Further Evidence for the Influence of Photoperiod at Birth on Chronotype in a Sample of German Adolescents

Abstract

Individuals differ in their circadian preferences (chronotype). There is evidence in the literature to support a season-of-birth effect on chronotype but the evidence is not convincing. In part, the relationship is obscured by a number of methodological differences between studies, including the measures used to define morningness, the way in which the seasons were categorized, and the sample size. This study adds to the literature in several ways. First, we adopt a new approach to categorizing the photoperiod rather than the calendar season; thus we prefer to use the term photoperiod at birth. Second, we used two measures of morningness. Third, we used a large and homogeneous German sample. The results show that adolescents (n = 2905) born during the increasing photoperiod (Feb–Apr) had a significantly later midpoint of sleep (MSFsc) than those born during the decreasing photoperiod (Aug–Oct). A similar pattern was found for the Composite Scale of Morningness (CSM). Furthermore, both measures of chronotype demonstrated a significant quadratic function over a 1-yr cycle. When looking at each of six consecutive years separately, the Composite Scale of Morningness suggests a cosine rhythm linked to increasing and decreasing photoperiods that becomes weaker in amplitude with increasing age. Despite the strengths in our study, the effect of photoperiod at birth on chronotype remains small. Future studies may require larger sample sizes, may need to explore how neonatal light exposure modulates chronotype, and may need to track how puberty and adolescent lifestyle habits mask the photoperiod effect. (Author correspondence: [email protected])

Keywords:

• Composite Scale of Morningness
• Day length
• Human circadian rhythms
• Midpoint of sleep
• Month of birth
• Morningness-eveningness
• Season of birth
• Sleep

ACKNOWLEDGMENTS

We are grateful to principals, teachers, parents, and pupils for participating. Many thanks go to Farina Pötsch for visiting participating schools and Caroline Buhmann, Farina Pötsch, and Lena Saliger for feeding the computer.

Declaration of Interest: This study was supported by grant 70131211 of the University of Education Heidelberg.

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.