Examining sex differences in morningness-eveningness and inter-individual variability across years of age: A cross-sectional study

This study investigated the impact of age on morningness-eveningness (ME) and its inter-individual variability, with a focus on sex-specific patterns. A sample of 2890 participants aged 12-94 years (55.85% female) completed the Composite Scale of Morningness. Multiple linear regression analyses revealed a significant Age × Sex interaction, indicating distinct age-dependent patterns for males and females in both ME and its inter-individual variability. We conducted segmented regression analyses to explore these dynamics further and identify breakpoints. Eveningness increased across adolescence, with both males and females reaching peak lateness at 20 years. Morningness increased thereafter until 64.94 years for females and until 59 years for males. Following these sex-specific breakpoints, morningness remained constant for females and increased at a reduced rate for males. After the age of 48, males exhibited greater morningness than females. Inter-individual variability in ME (VME) changed with age, increasing until 33 years for men and 36.80 years for women, followed by a decrease for both sexes. From 50.30 years onward, females showed greater VME compared to men. This study suggests that ME and VME shift across age, and differences between sexes were observed from middle age onwards. Diurnal preferences are likely influenced by internal and environmental variables.