To Everything A Season (Including Sex)
July 9, 2013

To Everything A Season (Including Sex)

Got babies, babies, and babies on the brain? You’re not alone. Summer marks the start of baby season, with the peak in the baby craze due in September. Scientists have known for years that human births are seasonal, but the reason remains a mystery.

A time to embrace.

In other mammals, seasonal births are controlled by circadian rhythms. In hamsters, for example, the shortening of days triggers their sex organs to recede, allowing them to conserve energy for the winter. In humans, circadian circuits are well known to control sleep patterns, but do they influence the human sex drive, too? In North America, conceptions follow a six-month rhythm, peaking in mid-summer and early winter. Condom sales follow the same pattern. Abortions peak a few months later, in early autumn and late winter, while STD diagnoses peak in early autumn. Internet searches for porn, prostitutes, and online dating sites also shoot up in mid-summer and early winter.

These peaks in sexual interest start just after the spring and autumn equinoxes. This could mean circadian rhythms play a role, but these days, seasons are as much defined by chocolate bunnies and Santa suits as they are by the length of the days. The December peak, for example, coincides with the winter holiday season and has been dubbed “the Christmas Effect.” On the other hand, European conceptions peak in late-August. And non-Christian countries, like Israel and parts of India, see an uptick in winter conceptions, similar to peaks seen in the Americas.

Other environmental effects, like temperature, throw another twist into the equation. Extreme temperatures are well known to hurt fertility, so conceptions may drop in the heat of summer. This might explain why August conceptions drop in America, but not in Europe, where the summers are milder.

A time to be born.

But why have a built-in timer for sex at all? Unlike other mammals, human babies are reared over several seasons, not just one, so does it really matter when they’re born? Turns out, your month of birth does have some effect on your health. Winter babies have higher incidences of schizophrenia, bipolar disorder, and neurotic tendencies. Winter/spring babies are also more likely to suffer from diabetes, Hodgkin disease, alcohol abuse, autism, eating disorders, personality disorders, as well as certain neurological illnesses, like Alzheimer’s disease and narcolepsy.

Babies born in different seasons even differ in personality. A study conducted in Sweden found that adults born between February and April have higher rates of novelty seeking and impulsiveness than babies born between October and January. The same author also connected season of birth with differences in monoamine metabolism. Monoamines — like serotonin, dopamine, and norepinephrine — are neurotransmitters that are known to affect mood and anxiety, as well as personality traits like novelty seeking, persistence, and harm avoidance.

But before you pick up that astrology forecast, none of this has anything to do with stars. Susceptibility to illness could be due to seasonal variations in pregnant women’s nutrient intake. Exposure to infectious disease also varies by season, and some researchers have even linked schizophrenia with a baby’s exposure to respiratory infections in the second trimester. Family planning might also play a role. Planned births are often timed away from winter. So, non-winter babies are more likely to be planned, and planned babies often have better prenatal and natal care.

Birth season also influences our circadian rhythms. Can’t seem to get up in the morning, no matter how many alarms you set? You might be a late chronotype. Falling asleep in your Cosmo during a night out with the girls? That’s typical of early chronotypes. It turns out, winter-born babies are more likely to prefer mornings, while summer born babies tend to like evenings.

But our natural rhythms influence more than just a morning/evening preference. In mice, circadian circuits are imprinted at birth, and it changes the way they respond to the seasons. Summer-born mice tend to have more stable behaviors. They synch their activity to the start of dusk, regardless of the season. Winter-born mice synch their winter activity to dusk, too, but in the summer, their timing changes. To researchers, this change in behavior looks a lot like seasonal affective disorder, or SAD. If similar imprinting happens in humans, it might explain why winter babies are more vulnerable to SAD and other mood disorders.

A time to every purpose.

The upshot of it all is that babies born in late summer/early autumn have a slight advantage when it comes to mental and physical health. But the effect is tiny. It might be just enough to maintain a little bit of seasonality in human births, but certainly not enough to make seasonal reproduction widespread. Even the glut of September babies is just a small bump in an otherwise flat distribution of birthdates. This vestige of seasonal behaviors is probably an evolutionary artifact, a throwback to some mammalian ancestor whose life hinged on the turning seasons. In humans, evolution has traded seasonality for the ability to adapt to any change in the environment – whether it stems from the seasons, or not.

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