Ankylosing Spondylitis is a chronic inflammatory disease that has a negative effect on the quality of life [21]. The findings of previous studies indicate the existence of disorders in the quality, duration, and efficiency of sleep, difficulties in falling asleep, the presence of restless leg syndrome, and obstructive sleep apnea among AS patients [37, 38, 46,47,48,49].

This systematic review and meta-analysis were conducted to pool the prevalence of sleep disorders in patients with AS. As a result, 18 studies with a sample size of 5,840 people were evaluated, and their reported results were analyzed. Accordingly, the pooled prevalence of sleep disorders among AS patients is 53%. Moreover, considering our meta-regression analyses, it was found that with the increase in ‘sample size’ and ‘year of publication’, the prevalence of sleep disorders significantly decreased.

In a systematic review by Leverment et al., poor sleep was reported in 35–90% of patients with axial spondyloarthritis [50]. Jiang et al. reported the prevalence of sleep disorders among AS patients as 31% [16]. In 4 other studies conducted in China, the prevalence of sleep disorders was reported from 35.4% in the study of Li et al. up to 67.6% in Nie et al. analysis. [20, 36, 38, 40]. The lowest prevalence (19.2%) among the retained studies was related to the work of Tymms et al. in Australia [10]. The differences in reported results of existing literature can be due to the variations in the tools adopted for assessing sleep disorders, alternative approaches to defining the cut-off point of the tools used, and the working methods within the studies.

As highlighted earlier, the findings show that the prevalence of sleep disorders decreases with the increase of ‘sample size’. Among the included studies, the highest prevalence belonged to the two studies of Abdulaziez and Asaad and Yüksel et al., with sample sizes of 20 people, 90% and 80%, respectively [19, 42].

In the study of Aydin et al., the scores of 5 out of the seven subscales of PSQI, i.e., subjective sleep quality, sleep duration, habitual sleep efficiency, sleep disturbances, and daytime dysfunctions in patients, were significantly higher than in healthy individuals. However, the scores of the other two subscales, sleep latency and the use of sleeping pills, were insignificant between the two groups. In fact, in the study, the main concern was related to the continuation of sleep and not the sleep onset; this may be because the inflammatory back pain intensifies in the second half of the sleep duration [46]. Inflammatory back pain and axial pain are among the main causes of sleep disorders in patients with AS [21]. In the study of Da Costa et al., 88% of patients with spondyloarthropathy had difficulty staying asleep [51].

In a report by Li et al., AS patients had higher scores than the control group in the subscales of subjective sleep quality, sleep latency, sleep efficiency, sleep disorders, and medication use. However, the difference between the two groups was insignificant regarding sleep duration and disruption in daily functioning [53]. Compared to that study, the current research has selected more databases for search and covers a longer time range (until 2022), and accordingly, the sensitivity of the present study is higher. Also, the analysis of factors causing heterogeneity based on meta-regression is considered so that an analysis can be performed on the high heterogeneity of the study.

Abdulaziez and Asaas conducted a study using polysomnography (PSG) to objectively evaluate sleep quality in patients with AS. They reported that compared to the healthy individuals, patients had a lighter sleep with an increase in Non-rapid eye movement (NREM) stage I and II, which means a reduction in deep sleep. In addition, slow-wave sleep was reduced among the patients, indicating a reduction in deep sleep [38, 43,44,45].

The pro-inflammatory cytokine, tumor necrosis factor-alpha (TNF-α), is one of the important cytokines in the inflammation process in patients. Several TNF-α inhibitors have been developed to reduce spinal pain and inflammation in this disease [54,55,56]. The findings of the study of Karatas et al. demonstrate that in patients with severe AS activity who underwent anti-TNF therapy, compared to patients who were in remission and were not treated with anti-TNF-α drugs three months after the treatment, the PSQI scores decreased significantly, which means that their sleep quality improved meaningfully compared to the healthy group. However, PSG-related parameters such as NREM stage I and II after three months of treatment did not show a difference between the two groups [37]. In the study of In et al., After evaluating the sleep quality of the patients using PSG, the total sleep time and its efficiency in the anti-TNF group was significantly higher than the group receiving NSAIDS, stage I was considerably shorter, and the rapid eye movement (NRM) stage was markedly longer than the NSAIDs group [57]. Therefore, according to the literature, the type of treatment can affect the quality of sleep and the prevalence of sleep disorders.

