Season of birth defines personality, but it’s “not astrology,” say researchers

The season in which babies are born can have a dramatic effect on their future risk of neurological disorders, including seasonal affective disorder, bipolar depression and schizophrenia. The findings, published in the journalNature Neuroscience, come from a study into the seasonal imprinting that occurs in babies at birth.

“Our biological clocks measure the day length and change our behavior according to the seasons. We were curious to see if light signals could shape the development of the biological clock,” said study leader Douglas McMahon, from Vanderbilt University.

In the experiment, groups of mouse pups were raised from birth to weaning in artificial winter or summer light cycles. After they were weaned, they were maintained in either the same cycle or the opposite cycle for 28 days. Once they were mature, the mice were placed in constant darkness and their activity patterns were observed.

The winter-born mice showed a consistent slowing of their daily activity period, regardless of whether they had been maintained on a winter light cycle, or had been shifted to summer cycle after weaning. When the researchers examined the master biological clocks in the mouse brains, using a gene that makes the clock cells glow green when active, they found a similar pattern: slowing of the gene clocks in winter-born mice compared to those born on a summer light cycle.

“What is particularly striking about our results is the fact that the imprinting affects both the animal’s behavior and the cycling of the neurons in the master biological clock in their brains,” noted co-researcher Chris Ciarleglio.

Interestingly, the experiments found that the imprinting of clock gene activity near birth had dramatic effects on the reaction of the biological clock to changes in season later in life. The biological clocks and behavior of summer-born mice remain stable and aligned with the time of dusk while that of the winter-born mice varied widely when they were placed in a summer light cycle.

The study raises an intriguing possibility: seasonal variations in the day/night cycle that individuals experience as their brains are developing may affect their personality. “We know that the biological clock regulates mood in humans. If an imprinting mechanism similar to the one that we found in mice operates in humans, then it could not only have an effect on a number of behavioral disorders but also have a more general effect on personality,” said McMahon. “It’s important to emphasize that, even though this sounds a bit like astrology, it is not: it’s seasonal biology!”

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Source: Vanderbilt University

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