Associations between Nonrestorative Sleep, Perceived Stress, Resilience, and Emotional Distress in Freshmen Students: A Latent Profile Analysis and Moderated Mediation Model

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Introduction
Research has shown that approximately 30% of college students sufer from symptoms of insomnia [1]. Since the outbreak of the COVID-19 pandemic, sleep problems such as insomnia, insufcient sleep, and unrefreshing sleep have increased among college students [2]. In recent years, nonrestorative sleep (NRS) has attracted increasing attention of researchers from multiple disciplines. NRS is a core symptom of insomnia [3] and is characterized by waking up unrefreshed and restless following a normal sleep duration [4]. Studies have found that individuals with NRS are more likely to develop depression [5] and other psychological disorders [6] compared to those without NRS. Terefore, it is crucial to explore the association between emotional distress and NRS, considering that anxiety and depression appear to worsen with NRS deterioration and may cause further harm to both physical and psychological health.
Emotional distress has been prevalent among college students during the COVID-19 pandemic [7], and 54.4% and 28.4% of college freshmen reportedly sufer from anxiety and depression, respectively [8,9]. Negative emotions have aggravated due to specifcity during pandemic of COVID-19. Freshmen students even response extremely nervous to anything written about COVID-19 [10]. Additionally, third year students are reported to have higher levels of life satisfaction, which is infuenced by year level [11]. In other words, anxiety and depression may spread more among freshmen students. High academic pressure and maladaptation since the beginning of colleges are risk factors for emotional distress which can cause serious daily impairment [12]. Additionally, NRS is a strong indicator of depression [13,14]. A large body of research confrms that the relationship between insomnia symptoms and emotional distress is bidirectional and complex [15,16]. Clinical studies have shown that emotional distress is easier to detect in those with insomnia symptoms [17]. However, little is known about the association between NRS and emotional distress in freshmen students.
Perceived stress is the subjective evaluation of exterior or interior stress [18], which can afect an individual's emotional state by adjusting cognitive function [19]. Previous studies have confrmed that perceived stress has a signifcant efect on emotional distress [20]. According to a transactional model of stress [21], perceived stress among students is afected by personal resources such as joy and selfefcacy, transforming perceived stress into tension and worries, which eventually cause emotional distress. In other words, higher perceived stress is positively related to higher emotional distress. Academic stress and excessive homework have disrupted the sleep patterns of freshmen worldwide: they go to bed late and get up early, which raises the risk of sleep defciency [22]. A study found that lower sleep quality was related to higher perceived stress [23]. Additionally, the degree of perceived stress varies across gender and living conditions [24]. However, the role of perceived stress between NRS and emotional distress in freshmen students remains unknown.
Te efect of NRS on emotional distress is still unclear, and resilience may play a moderating role in this association. According to resilience theory [18], the higher the dynamic adaptability in the face of stress or adversity [25], the better the mental state after resilience restructuring [26,27]. Low resilience has been associated with increased perceived stress [28] and insomnia symptoms [16,29]. Furthermore, previous study manifested a strong link between resilience and anxiety symptoms, and the lower the levels of resilience are, the more negative emotions they experienced [30]. Tus, it is reasonable to postulate that resilience may play an important role in the associations between NRS, perceived stress, and emotional distress.
Existing literature shows that NRS, perceived stress, and resilience have a signifcant infuence on emotional distress. However, the mechanisms of their interactions are to be elucidated; therefore, this study aims to explore further the mediating role of perceived stress and the moderating role of resilience in the associations between NRS and emotional distress.
To this end, we put forward the following hypotheses (Figure 1(a)): (1) NRS is signifcantly associated with perceived stress, emotional distress, and resilience among college freshmen. (2) Several distinct perceived stress patterns can be identifed by latent profle analysis (LPA). (3) Perceived stress may play a signifcant mediating role between NRS and emotional distress.
(4) Resilience may play a signifcant moderating role among NRS, perceived stress, and emotional distress.

Design and Participants.
Te study had a cross-sectional design and included 851 freshmen recruited using simple random sampling from two universities in June 2022. Te inclusion criteria were as follows: (1) newly enrolled freshmen students in 2021; (2) willing to participate in this study; and (3) could communicate fuently in Chinese. Te exclusion criterion included college freshmen with mental illness. Te participants flled out paper questionnaires which took 15-20 minutes and were collected and sorted by the investigator. After excluding 33 questionnaires with missing data, a fnal sample of 818 freshmen (response rate: 96%) was analyzed. All participants provided informed consent before flling in the questionnaire. Trained research assistants were available to address participants' instrumentrelated problems.

Sample
Size. Te minimum sample size was 274, considering a confdence interval of 95%, a standard deviation of 15, an allowable error of 2.0, and a 20% sample attrition rate [31]. Tus, the fnal sample of 818 was appropriate for analysis.

