The blossoms of spring smell especially sweet this year after a brutal winter that brought record-breaking snow and cold to most of the country. Casting off cold-weather lethargy along with our down jackets, we’re ready to burst forth with new projects and exercise regimens.

But though we may attribute this urge for action to the weather, it’s actually our bodies’ reaction to the sun. Scientific research shows that more hours of daylight, not warm breezes, tell our minds and bodies to get moving.

Though recognized since ancient times, mental states that change with the seasons were first scientifically described in 1984 in the Archives of General Psychiatry. Dr. Norman E. Rosenthal of the National Institutes of Health reported that some patients started feeling down in the fall but recovered in the spring, and the mood swings were more pronounced the farther people lived from the equator.

Since then, about 90 percent of people responding to surveys in higher latitudes have reported mood and behavior changes synchronized with the seasons. When the days lengthened in spring, their minds and bodies switched to high gear. They woke up earlier, ate less and felt driven to be active.

The biological mechanism behind seasonal change in humans is still unclear after nearly three decades of study, but most researchers agree that it’s an interplay of light, body chemistry and genetics.

Exposure to bright light appears to drive the system. A dedicated nerve pathway carries data from the retina to the brain’s internal clock, the suprachiasmatic nucleus (SCN). The SCN in turn sends a message to the pineal gland, which produces melatonin, the sleep hormone that also switches off activity hormones like adrenalin. More light makes the pineal gland produce less melatonin.

At the same time, light increases the brain’s supply of serotonin, a chemical that makes the brain more active and promotes upbeat feelings.

The dance between melatonin and serotonin stimulates the familiar call to action when the days get longer.

It all makes evolutionary sense, according to Paul Desan, a psychiatrist and researcher at Yale University and director of Yale’s adult mental health center.

Timing activity and breeding with the seasons is a good survival strategy for mammals. Many slow down or even sleep through the harsh winter and give birth in the spring when food becomes more abundant.

“We’re not seasonal breeders, but vestiges of it are still hardwired into our brains,” said Desan. “There is a general human tendency in the winter to be somewhat like hibernating bears.”

But when our eyes start transmitting more light to the SCN, our brains emerge from hibernation. Our energy, curiosity and sociability increase.
Spring body chemistry promotes optimism, and that, in turn, increases our willingness to take risks and try new things.

The connection between optimism and risk-taking is well documented. Daniel Kahneman and Dan Lovallo, researchers in psychology and business at the University of California Berkeley, say that optimistic people believe they will be successful. They are willing to accept risk because, deep down, they have faith in their plans and don’t think they will have to bear the potential negative consequences.

When it comes to risk assessment, George Loewenstein of Carnegie Mellon University showed that feelings trump intellect. A person hesitates to cross a bridge that doesn’t “feel right” even if evidence shows it’s perfectly safe. This clash between perceived and actual safety is what makes roller coasters so exciting.

Optimistic people are more eager to get on the roller coaster. It’s not just that they crave the excitement. They also have the emotional resources to set aside instinctive fears and enjoy the ride.

Looking at the evidence, perhaps we should start our New Year’s resolutions in April. Our built-in seasonal enthusiasm might help us keep them for more than a week or two.