Association between social jetlag and weight and fat reduction in dieting.

Objectives: Social jetlag (SJL), the discrepancy between an individual's inherent circadian rhythm and external social schedule, is associated with obesity. This study aimed to investigate whether SJL also influences body weight and body fat loss during dieting. Methods: This was an observational study from 2015 to 2018 with participants who had joined an exercise and nutrition program at a private personal training gym. Data from 11,829 individuals provided by the gym along with their sleep logs were analyzed. Differences in change in body mass index (BMI) and body fat percentage (%body fat) were compared by the degree of SJL. Regression was conducted for the change in BMI and %body fat on SJL, adjusted for gender, age, engagement duration in the program, initial BMI, initial %body fat, chronotype, and dietary intakes. Results: The subjects comprised 3,696 men and 8,133 women with a mean age of 40.4 years. Greater SJL was associated with a lower efficacy of BMI and %body fat reduction. The change in BMI (+ 0.56 / hour: SJL) and %body fat (+ 1.40 / hour: SJL) was associated with SJL after adjusting for each variable including dietary intake. Conclusion: SJL was associated with the effect of exercise and nutrition instruction on BMI and body fat reduction, even after adjustment for covariates related to dietary intake. Maintaining consistent sleep–wake rhythms may be crucial for enhancing the efficacy of weight loss programs. [ABSTRACT FROM AUTHOR]