Associations of social jetlag with depressive symptoms among adults in Southern China: a cross-sectional study

ABSTRACT To investigate the associations between social jetlag and depressive symptoms and further analyze the interaction of social jetlag and chronic diseases on depressive symptoms among adults. Data were derived from a cross-sectional survey. Information on social jetlag, depressive symptoms, demographic characteristics, lifestyles, and health situations were collected via a structured self-administered questionnaire. The multivariable logistic regression analyses and restricted cubic splines regression were performed to test the association between social jetlag and depressive symptoms. The multiplicative model was applied to analyze the interaction of social jetlag and chronic disease on depressive symptoms. A total of 5818 adults were included and the prevalence of social jetlag was 22.95%. After multivariable adjustment, adults who experienced more than 2 hours of social jetlag had a significantly greater risk of depressive symptoms (OR = 1.43,95%CI:1.00–2.05, P = 0.049). Compared with participants having<1 hour of social jetlag and no chronic disease, participants having chronic disease were more likely to report depressive symptoms(OR = 3.22,95%CI = 2.59–4.00, P < 0.001). Moreover, among those who have chronic disease, the greater the social jetlag they reported, the greater risk of depressive symptoms they have (OR = 3.81,95%CI = 2.77–5.33, P < 0.001; OR = 4.08,95%CI = 2.24–7.43, P < 0.001). Social jetlag was linearly associated the with risk of depressive symptoms, according to restricted cubic splines. This study shows that 22.95% adults experience more than 1 hour of social jetlag. Social jetlag was positively associated with an increased risk of depressive symptoms. Among those who have chronic disease, the greater the social jetlag they reported, the greater risk of depressive symptoms they have.


Introduction
Depression, a common mental health disorder characterized by a predominantly depressed mood (Bos et al. 2019;Yan et al. 2019), is a leading cause of non-fatal disability worldwide Pfau et al. 2018). It is estimated that 10-15% of the global population will experience depression during their lifetime (Lohoff 2010). The increasing global health burden and prevalence of depression result in a growing body of evidence about its risk factors (Wu et al. 2022;Zhang et al. 2023).
Recent studies have indicated that social jetlag is an important risk factor for depressive symptoms (De Souza and Hidalgo 2014;Knapen et al. 2018;Mathew et al. 2019;Min et al. 2023). Social jetlag is one of the parameter values used to present the degree of a circadian rhythm disorder (Johnson et al. 2020;Roenneberg et al. 2012), measuring the absolute value of the difference between the midpoint of sleep during weekdays and weekend days (Islam et al. 2020;Qu et al. 2023;Wittmann et al. 2006). Individuals, who prefer a late chronotype usually go to bed later, but due to work requirements or other reasons, they have to wake up relatively early on weekdays, resulting in short sleep duration during the weekdays, and they may attempt to compensate for their lack of sleep on weekdays by delaying their wake-up time to get more sleep on the weekends (Rutters et al. 2014).
Up to now, studies on the relationship between social jetlag and depressive symptoms were mostly focused on adolescents (De Souza and Hidalgo 2014;Mathew et al. 2019). While studies in adults are relatively rare which were mainly conducted in developed countries, and the findings were inconsistent (Islam et al. 2020;Knapen et al. 2018). A study from Japan showed that greater social jetlag was associated with higher levels risk of depressive symptoms among non-shift workers in Japan (Islam et al. 2020). A study from the Netherlands showed that patients with major depression did not present higher social jetlag compared to the healthy control group (Knapen et al. 2018). However, there has been no data on the association of social jetlag with depressive symptoms among adults in China.
To fill this research gap, a cross-sectional survey was conducted in China to explore the association between social jetlag and depressive symptoms among Chinese adults. Meanwhile, there is evidence that chronic diseases are affected by social jetlag (Anothaisintawee et al. 2018;Mota et al. 2019;Parsons et al. 2015), which in turn may affect mental health such as depressive symptoms (Lu et al. 2018). But there is no study that reveal the interaction of social jetlag and chronic diseases on depressive symptoms among adults. Therefore, this study further analyzed the interactive effect of social jetlag and chronic diseases on depressive symptoms.

