Association Between Underweight Status and Social Jet Lag Among Japanese University Students: A Cross-Sectional Study...

The prevalence of underweight among young adults in Japan is approximately 20% in both males and females. Despite its potential health risks, underweight in males has received limited attention. Social jet lag (SJL), the misalignment between biological and social time, has recently been associated with various health outcomes, including obesity and circadian rhythm disturbance. However, evidence on the relationship between SJL and underweight, particularly regarding gender differences, remains limited. This study aimed to examine the association between SJL, lifestyle factors including internet use, and underweight status among university students, focusing on sex-specific differences.

A cross-sectional study was conducted among 3,102 students (54.6% male) at Shimane University in 2022. Participants were classified into underweight (< 18.5 kg/m²) and non-underweight groups. Lifestyle data, including sleep patterns, diet, physical activity (PA), internet use, and gaming time, were obtained from health examinations and questionnaires. SJL was calculated as the difference in sleep midpoint between weekdays and weekends.

For multivariate analysis, logistic regression was performed with underweight as the dependent variable. Age was included as a covariate, and variables significant in univariate analyses were entered into sex-stratified models.

The prevalence of underweight was similar in males and females (17.2% vs. 17.3%). In both sexes, underweight students exhibited lower systolic blood pressure, higher resting heart rate, and lower PA than their non-underweight counterparts. Among males, the underweight group exhibited a greater median SJL (1h) and a higher prevalence of internet addiction (17.1%). Multivariate analysis revealed that SJL was independently associated with underweight only in males (OR = 1.14, P < 0.05).

Although underweight university students exhibited similar physiological characteristics across sexes, SJL was independently associated with underweight only in males, suggesting that irregular sleep-wake patterns may constitute a novel behavioral risk factor for underweight in this population.