Ethical Approval.
Tis study is part of the Be Resilient to Nursing Career program (BRNC, registration number: NFYKDX002). More details about BRNC are available from previous studies [32][33][34][35][36][37]. Informed consent was obtained from the participants before flling in the questionnaire. Te personal information of the respondents was kept strictly confdential.  [44]. Te scale consists of 10 items, and the score ranges from 0 to 40, with a higher score indicating a higher level of resilience. Te scale has been used extensively among college students with outstanding reliability and validity [34]. Cronbach's alpha in this study was 0.920. . PSS-10 is the most widely used tool for measuring stress, which was developed by Cohen and colleagues [45], and its Chinese version was validated by Zhen et al. [46] (GFI � 0.925, NFI � 0.914, CFI � 0.965, RMR � 0.026, and RMSEA � 0.043). Te scale has 10 items divided into two domains: negative feelings (items 1, 2, 3, 6, 9, and 10) and positive feelings (items 4, 5, 7, and 8) [46]. Te score ranges from 0 to 40, with a higher score indicating higher perceived stress. Cronbach's alpha in this study was 0.815.

Data
Analysis. Descriptive statistics were described as mean (SD) and proportion (%). Data distribution was checked before logistic regression, and univariate analysis was employed to explore the potential factors infuencing emotional distress. Pearson's analysis assessed the associations among NRS, resilience, perceived stress, and emotional distress. Te strength of the relationship was divided as follows: weak (|r| < 0.3); moderate (0.3 ≤ |r| < 0.5); and strong (|r| ≥ 0.5) [50,51]. LPA was conducted to explore potential profles of perceived stress. We estimated models based on log-likelihood H0 value (Log), Akaike information criterion (AIC), Bayesian information criterion (BIC), sample-size adjusted Bayesian information criterion (aBIC), bootstrapped likelihood ratio test (BLRT), and entropy [52]. In addition, theory-based reasons for LPA-related profles were also taken into consideration [52]. Univariate and multivariate logistic regressions were applied to identify Perspectives in Psychiatric Care potential indicators for diferent LPA-based profles. Bayesian independent samples t-test was performed to compare emotional distress among freshmen based on LPA profles (alternative hypothesis H1: diferences exist between groups; null hypothesis H0: no diferences between groups). Harman's one-factor model was constructed to estimate the potential existence of the common method bias [53]. A mediation analysis was performed to examine the mediating role of perceived stress between NRS and emotional distress.

Demographic Characteristics.
In total, 27.6% of freshmen students were male and 49.5% came from cities. Signifcant diferences were identifed between emotional distress based on gender (P < 0.001) and whether parents live separately (P < 0.05). Detailed information is provided in Figures 1(a) and 1(b).
Pearson correlation heatmap and other details are demonstrated in Figure 2.

LPA of Perceived
Stress. LPA models with 1 to 5 classes were analyzed to fnd the optimal model of perceived stress (see Figure 3(a)). A 3-class model was chosen in consideration of the following reasons: (1) sample size was more than 10%; (2) ftting index including AIC, BIC, and aBIC was relatively small; (3) Lo-Mendell-Rubin likelihood ratio test (LMR) and BLTR were signifcant (P < 0.05); and (4) variability existed between diferent profles. Details about LPAbased profles are presented in Figure 3(b). Te three profles were named as low ability-middle stress (10.5%, class 3), middle ability-high stress (65%, class 2), and high ability-low stress (24.5%, class 1). Signifcant diferences were identifed in emotional distress among LPA-based subgroups. Univariate logistic regression indicated that only gender was a signifcant indicator to profle types. Other details are described in Figure 3

Mediation Role of Perceived Stress Based on LPA (Perceived Stress as Category Variable).
Harman's one-factor model indicated that the frst factor accounted for 31.2% of total variances and the common method bias could be ignored in this study. All potential confounding variables were controlled in advance. Taking high ability-low stress (class 1) as the reference, middle ability-high stress (class 2) and low ability-middle stress (class 3) subgroups were