Study design and sample selection
A cross-sectional survey on the Mental Health Status of Zhuhai Residents was conducted in Zhuhai City, China from October to December 2021. This survey aims to investigate the mental health status and to determine the factors of related to psychological symptoms among permanent residents aged 18 years (i.e., born before September 1, 2003) and above. Permanent residents refer to those who were born and have lived in Zhuhai for more than 6 months in the past 12 months.
A multi-stage cluster random sampling method was applied to recruit participants, as illustrated by Figure 1. In the first stage, 323 communities (villages) in all districts of Zhuhai were included. In the second stage, communities (villages) were taking using probability proportionate to size sampling according to the number of communities (villages) in each district and a total of 90 communities (villages) were sampled. In the third stage, at least 65 households were selected from each community (village) according to a systematic sampling method, starting point and interval determined by door numbering. In the final stage, one adult resident was selected from each sampled household using a random sampling method (i.e., select the adult whose birth date is the closest to the 15th day of each month).
The questionnaire survey were carried out according to the actual situation of the community (village), with the centralized survey supplemented by a household survey. 90 investigators from primary healthcare institutions have undergone unified training concerning the study background, questionnaire contents and social survey techniques. Face-to-face interviews were conducted to collect the data using the electronic questionnaire. Participants should fill in the items of the questionnaire one by one guided by the investigators.
A total of 6022 participants were initially chosen and interviewed. However, 204 of them were excluded from the sample due to obvious logical errors were found in their questionnaire. A total of 5818 individuals were eventually included in the database.

Ethical approval
This study was approved by the Research Ethics Committee of the Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China (No: S027), and performed in accordance with the principles of the Declaration of Helsinki. All participants had signed consent form before filling out the questionnaire, and personal information of all participants was kept confidential.

Quality control
Three main quality control measures were adopted to guarantee the survey quality. Firstly, to prevent repeated submissions of questionnaires from the same participant, each mobile phone number could only answer once through web cookies. Secondly, we distributed four quality control questions at different places in the questionnaire to identify the participants who filled the questionnaire at random, including "Where is the capital of China?," "What is 5 plus 3?," "What is 1 plus 2?" and "What is 9 minus 7?." Each question had 4 alternative answers, of which only one was correct. The electronic questionnaire system would mark the questionnaire as invalid if incorrect quality control questions presented. Thirdly, participants whose questionnaires were completed in less than twelve minutes would be marked as invalid by the electronic questionnaire system.

Instrument and measurement
The content of the questionnaire included the demographic characteristics, lifestyles, depressive symptoms and health situations. Specifically, the demographic characteristics of participants included age (years, continuous), gender (male, female), BMI (kg/m 2 , continuous), region (urban, rural), education (elementary school and below, junior high school, senior high school/professional high school, junior college and above), marital status (unmarried/widowed/divorced/ separated, married), occupation (manual workers, mental workers, retired, unemployed/other workers), monthly per capita household income (CNY) (0-6000, 6001-12000,12001 and above).

Social jetlag
Participants were asked to report bedtime and wake-up time during the weekdays (Sunday -Thursday nights and Monday -Friday mornings, respectively) and bedtime and wake-up times during the weekends (Friday -Saturday nights and Saturday -Sunday mornings, respectively). Bedtime and wake-up time on weekdays were assessed with the question: "At what times (hours: minutes) do you usually go to bed and wake up on weekdays?" Bedtime and wake-up time on weekends were assessed with the question: "At what times (hours: minutes) do you usually go to bed and wake up on weekends?" Social jetlag ( (For example, if you go to bed at 0:00 and wake up at 7:00, the sleep midpoint is 3:30.) According to social jetlag, we classified participants into three groups, social jetlag <1 hour, 1-2 hours, and ≥2 hours, and the reference category was <1 hour, as was done in previous studies (Liang et al. 2022;Zerón-Rugerio et al. 2019;Zhang et al. 2019).

Depressive symptoms
Depressive symptoms was measured by the Patient Health Questionnaire (PHQ-9) (Wang et al. 2014), a self-reporting instrument with nine questions. The PHQ-9 was carried out based on a 4-point Likert scale, ranging from 0 (not at all), 1 (several days), 2 (more than half the days), and 3(nearly every day) with a total score ranging from 0-27. The scores <5 represent no depressive symptoms and the scores ≥5 represent depressive symptoms. The reliability of the scale was fairly strong as well (Cronbach's α = 0.912).

Statistical analysis
Descriptive analyses included means and standard deviations (SD) for continuous variables and percentages for categorical variables in this study. χ2 tests were conducted to compare the distribution of categorical variables among social jetlag groups and one-way ANOVA was conducted to compare the continuous variables among social jetlag groups. Multivariable logistic regression analyses were performed to test the association between social jetlag and the risk of depressive symptoms. Moreover, the multiplicative interaction model was applied to analyze the interaction of social jetlag with chronic diseases on depressive symptoms. We also conducted restricted cubic splines regression with three knots at the 10th, 50th, and 90th percentiles to flexibly model the association between social jetlag and the incidence of depressive symptoms (Liang et al. 2022). Significance level was accepted as P < 0.05 (twotailed) for all tests.
The SPSS version 23.0 was employed in the descriptive analyses. The Stata version 14.0 was employed in the multivariable logistic regression and the multiplicative interaction model analyses. The R version 4.0.2 was employed in the restricted cubic splines regression.

