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.