3.1 Design, setting and participants

Our study was a cross-sectional survey. Participants were recruited from September 2020 to December 2021 by using convenience sampling taken from the vascular surgery department of a tertiary hospital in Nanchang, China. Patients were considered eligible if they not only were diagnosed with ASO during colour Doppler flow imaging (CDFI) or CT angiography (CTA) and were aged ≥45 years, but also if they were able to understand and complete the questionnaires. The elimination criteria were patients who clearly did not carefully or correctly answer the questionnaire. Based on an N:q ratio of 10:1, the target sample size was calculated as 150 patients with ASO. A total of 181 patients were included in this study. However, nine patients did not correctly answer the questionnaire. Therefore, this study ultimately included 172 patients with ASO (Figure 1).

3.2 Data collection

The researcher conducted the investigation into the hospital through face-to-face interaction. Before distributing the questionnaire, we informed the patients that it was anonymous, and informed them of the research purpose as well as the directions for filling it in. The patients filled out and returned the questionnaires on the spot, and informed consent was obtained from all participants. As some elderly people in this survey had a low education level and decreased eyesight, the researcher combined the data collection with the interview survey method and helped them to fill in the questionnaire. A total of 181 questionnaires were distributed and 172 were received with an effective recovery rate of 95.03%.

3.3 Measurements

3.4 Data analysis

SPSS 25.0 was used to conduct statistical analysis. Descriptive statistics, which include frequencies, percentages (%), mean and standard deviation (SD), were used for the demographic characteristics of ASO patients and the scores of each scale. Two independent samples t tests or one-way ANOVA were used to compare the sleep quality of patients with ASO to identify differences in demographic characteristics, and statistically significant variables were included in linear regression analysis. Pearson′s correlation coefficient was used to analyse the correlations between sleep quality, PSS and PF. A simple mediational model (Model 4) with 5,000 bootstraps using PROCESS of SPSS 25.0 was implemented to test the mediating effect. Statistical significance was inferred when the 95% confidence interval (CI) did not include 0. In addition, p < .05 for all tests was considered statistically significant.

3.5 Ethical considerations

The study was approved by the ethics committee of the hospital (NO. 202060).

4.1 Description of participants

Table 1 shows the demographic characteristics of the participants. The mean age of patients was 71.99 ± 5.96 years, and 75.6% were males. Half of the patients were a normal weight. A total of 22 patients (12.8) were college graduates. There were 108 patients (62.8%) who earned 2,500–3,500 RMB per month, and 101 patients (58.7%) lived in villages. Approximately 64.5% of patients were married, while 47 patients (27.3%) lived alone. There were 60 patients (35.0%) who were in mild pain. The number of patients diagnosed with hypertension and diabetes were 111 (64.5%) and 110 (64.0%), respectively. There were 65 patients (37.9%) who were current smokers.

4.2 Comparative analysis of sleep quality among ASO patients with different characteristics

The PSQI mean scores of all patients was 12.12 ± 3.01. There were statistically significant differences in age, per capita monthly income, residence, marital status, living situation and pain level (Table 1).

4.3 Correlation analysis of PIPS, PSQI and PSSS

Patients' sleep quality was positively correlated with PF (r = .553, p < .01). However, it was negatively correlated with PSS (r = −.577, p < .01) (Table 2).

4.4 Mediating effect analysis

Mediating effects were tested for the relationship between PF in PSS and sleep quality, controlling for age, monthly per capita income, residence, marital status and living situation. The results showed that PSS had a significant effect on sleep quality (β = −.211, p < .001). The direct effect of PSS on sleep quality remained significant after the mediation of PF was included (β = −.147, p < .001). PSS had a significant negative impact on PF (β = −.376, p < .001), and PF had a significant positive impact on sleep quality (β = .169 p < .001), as shown in Table 3 and Figure 2. The upper and lower limits of bootstraps 95% CI of the mediating effect of PF did not include 0, indicating that PSS could not only directly predict sleep quality but also indirectly predict sleep quality through the mediating effect of PF. The mediating effect (β = −.064) accounted for 30.33% of the total effect (β = −.211), as shown in Table 4.

5.1 Sleep quality of ASO patients

This study showed that the average sleep quality score of ASO patients was 12.12 ± 3.01, which was worse than that of other patients (Alhalal et al., 2021). There were 35% of patients with mild pain, but their sleep quality score was 11.43 ± 3.10, which still indicated sleep disturbance. This study (Haack et al., 2020) shows that pain is a significant influencing factor of sleep quality, chronic pain is highly comorbid with sleep disturbance, and there is a well-established bidirectional relationship between sleep disturbance and pain. This may be related to the nocturnal resting pain symptom of ASO, which causes patients to experience disrupted sleep at night, making it difficult for them to fall asleep again. Meanwhile, sleep deprivation lowers pain thresholds, decreases cognitive ability to cope with pain and increases pain perception, which exacerbates poor sleep (Duan et al., 2021). Therefore, these patients' sleep quality is poor in comparison with other patients. This study also found that women had poorer sleep quality than men. However, there was no significant difference in overall sleep quality between men and women, which was consistent with Tang (Tang et al., 2017) and Keilani (Keilani et al., 2018). We also reached an interesting conclusion, which was that hypertension had no significant effect on sleep quality in this study, which was inconsistent with Yang (Yang et al., 2021). This may be related to the pathogeny of ASO. In addition, as hypertension is one of the important causes of atherosclerosis, as a risk factor for this disease, 64.5% of patients had hypertension, which did not demonstrate a significant difference.