Discussion
In the current study, there existed heterogeneity in freshmen students' perceived stress. Perceived stress played a signifcant mediating role between NRS and emotional distress, while resilience could not signifcantly moderate the associations among NRS, perceived stress, and emotional distress. Although NRS has often been included in insomnia research, NRS does not necessarily mean insomnia [3]. Additionally, NRS has been relatively less explored in emotional distress-related studies based on a student sample [6,54,55]. Te current study provides some insights for early identifcation and intervention.
First, in the current study, females had more anxiety and depression symptoms than males, which was consistent with previous research [56,57] and could be attributed to the gender-based diferences in sensitivity and self-cognition [58]. In addition, parental confict resulting in separation is believed to put psychological burden on children, which was also confrmed in the current study [59].
Second, consistent with our frst hypothesis, NRS was positively associated with emotional distress, which was also confrmed in a previous study [60]. In a two-year longitudinal study, NRS was found to have the ability to predict long-term depression [61]. Robust evidence has indicated that NRS causes disturbance in the circadian rhythm of cortisol levels, which can change the 24-hour sleep-wake cycle [62], and is related to the etiology of emotional distress [63], suggesting that interventions for emotional distress should focus on NRS. Psychotherapy can improve NRS [64]. Additionally, universities should organize orientations to help students adjust to the transition from high school to college.
Tird, congruent with our second hypothesis, heterogeneity of perceived stress was verifed in freshmen students and LPA recognized three profles: high ability-low stress,  Perspectives in Psychiatric Care middle ability-high stress, and low ability-middle stress. According to the transactional model of stress and coping, primary and secondary appraisals occur after exposure to stress, and this cognitive process is moderated by perceived stress [65]. High perceived stress can lead to anxiety-depression-oriented cognition and behavioral outcomes [66]. Tus, more attention should be given to the middle abilityhigh stress group. In addition, females were more likely to be classifed into high perceived stress subgroups. As their social status and family role are complex relative to males, females seek more external support and are considered more emotional and sentimental [67,68], with diferent stressors contributing to higher perceived stress which is consistent with previous research [69,70]. From a physiological perspective, the gender-based diference in perceived stress might be attributed to the stronger high-sensitivity C-    Perspectives in Psychiatric Care reactive protein (CRP) regulation in males than in females [71]. Gender diferences call for parallel but distinct developmental schemes for female and male students [72]. Hence, teachers need to transform their communication and teaching styles to suit the needs of male and female students.
For females under stress, psychological counseling or activities can promote socialization and release of stress. Fourth, consistent with our third hypothesis, the mediating role of perceived stress (as both a continuous variable and category variable) was confrmed between NRS and emotional distress. Tus, the new pathway of NRS-perceived stress-emotional distress was established in the current study, which had been less explored in previous research. Tis pathway shows that individuals with diferent perceived stress profles will respond diferently to the same level of NRS, resulting in varied levels of anxiety or depression [73]. Relevant research has attributed this fnding to the corticotropin-releasing hormone (CRH). High perceived stress will stimulate chronic hyperactivity of the stress system and release CRH antagonists, causing human pathologic states, such as depression and chronic anxiety [74]. Te mediation role of perceived stress also provides insights for developing interventions for NRS-induced anxiety and depression.
Fifth, contrary to our fourth hypothesis, resilience did not signifcantly moderate NRS, perceived stress, and emotional distress. Tis phenomenon might be attributed to the inclusion of LPA-based category variables in the moderation analysis, which was less explored in previous research. Additionally, the current moderating efect was  based on LPA, and the sample distribution of each subgroup was uneven, resulting in biased results. Too few samples under the profles may also lead to an insignifcant moderation efect. Also, the excessive infuence of other factors on the dependent variable may afect the moderating efect.
In the current study, heterogeneity of perceived stress resulted in three distinct profles, and the results could be theoretically explained by the diference among diferent subgroups. In addition, the gender variable in this study was not balanced, resulting in potential selection bias. Tese fndings should be replicated in males and females separately with a large sample.
In summary, this study demonstrated a better understanding of associations among NRS, perceived stress, emotional distress, and resilience based on the sample of Chinese college freshmen students. Te fndings showed that (1) there was heterogeneity in college freshmen's perceived stress; (2) the association between NRS and emotional distress was signifcantly mediated by perceived stress; and (3) resilience did not play a signifcant moderating role among NRS, perceived stress, and emotional distress.

Limitations
Some limitations of the current study should be considered. First, considering the cross-sectional design, the fndings derived from the present study should be further validated in intervention studies. In future research, we will provide insights into the mechanisms infuencing emotional distress in Be Resilient to Nursing Career program (BRNC) and, at the same time, expand the scope of our study population to facilitate generalization of the fndings. Second, the sample is recruited from two universities in Guangzhou, resulting in potential selection bias, and these fndings might not be generalized to individuals with diferent races or cultural backgrounds. Tus, the results should be explained with caution. Tird, the gender variable is not balanced in the current study, and these fndings are recommended to be replicated in males and females separately, which could not be achieved here due to the small sample size of males. Te current study found signifcant diferences in emotional distress by gender. Terefore, gender-specifc psychological intervention strategies can be recommended for college counseling units. To avoid the interaction of negative emotions among individuals, a group mindfulness dialogue format can be recommended to regulate individual emotional states.

Conclusion
Heterogeneity exists in perceived stress among freshmen students. Perceived stress plays a signifcant mediating role between NRS and emotional distress while resilience cannot signifcantly moderate the associations among NRS, perceived stress, and emotional distress.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon reasonable request.  Figure 6: Te moderating efect of resilience among NRSS, perceived stress, and emotional distress. 8 Perspectives in Psychiatric Care which was registered online prior to recruitment at https:// www.chictr.org.cn/index.aspx (ChiCTR2000038693).

Conflicts of Interest
Te authors declare that they have no conficts of interest.