Month of birth and height of Spanish middle-aged men.

We examined whether body height differs in Spain, a country having a high number of hours of sunlight, between middle-aged males and females born during the months with the longest and those born in the months with the shortest periods of daylight hours. We used data from a cross-sectional survey that documented standardized measurements of height and other variables for a 2021-person random multi-stage sample representative of the Spanish population aged 35-64 years. The relationship between height and month of birth was analysed by multiple linear regression, adjusting for age, occupation and type of residence. Male adults born in summer proved to be 1.7 cm taller than their counterparts born in winter (95% confidence interval 0.2-3.3 cm, p = 0.03). This relationship was most marked for those whose occupation was non-manual (2.1 cm, p = 0.04 vs 1.4cm, p = 0.2 in manual occupations). No significant (p = 0.8) or relevant (0.16 cm) summer- and winter-related differences in height were found in women. These results are compatible with the hypothesis that adult male height could be partially influenced by physical environmental factors such as the action of sunlight during the early stages of life.