5.2 Psychological flexibility and perceived social support are strongly associated with sleep quality

The results of this study showed that when ASO patients have lower levels of PSS, they tend to have worse sleep quality. The two factors are negatively correlated (r = −.577, p < .01), which is consistent with previous studies (Hao et al., 2021; Seol et al., 2021). The reason for this if patients receive less understanding and support from their society, family and friends, they are more likely to deal with this disease negatively, which reduces their confidence in disease recovery (Fivecoat et al., 2018; Liu et al., 2021) It also amplifies their negative emotions during the disease progression, which reduces their sleep quality (Grey et al., 2020). The results of this study indicate that the sleep quality of ASO patients was usually worse when their PF was lower, and these two factors were positively correlated (r = .553, p < .01), which is consistent with the results of previous studies (Daly-Eichenhardt et al., 2016; Kato, 2020). Avoidance behaviour is a natural human reaction, but excessive avoidance leads to psychological rigidity (Dawson & Golijani-Moghaddam, 2020). Patients with low PF are sensitive to insomnia, pay close attention to their sleep and try to eliminate it. However, when individuals do this more deliberately by focusing on sleep while aggravating their psychological inflexibility, they are more likely to use pessimistic and avoidance coping methods, which aggravates their sleep problems and forms a vicious cycle. High PSS and PF are closely related (r = −.405, p < .01). The physiological function and adaptability of ASO patients decreased with their increased age, and the support from their surrounding care was less sensitive, which decreased their PSS level. Furthermore, their cognitive functions weakened, and their comprehension ability gradually declined, causing them to easily become depressed and worried about their disease prognosis, which aggravated their psychological inflexibility (Shi et al., 2016). If patients can receive enough material and emotional support from society, family and friends, they can maintain a good healthy mindset (Fu et al., 2022). This suggests that medical staff can establish a social support path that involves acquiring both inpatient and outpatient core knowledge, strengthening inpatient ASO-related health education and treatment knowledge, and reducing fear resulting from diseases. Strengthening psychological counselling for ASO patients can relieve their disease stress by providing them with relaxation training and other methods. Other approaches include urging family members, relatives and friends to give prompt emotional and material support to patients, improve patients' confidence in their disease recovery, maintain their healthy psychological state and improve their sleep quality.

5.3 Psychological flexibility partially mediated the relationship between perceived social support and sleep quality

The mediating effect analysis showed that PSS negatively predicted sleep quality (β = −.211, p < .001), PF partially mediated the relationship between PSS and sleep quality (β = −.064, p < .001) and the mediating effect accounted for 30.33%. It also showed that PSS can not only directly and negatively predict sleep quality of ASO patients but can also indirectly and negatively predict sleep quality through PF. Disease is a negative life event, and some patients find it difficult to accept. A higher perceived vulnerability to disease leads to avoidant behaviour and social withdrawal, which results in lower psychological well-being (Mallett et al., 2021). At the same time, ASO impairs self-care ability and also needs family care and psychological support. However, unaccompanied care or family′s anxiety about causing inconvenience and creating economic pressure for their children leads to severe cognitive distortions in patients (Fu et al., 2022). This affects their social interactions and working conditions and also reduces their degree of social support and increases their psychological burden, which ultimately reduces their sleep quality. On the contrary, social support is an important external resource for maintaining an individual′s physical and mental health and can reduce the negative effects of stressful events (Zheng et al., 2021). When patients receive social support such as material security, emotional support and psychological assistance from spouse, children, friends and relatives (Xie et al., 2020), which can increase their sense of security and confidence in restoring their health, thus improving their sleep quality. With the dramatic increase in the number of empty nesters, mental health problems caused by the lack of spouse′s or children′s companionship for the elderly are frequent, so patients with low social support are more likely to have negative emotions.(Tang et al., 2022). These cause the patient to obsess about the fear created due to the disease, making them psychologically inflexible. Their subsequent mental stress creates high levels of activity in the hypothalamic–pituitary–adrenal (HPA) axis, which tends to impede sleep onset and maintenance (Peltz et al., 2020). Therefore, reasonable and effective measures must be taken to continuously improve PSS and reduce PF to improve the sleep quality of ASO patients. In terms of PSS, the hospital can create a healthy environment for patients, patients can communicate with and encourage each other, family and friends can strengthen their emotional care and material help for these patients, and they can constantly improve the level of PSS for patients. Based on the premise that physical conditions allow, patients should actively participate in outdoor activities. These can not only enable them conducive to recover their physical functions but can also aid them with actively integrating into society, maintaining a positive attitude, reducing their negative emotions and improving their psychological inflexibility, which can improve their sleep quality.