Disease activity can be another factor in reports on the severity of sleep disorders. In this regard, the study of Tymms et al. showed that patients with a more severe AS experience the symptoms of insomnia seven times more [10]. The findings of other studies also indicate the negative impact of more severe disease activity on the sleep quality of AS patients [6, 14, 18, 21, 37]. More disease activity causes increased structural damage and disruption of spine mobility, which can cause difficulty in changing position while sleeping and thus disturb the patients’ sleep [18, 40]. Also, the findings indicate a significant positive correlation between sleep disorder scores and metrics (measurements related to the pelvis and spine) measured by Bath Ankylosing Spondylitis Metrology Index (BASMI) [6, 19, 28]. However, reported results in an article by Li et al. found no significant relationship between metrological indicators and sleep quality [38].

Night pain is one of the prominent features of AS disease, which can affect the quality of sleep among patients [16]. The findings of existing literature indicate a significant relationship between pain experience and sleep quality disruption [18, 20, 22, 37, 38]. In a study by Nie et al., nighttime back pain was one of the key factors resulting in sleep disturbance. Experiencing pain before and during the night causes problems in falling asleep and reduces the duration and quality of sleep [20].

AS also significantly impacts a patient’s psychological state [23]. In the study by Zhang et al., the prevalence of depression among AS patients was reported about 35% [23]. Furthermore, the findings of Jiang et al. and Nie et al. showed that 31.6% and 48% of AS patients experience anxiety, respectively [16, 20]. Additionally, the prevalence of anxiety disorders in patients suffering from insomnia and patients without insomnia was reported as 76.1% and 33.3%, respectively [22].

The findings of previous studies indicate the existence of a significant relationship between sleep disorder and variables such as older age [19, 20, 38], experiencing more fatigue [19, 20, 27, 34, 46], delay in diagnosing the disease [20], longer duration of the disease [18,19,20], greater severity and duration of morning stiffness [18, 38, 46], lower quality of life [18, 19, 22, 28, 37, 61], higher CRP values [6, 18, 38, 46], presence of extraspinal manifestations of the disease [20], presence of functional limitations [6, 19,20,21,22, 37, 38, 46], lower level of education [38] and female gender [6] among AS patients. In the study of Wadeley et al., it was found that women with axial spondylarthritis experience sleep disorders three times more than men [52]. Further, in the study of Hultgren et al., prevalence of sleep disorder in women and men with AS was reported as 81% and 50%, respectively [35].

Regarding the relationship between educational level and sleep quality, the results of the study by Li et al. showed that there is a significant negative relationship between the overall score of sleep quality and years of education, which means that the higher a patient’s education, the less likely she/he is to experience a sleep disorder [38]. Also, in the study of Jiang et al., higher education patients experienced less anxiety and depression [16]. Therefore, according to pertinent literature, higher education seems to enable a patient to gain sufficient knowledge and awareness about the disease and its associated considerations. This also can help the patient to be able to control the disease and its symptoms. Nonetheless, in the findings of the study of Nie et al. no relationship between sleep disorder and the level of education of a patient was found [20].

The findings of the study of Günaydin et al. showed that the experience of fatigue in AS patients is influenced by sleep disorder [27]. Moreover, as sleep disturbance can increase the experience of pain, it seems that sleep disturbance lowers the threshold of tolerance and strengthens the pain signals, thus resulting in the person focusing more on the feeling of pain [38]. In the study of Purabdollah et al., it was reported that a significant relationship was found between sleep disorders and pain [58,59,60,61,62]. This study states that the severity of pain and sleep problems can predict inflammatory markers that can be clues to the severity of the disease. Therefore, relieving pain and improving sleep can reduce the severity of the disease [63].

This study found a relatively high pooled prevalence of sleep disorders among AS patients. Also, it is known that sleep disorders affect various aspects of patients’ lives. Therefore, regular follow-ups and therapeutic interventions, including non-pharmacological treatments, as well as effective and quality education offered by healthcare providers, are vital to improving the patients with AS physical and mental health. Moreover, pertinent interventions and policies will be instrumental in treating and preventing sleep disorders among patients.

One of the limitations of the study is that we included a number of researchers that had not used specific sleep disorder questionnaires to report the prevalence of sleep disorders. Additionally, two used PSQI cut-off points were used for measuring sleep disorders within the included studies. Moreover, the included studies used different sleep disorder questionnaires to report the prevalence. The report of sleep disorder was based on self-report questionnaires that provide the possibility of subjective evaluation of sleep quality. The mentioned cases may have affected the accurate reporting of the prevalence of sleep disorders in AS patients. Another limitation of the current research was that many studies could not be included due to either lack of access to their full text or their omissions after the quality evaluation stage.