Seasonal Biology Comes of Age: Could Daylength Affect Personality?

Could one of the cheesiest pickup lines ever, “What’s your sign,” suddenly become a useful job recruiting tool? Or maybe even the very information you want in determining your compatibility with a person you’ve just met, whether for a work or personal relationship?

landscape covered with snow

In a report published in Nature Neuroscience online 6 December 2010 (1), we learn that the season in which we were born does indeed affect our biological clock. More precisely, the length of the photoperiod or amount of light under which baby mice were born, was shown in this work to affect their biological clocks.

The research comes from Vanderbilt University and C.M. Ciarleglio et al. The authors note that “Environmental factors, particularly light, can markedly influence neural development.” It has previously been shown that seasonal light changes can acutely reorganize the biological clock of mature animals, located in the suprachiasmatic nuclei. However, the influence of light in the development of young animals’ and their circadian clock is unknown.

The suprachiasmatic nuclei (SCN; 2) is a tiny region in the midsection of the mammalian brain that is responsible for controlling circadian rhythms. The SCN generates neuronal and hormonal activities that in turn regulate multiple bodily functions, on a 24-hour clock. Mammalian behaviors such as physical activity and sleep, plus physiological features such as body temperature, immune function and digestion are controlled by the tiny SCN and its neuronal/hormonal messengers. If the SCN is damaged, these diverse functions are lost.

The study examined mice raised in light conditions that mimicked either short winter days or the light periods characteristic of long summer days. After weaning the mice were studied an additional 4 weeks under either the same light conditions (photoperiod) or conditions opposite of their initial photoperiod exposure. The mice were then housed in darkness and their activity studied.

Mice raised under winter-like conditions, no matter whether maintained under those short light periods or then moved to longer photoperiod days, showed a slowing of their daily activity time. When the researchers studied a gene connected to the SCN or biological clock in the mice, they found a corresponding slowing of the gene activity.

In contrast, the mice grown under summer light-like conditions show neither the slowing of daily activity nor of gene activity.

Ciarleglio et al. called these results “particularly striking”, noting that the light imprinting affected both the behavior of the mice and the neural activity in that biological clock region, the SCN.

The researchers comment that this study raises intriguing, though highly speculative possibilities about whether the day/night cycle during human development could affect personality. It is known that the biological clock affects mood in humans. However, as reported by , senior author D.G. McMahon notes that “even though this sounds a bit like astrology, it is not; it’s seasonal biology  (3)!”

That’s all very good and well. Obviously hiring or not hiring a person based on the month of their birth could be a form of discrimination. As well, you and I both know highly functioning persons born in December, as well as June and other months. I wonder what similar research would show for those mice (or people) born during months that are characterized by neither the longest nor shortest days.

While “What’s your sign” may be off-putting to most of us, this writer is an amateur fan of astrology, and have friends from other parts of the world, India and Trinidad in particular, for whom astrology is a oft-quoted consideration when discussing personality. I tell them about so-and-so and they ask, “When was she/he born?” Perhaps this research will to put a more solid, scientific footing to what some people already consider biological science?


  1. Christopher M Ciarleglio, John C Axley, Benjamin R Strauss, Karen L Gamble, Douglas G McMahon. Perinatal photoperiod imprints the circadian clock. Nature Neuroscience, 2010; DOI: 10.1038/nn.2699
  3. Vanderbilt University (2010, December 6. Season of birth may have long-term effects on personality, study suggests. ScienceDaily. Retrieved December 7 2010, from
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Kari Kenefick

Kari has been a science writer/editor for Promega since 1996. Prior to that she enjoyed working in veterinary microbiology/immunology, and has an M.S. in Bacteriology, U of WI-Madison. Favorite topics include infectious disease, inflammation, aging, exercise, nutrition and personality traits. When not writing, she enjoys training her dogs in agility and obedience. About the practice of writing, as we say for cell-based assays, "add-mix-measure".

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