How our season of birth impacts who we are

This post is also available in Dutch.

Scientists have collected a convincing amount of evidence that your season of birth might have substantial influence on your health and character for the rest of your life.

You are probably familiar with the concept of horoscopes, which sometimes also include a description of character and personality traits that go along with a particular zodiac sign. While many people, especially scientists, are not convinced that our personality is in any way related to astrology, there is some evidence that the season you are born in could have an effect on your health and personality.

The line of research was sparked by multiple studies that showed that an unexpectedly large proportion of schizophrenia patients was born in winter months. Similar results were found for other disorders or childbirth complications. Only a few years later, a possible link between seasons and prevalence of personality traits was investigated, showing that the occurrence of certain temperament profiles could also fluctuate throughout the year.

The exact mechanisms behind these phenomena are still unknown. Researchers have suggested that external factors such as likelihood of (viral) infections, food availability and sunlight exposure affect the development of a baby in the womb, as well as after birth, to the extent that long-term changes occur. Research done in mice has shown that this is not entirely far-fetched.

A study from 2010 showed differences in the intrinsic biological clock of mice who were born and weaned under different artificial light cycles. The biological clock in this case refers to a number of compounds and processes that control whether we are awake and active or sleeping and resting, that is, our pattern of daily activity. In this study, mice were born and weaned under artificial light cycles simulating either summer (“summer mice”) or winter (“winter mice”). Then, after being weaned, the mice were maintained in either the same or the opposite cycle for 28 days. Afterwards, the by-now mature mice were placed in constant darkness to investigate their intrinsic daily activity patterns.

The intrinsic biological clock of the winter pups seemed to overreact to changes in the light cycle after weaning. In darkness, the daily activity patterns of those mice were not only extremely variable, but also different from the daily rhythm shown before under normal (seasonal) light conditions. This was not the case for summer mice, who showed little variation in daily rhythms. Moreover, winter mice who were shifted to a summer cycle remained active for an extra hour and a half every day compared to all other groups and according to their intrinsic biological clock days lasted slightly longer. This response is strikingly similar to changes in behavior shown by human patients suffering from seasonal affective disorder (winter depression), which is also more prevalent in humans born in the winter. Interestingly, these changes were not only visible on a behavioral level, but researchers observed the same changes in the activity pattern of neurons regulating the biological clock in these mice.

This shows that certain environmental conditions and influences early during life indeed can affect biological processes in the long-term and subsequently also shape who we are. Of course, that does not mean that your birth month is in any way your destiny. However, people who believe in horoscopes may be onto something; our personality traits may be influenced by our date of birth… just not for the reasons commonly thought.

Written by Eva, edited by Monica.

Image by LukePricePhotography (CC BY 2.0)

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