In this study, we demonstrated that SJL was associated with a decreased effectiveness of exercise and nutritional instruction on weight and body fat loss. The effect of SJL on the decrease in body weight and fat was found to be significant even after considering the effects of gender, age, duration of observation, initial body weight and fat, chronotype, and dietary intake. SJL can be improved through increased awareness, and that it could be crucial in the pursuit of efficient dieting. Numerous previous studies have demonstrated an association between SJL and obesity; however, most of these studies were cross-sectional. Therefore, a longitudinal evaluation was necessary to clarify the association between SJL and obesity [27]. The present study, which analyzed outcomes after a certain period, offers a more definitive insight into the association than cross-sectional designs. Improving dietary habits and physical activity is crucial in addressing obesity. In this study, it was observed that a lower SJL is associated with the effects of these interventions, suggesting a potential public health implication.

Previous studies reported that individuals with a night-type chronotype are at a higher risk of obesity [28]. However, the mechanisms linking chronotype with obesity have not been clarified to date. In the present study, a multivariable analysis was performed with SJL and chronotype as explanatory variables, and MSFsc was not found to be a significant variable. Individuals with an eveningness chronotype are likely to have a greater SJL, which could act as a confounding factor. It is possible that the increased SJL associated with eveningness contributes to weight loss resistance, rather than the mere presence of an eveningness chronotype. On the other hand, being a night-type or having short sleep duration is reported to contribute to weight gain through increased food intake [29]. In this study, the diet program provided detailed guidance on food intake, and the lack of a significant link between chronotype and food intake might have reduced its impact on changes in body weight and body fat. Additionally, sleep duration, which was also not detected as a significant factor in the sensitivity analysis, despite many studies indicating that short sleep duration tends to lead to obesity [30, 31], may not have shown significance for similar reasons.

Compared with recent studies, the mean value of SJL in this study population is low. In China, it has been reported that 17% of participants had an SJL exceeding 1 h [23], and in Germany, within a work environment that includes shift work, an average SJL of 1.96 h [32] has been documented. In a recent survey of the general Japanese population, the average SJL was 0.91 h, with 40% of participants exceeding 1 h [24]. However, in this study, the average SJL was 24 min (0.6 h), and only 10.3% exceeded 1 h. The reason for this difference is unknown. These studies indicate that younger individuals tend to have larger SJLs, our study included a few younger participants, which might have influenced the lower observed value. Additionally, there could be a sampling bias, as individuals able to attend a personal training gym might not be engaged in occupations that require much overtime work, potentially excluding those with extreme SJL values. This study has some more limitations. First, the data used in this study was self-reported, not using actigraphy or any other objective methods, which may compromise its accuracy. SJL and chronotypes were evaluated by using self-administrated clock times, and the objective internal circadian rhythm indicators, such as the time of dim light melatonin onset, rectal temperature, or clock gene expressions, were not directly measured. Moreover, when using MTCQ methods to calculate SJL and MSFsc, the data from potential shift-workers were not excluded since the occupation and work styles of participants were not assessed in this study. Although the distribution of SJL was in a relatively lower and narrow range, the inaccuracies of these indicators still remain. Seconds, the adequacy of dietary restrictions and the heterogeneity of training loads among participants is unknown. The mean self-reported average daily total caloric intakes were low, suggesting high compliance with this weight loss program. However, the fidelity of the intervention was not directly assessed in this study and was not adjusted in the analyses. Furthermore, the baseline dietary intakes and physical activity levels before the intervention were also unknown. These data were not adjusted in this study. Third, there may be other confounders. The absolute value of SJL is likely to be larger among shift workers or participants with long working hours [33]. Those with limited free time may not have undergone sufficient physical activity in their daily life outside of the program’s training. Furthermore, not only SJL based on sleep–wake schedules, but also whole life schedule including mealtime schedules could affect dietary outcomes. Variable eating patterns cause adiposity and worsen glycemic control [34], which might contribute to obesity and adverse dietary outcomes. There may also be other lifestyle factors that increase SJL and contribute to the worsening of obesity, but not all potential confounding factors were investigated in this study. Fourth, this this study consists of a large sample size with over 10,000 participants, and not restricted to any specific clinical populations other than obesity, suggesting that its external validity may be relatively robust. However, the study was conducted among Japanese individuals, introducing an inherent racial bias. The extent to which these findings can be directly applied to other racial or ethnic groups remains uncertain, highlighting the need for future research in diverse populations.

Several limitations underlie in this study. First, a significant challenge is the unclear mechanisms of these results. Existing studies have suggested a correlation between SJL and increased insulin resistance and insulin secretion [15], as well as elevated cortisol levels [16]. However, this study did not investigate the physiological changes associated with increased SJL. Second, it is unclear whether these results can be generalized to all populations. This is because the study was conducted only in Japan, mainly composed of East Asian people, and did not examine the influence of ethnicity on the negative effects of SJL on obesity and metabolism. Third, although food consumption was adjusted for in the analyses, other factors that may affect diet, obesity, and health, such as alcohol intake, tobacco use, or any other supplement intake, were not examined. Especially in the case of alcohol intake, weekend drinking could be associated not only with obesity and dieting but also with the occurrence of social jet lag (SJL). Measuring and adjusting for these factors in future studies will be needed. Forth, this study is an observational survey and does not involve a randomized comparative trial. Therefore, it does not provide complete proof of a causal association between SJL and obesity. Specifically, a randomized controlled trial comparing a group in which SJL is reduced through guidance with a control group, while controlling for various conditions, is needed to determine whether improving SJL promotes improvement in obesity.