Participants' characteristics for demographic variables, lifestyles, and health situations
The main characteristics of the participants are reported in Table 1. The mean age of the total 5818 participants was 45.98 (SD = 16.84) years. Among them, 3127 (53.75%) were female, more than 60% were urban residents. 39.50% of participants' education level were junior college and above, more than 40% were manual workers, and nearly 70% of them with monthly per capita household income were 0-6000 CNY. Moreover, 84.53% of the participants reported that they have nap habits. Overall, 17.02% of them reported depressive symptoms, 33.76% of them reported have at least 1 chronic disease. The top five chronic diseases were hypertension, diabetes, intervertebral disc disease, chronic gastroenteritis ulcers, rheumatoid arthritis and gout. And the mean social jetlag of the total 5818 participants was 31.84 (SD = 37.76) minutes. Participants who had 1-2 hours social jetlag were younger, living in rural (73.39%), with junior college and above education level (59.77%), married (68.01%), mental workers (42.92%). And more likely never smoking (80.82%), never drinking (85.93%), to regularly eat three meals a day (86.74%) and to nap (87.19%).

Prevalence rate of social jetlag and its association with demographic characteristics, lifestyles, and health situations
219 participants had ≥2 hours of social jetlag. Among them, nearly 70% were rural residents. 56.16% of participants' education level were junior college and above, more than 55% were married, and 65.75 % of them with monthly per capita household income were 0-6000 CNY. Moreover, 74.43% of the participants reported that they never smoking and 78.54% never drinking. Table 3 represented the association between social jetlag and depressive symptoms. Compared with <1 hour of social jetlag, after demographic characteristics adjustment, participants who reported having 1-2 hours social jetlag showed no significant risk of depressive symptoms (adjusted OR = 1.02,95%CI:0.85-1.24, P = 0.812),

The interaction of social jetlag and gender on depressive symptoms
As shown in Table 4, with the increase of social jetlag, female more likely to report a higher risk of depression compared to male (P < 0.05). Table 5 showed the interaction of social jetlag and chronic diseases on depressive symptoms among Chinese adults. After adjusting for demographic characteristics, lifestyles, and health situations (except for chronic diseases and the number of chronic diseases), compared with <1 hour of social jetlag and no chronic disease, participants having chronic disease were more likely to report depressive symptoms (OR = 3.22,95%CI = 2.59-4.00, P < 0.001). Moreover, among those who have chronic disease, the greater the social jetlag they reported, the greater risk of depressive symptoms they have (OR = 3.81,95%CI = 2.77-5.33, P < 0.001;OR = 4.08, 95%CI = 2.24-7.43, P < 0.001). And participants who suffered more chronic diseases were more likely to report depressive symptoms (P < 0.05). Meanwhile, we analyzed the interaction of social jetlag and the top five chronic diseases on depressive symptoms, but the results showed no statistically significant (all P ≥ 0.05). (The interaction of social jetlag and the top five chronic diseases on depressive symptoms for this study was shown in Table S1 in the Online Supplementary Document.)

The linear relationship between social jetlag and depressive symptoms
In Figure 2, we used the restricted cubic splines to flexibly model and visualize the relationship between social jetlag with depressive symptoms among adults in China. Social jetlag (as a continuous variable) was linearly associated with an increased risk of depressive symptoms.

