There’s something in the air, and it’s not just pollen. Spring break, spring vacation, spring weddings — spring fever. We want to get out, wear less, mingle lots. Kids feel it, too. Talk to any teacher, you’ll likely hear there’s craziness in the classrooms.

That energy surge, in whatever form it takes, is a function of longer days and lots more sunlight, says Michael Smolensky, PhD, professor at the University of Texas-Houston School of Public Health. He is co-author of the book The Body Clock Guide to Better Health.

In fact, many facets of everyday life are governed by seasonal patterns as well as circadian rhythms — our internal biological clock, Smolensky tells WebMD.

“These are the rhythms of life, and we take them for granted,” he says. “People accept the fact that our bodies are organized in space — that our toes are at the end of our feet, and the hairs on our head stand up. But we give little thought to the fact that our bodies are structured in time.”

When seasons change, the retina — the inner layer of the eye that connects to the brain through the optic nerve — naturally reacts to the first subtle signs in the amount of daylight, says Sanford Auerbach, MD, director of the Sleep Disorders Center at Boston University. This reaction triggers hormonal changes, including an adjustment in melatonin, a hormone that affects sleep cycles and mood changes.

During the long darkness of winter months, the body naturally produces more melatonin. For people prone to seasonal affective disorder, all that melatonin triggers a winter depression. In spring, when melatonin production eases up, so does depression.

“There’s more daylight, so people have more energy, sleep a little less,” Auerbach says. “People who have manic-depressive problems [bipolar disorder] may be more manic in springtime.”

Body image springs into our consciousness this time of year. We’re shaking that craving for carbohydrates that makes us put on weight, says Smolensky. “It’s likely a carryover from our ancestors who had a hibernation-type biology. In the fall, they began putting on weight to get through the lean times of winter.”

We may have more energy in springtime, but it won’t necessarily play itself out in the bedroom, Smolensky tells WebMD. “When we look at couples who have kept diaries of sexual encounters and single males who have kept their own data, sexual activity is really rather low in the spring. The peak is in the fall.”

The reason? Testosterone levels peak in summer and autumn — not in springtime, he says. The evidence: More women conceive in late summer and early autumn than in spring, he says. The pattern also shows up in the CDC’s data on two common sexually transmitted diseases, syphilis and gonorrhea. The peak is in late fall and early winter.

Our long-ago ancestors chose to procreate in the fall, he says. “Given the fact that mammals have a longer gestation period, it’s best to conceive in the fall and deliver in the spring, when the supply of nourishing food supplies would be supportive of offspring. It may be a naturally selective thing that became ingrained in the genetics of human beings.”

However, our more recent ancestors created Valentine’s Day “as a pagan ritual whose sole purpose is to celebrate sexual activity,” Smolensky tells WebMD. “Was this society’s primitive way a means to stimulate sexual interest in a time when interest really wasn’t there?”

Spring breaks and vacations may have evolved similarly, he says. “Given enough alcohol and nudity, sex is a normal response that can occur at any time of the year.”

Just take note: There are more unplanned babies conceived during spring months, Smolensky tells WebMD. He’s analyzed studies in which IUDs and birth control pills were used and found that unintended conceptions occurred twice during the year — in May and in September/October — “even though the women claimed they were consistently adherent to birth control. There likely are seasonal biological factors at work, but what they are, we don’t know.”

Another word of caution: “Sperm counts are higher in spring,” he says. “In sexually active males, sperm count is affected by two factors — environmental temperature and sexual activity. When they are sexually active, sperm count goes down somewhat. When they’re not sexually active, they’re not using it, so it goes up.”