If the clock is stable, things go according to plan: your body makes all its appointments. The problem is that no matter how perfectly genetically calibrated your clock, it’ll have to face the outside world, where a slew of environmental factors can permanently (and potentially negatively) alter how well it works. That’s where the season of your birth comes into play.

The McMahon Lab team took a bunch of newborn mice and put them in one of three different environments, each corresponding to a different “photoperiod” of the year: a 12 hour/12 hour split between light and dark (a.k.a. the spring and fall equinoxes), an 18/6 split (the summer solstice), or a 6/18 split (the winter solstice). After 21 days—quite a while in mouse-time—the mice were moved into different photoperiod environments and the researchers looked at how they reacted. Most of the mice adapted to the change just fine, with one exception: The clocks of the “winter”-raised mice became extremely labile—seemingly, in fact, unstable.

The instability of the winter-raised mind isn't a new concept. Over the years, many studies have shown some correlation between various diseases and disorders and seasons of birth, but one grouping stands out. This particular correlation was found as early as 1929, and more than 200 studies have confirmed the results, including a large meta-study in 2003 accounting for more than 86 million births from 27 different parts of the world. The bottom line: If you were born in the winter, you’re at a higher risk for schizophrenia than those born in the sunnier months.

Similar results were found when researchers studied bipolar disorder and Seasonal Affective Disorder. There’s a reason why.

“Disorders like depression, schizophrenia, or autism—developmental disorders—are linked to the circadian clock,” Ciarleglio says. “The clock is hard-wired to other important areas of the brain that play major roles in how those disorders manifest.”

Light enters the eyes, and a signal is sent to the suprachiasmatic nucleus—the brain’s master clock—which, in turn, then sends signals down raphe nuclei in the brain stem. The main function of the raphe nuclei is to release serotonin to the rest of the brain. Serotonin, of course, helps regulate mood, appetite, and sleep, and an imbalance in the neurotransmitter is the basis of depression. Your biological functions all depend on light.

Traditional astrology was wrong: the moon does not cause insanity. But at the same time, astrology was right: those born in the winter season, when the moon has its longest reign in the skies, are more likely to suffer from mental disorders like schizophrenia and bipolar disorder. Modern science is catching up, and clarifying: Based on Ciarleglio’s and other researchers’ work, we now understand that the lack of sunlight in those months seems to contribute to long-term changes in how genes are expressed. In other words, we are inextricably tied to the specifics of our developmental environments.

In the past, scientists had to take the side of either nature or nurture in an often heated debate. These days, most have agreed that the better answer involves a little of column A and a little of column B. Yes, you are born with a single set of genes that determines many of your innate characteristics, but how those genes are expressed is manipulated by your environment and developmental experience.