The Relationship Between Digit Ratio and Circadian Typology Among Medical Students

Objectives: Morningness-eveningness preference stands for interindividual chronobiological differences. This study was designed to investigate if prenatal testosterone exposure is a candidate for having a causal role in sexual dimorphism observed in circadian typology.Materials and Methods: A total of 183 students (111 females, 72 males; mean age 20.7 years) were involved in the study. Circadian typology was assessed using the Horne-Ostberg Morningness-Eveningness Questionnaire. Digit lenghts were measured with digital calipers and digit ratio (2D/4D) was calculated by dividing the length of the second digit by that of the fourth digit.Results: Circadian typology assessment revealed 54 morning-(29.5%), 100 intermediate-(54.6%) and 29 evening-(15.8%) type students. Digit ratio of both hands in males were significantly lower than females (p<0.001). Digit ratio for both hands in morningness trait were significantly higher than intermediate (p<0.001) and eveningness (p<0.05 for the left and p<0.01 for the right hand) traits. Morning type was more common among females (35.1% vs 20.8%; p<0.05). Higher digit ratio was significantly associated with the morning type for both of the hands in both genders.Conclusion: Prenatal testosterone exposure, represented by digit ratio (2D/4D) as a proxy marker, seems to have a causal role in sexual dimorphism observed in the circadian typology preferences.