Discussion
In this cross-sectional study, we found that the prevalence of social jetlag (≥1 hour) was 22.95% among Chinese adults. Greater social jetlag was significantly associated with an increased risk of depressive symptoms, and the association appeared to be linear. In addition, among those who have chronic disease, the greater the social jetlag they reported, the greater risk of depressive symptoms they have.
We found that 22.95% of Chinese adults suffered from 1 hour or more of social jetlag, which was smaller than 36.5% reported in the Japanese working population Mental workers, as opposed to "manual workers," refer to workers who mainly consume brain power, including state and social administrators, office workers, and personnel and professional and technical personnel.  Continued) and much smaller than the 69% reported in the European population (Islam et al. 2020;Roenneberg et al. 2012). The main reason for the result was that, the essence of social jetlag is to compensate for the insufficient sleep on weekdays by delaying waking up on weekends (Im et al. 2017;Takaesu et al. 2021;Zerón-Rugerio et al. 2019). Generally, Chinese people have a 2 hours break at noon to eat lunch and then take a nap to refresh their energy for the activities in the afternoon, a habit they continue even after retirement (Li et al. 2017;Zhang et al. 2019). This may have buffered the lack of sleep on weekdays to some extent.   The results showed that adults who reported more than 2 hours of social jetlag showed a higher risk of depressive symptoms than those who reported fewer hours of social jetlag, which is consistent with Levandowski's findings (Levandovski et al. 2011). This could be explained by the circadian clock. Polymorphisms in the circadian clock genes of NPAS2 and CLOCK are associated with circadian rhythms (Bellivier et al. 2015;Kim et al. 2015;Partonen 2012). The interaction of those circadian clock genes may disrupt circadian rhythms, which could predispose individuals to a wide range of mood disorders, including depression (Allen et al. 2022;Barandas et al. 2015;Robillard et al. 2018). Thus, social jetlag, a type of circadian rhythm disorder, may cause depressive symptoms. There are also a few studies that have not found  The solid lines indicate multivariate-adjusted odds ratios and the shaded areas indicate the 95% CIs. The reference point is 0 minutes for social jetlag, with knots located at the 10th, 50th, and 90th percentiles. The model was adjusted for demographic characteristics, lifestyles, and health situations.
a correlation between social jetlag and depressive symptoms (Chang and Jang 2019;De Souza and Hidalgo 2014;Knapen et al. 2018). The inconsistency of the above findings may be related to differences in the study population, methodology, and individual factors (e.g., age, gender, and gene). It is therefore important that more studies are performed to further analyze the role of social jetlag in depressive symptoms. Although all of the above studies (Chang and Jang 2019;De Souza and Hidalgo 2014;Knapen et al. 2018;Levandovski et al. 2011) have explored the correlation between social jetlag and the prevalence of depressive symptoms, none of these studies have adjusted for potentially important confounding factors such as demographic factors, lifestyles, and health situations. This study indicated that the association between social jetlag and depressive symptoms was robust after adjustment for a wide range of potential confounding factors. The restricted cubic splines regression revealed that social jetlag was linearly associated with risk of depressive symptoms, with positive association. This finding was similar to Islam's study concluded that the association between social jetlag and depressive symptoms appeared to be linear (Islam et al. 2020). Therefore, motivating adults to build and maintain a regular sleepwake cycle both on weekdays and on the weekends is important for alleviating depressive symptoms.
Our study revealed that the interaction of social jetlag and chronic diseases on depressive symptoms among adults. Among those who have chronic disease, the greater the social jetlag they reported, the greater risk of depressive symptoms they have. Individuals who reported have chronic disease need long-term drug treatment, and with the growth of age and course, individuals with chronic disease often have different accompanying diseases and even complications, which will bring many problems in physiology, such as sleep quality (Yi et al. 2021). However, sleep problems could lead to various psychological disorders, including depressive symptoms (Zhao et al. 2019). Meanwhile, social jetlag has harmful effects on many of the body's physiological systems, including the cardiovascular, endocrine and central nervous systems (Islam et al. 2018(Islam et al. , 2020Kim et al. 2020). People with chronic disease may be more sensitive to the effects of social jetlag due to their physical condition, which increases the risk of depressive symptoms. In addition, social jetlag may increase the risk of the stress response and the psychological and physical burden among people with chronic disease, thus increasing the risk of depressive symptoms of them.
The strength of this study was that this is the first study to investigate the association between social jetlag and depressive symptoms among adults in China. Therefore, this study may supply more epidemiological evidences to evaluate the relevance between social jetlag and depressive symptoms, which was based on the population in Asia. Secondly, this is the first time to analyzed the interactive effect of social jetlag and chronic diseases on depressive symptoms. Some limitations should also be acknowledged. Firstly, social jetlag and depressive symptoms were collected based on selfreported measures, which might be a potential source of information bias. Secondly, the study was designed as cross-sectional, the results only suggested the observational correlation rather than the exact causality between social jetlag and depressive symptoms, which requires further verification by prospective studies. Thirdly, this study only questions bedtimes and wake times on weekdays and on weekends and did not take into account the latency of sleep and sleep inertia, which might lead to an underestimation of social jetlag. Fourthly, due to limitations in the design of the questionnaires, we did not collect the Chronotype of the participates, which may affect the accuracy of the results. In future researches, we will focus on the effect of the Chronotype on social jetlag. Lastly, the participants in this study were limited to one city, thus requiring the cautious extension of the study findings to regions with different economies, cultures, and geographic locations.
In conclusion, our results indicate that a higher social jetlag was associated with an increased risk of depressive symptoms among Chinese adults. Meanwhile, when the social jetlag was≥1 hour, adults with chronic diseases were more likely to report depressive symptoms. These relationships should be further investigated in longitudinal studies to reveal if the reduction of social jetlag should be part of prevention strategies.

Data availability statement
The data supporting the findings of this study are available from the corresponding author on reasonable request. The data are not publicly available due to privacy and ethical restrictions.