the body to change in ways that are appropriate for the seasons," she says. For instance, appropriate biological changes for ancient humans might have entailed more energy and enhanced focus for foraging in the summer. For the winter, we may have evolved to adopt a melatonin-induced, hibernation-like state to prevent unnecessary energy exertion.
Meanwhile, levels of serotonin, a hormone that contributes to feelings of wellbeing, rise. Various studies suggest that the more light there is on a given day, the higher serotonin concentration people have in their blood and key areas of the brain. "As the days get longer and brighter," says Norman Rosenthal, a psychiatrist at Georgetown University School of Medicine, "our brains release more serotonin."
We may be especially affected by this feel-good hormone in seasonal transitions. During the dark winter months, our brains release much less serotonin. Serotonin receptors consequently become "very sensitive" and "very responsive" to changes, Rosenthal says. When daylight lengthens in the spring, serotonin flushes these sensitive, starved receptors, thereby amplifying feelings of buoyancy and vitality.
Seasonal hormonal changes are more pronounced in people with seasonal affective disorder (SAD), a condition characterized by apathy, increased appetite, social withdrawal, and increased sleep and fatigue. But Rosenthal, who pioneered the diagnosis of SAD, thinks seasonal mood changes happen in everyone, just to a lesser extent. "It's so common for people to feel exuberant in the spring, a little bit down in the dumps in the wintertime. People need more sleep in the winter and less in the spring." He emphasized that "there are chemical changes going on in everybody's brain across the seasons, and those chemical changes may account for differences in behavior and how people feel across the seasons and across the year."
It's worth noting that not all chemical changes favor long days and warm weather. SAD may actually be more common in summer than in winter—by a factor of three to two, according to some research. Other research shows that children release more cortisol, a hormone associated with stress, in the summer than in the winter. Moreover, our moods depend not just on the seasons but on us as individuals.
Still, it's conceivable that on the whole we're happier in the spring, in transition between two seasonal extremes. We see this possibility on a smaller scale in research on lighting at work. One study found that workers' moods were lowest when they experienced the lighting as "too dark." Their mood improved as lighting increased, but then declined again when they experienced the lighting as "too bright." In other words, there seems to be a happy perceptual medium of "just right"—like May.
We love May in part because nature and our culture cue us with green grass, parties, and warm weather. But we also love May because our biology responds in an enjoyable way. It's a fluid combination. "It's the flowering of nature and the flowering of our brains coinciding," says Rosenthal.
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