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Effect of yoga on sleep quality in women: a systematic review and meta-analysis

independently evaluated bias by using tool suggested and conducting The main measure the

3 was beneficial in managing sleep problems. Despite certain disadvantages in methodology in the included studies, yoga may be recommended as an additional therapy to women in addition to pharmacological treatment.

Background
Sleep problems are one of the most common medical complaints. Lack of sleep is associated with significantly decreased work performance, impaired daytime function, and increased health care costs [1]. Sex-based differences in sleep problems have been widely published and discussed across sleep articles. Insomnia is approximately 1.41 times more common in women than in men [2]. Female populations at certain stages in their life span may be more vulnerable to insomnia. In these stages, hormonal changes associated with hormones, such as follicle-stimulating hormones (FSHs), luteinizing hormones (LHs), and progesterone, may play an important role in influencing women's sleep construction [3] during adolescence [4], pregnancy and postpartum [5] or menopause [6]. Several behavioral, psychological and pharmacological treatments are available for insomnia; however, their efficacy varies considerably. Although pharmacotherapy remains the most common treatment [7], hypnotics have been associated with many side effects, such as drowsiness, cognitive impairment, dependence, and tolerance.
Yoga, which originated from India thousands of years ago, is one of the most popular complementary and alternative medicines. Yoga has been widely adapted in the modern Eastern and Western world in a variety of ways. Yoga is an ancient form of exercise that focuses on strength, flexibility, and breathing to boost physical, mental and spiritual health [8]. There are many different styles of yoga, such as Tibetan, Iyengar, and Hatha Yoga. Some styles are more vigorous than others, whereas some may have different areas of emphasis, such as posture or breathing. The main components of yoga in Europe or America are mostly associated with physical posture (Asana) and breathing control (Pranayama) and meditation (Dhyana) [8]. In women, yoga can help not only to develop a healthy and regular menstrual cycle but also to cope with the psychological changes that they may experience due to physical instability [9]. Yoga is also safe and effective at improving fatigue severity, depressive moods, and sleep quality [10]. 4 Although systematic reviews and meta-analyses have assessed the efficacy and safety of yoga in specific groups of women, such as those with prenatal depression [11] and primary dysmenorrhea [12] in different stages, evidence for the efficacy of yoga in improving sleep quality has not yet been systematically assessed. Thus, the aim of this review was to systematically evaluate and perform a meta-analysis of the available data on the efficacy and safety of yoga in terms of improving sleep quality in women.

Methods
Before beginning the review, we followed the checklist for systematic reviews in concurrence with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines [13] and suggestions by the Cochrane Collaboration for programming and conducting systematic reviews and meta-analyses. [14].

Types of studies
Randomized controlled trials (RCTs), randomized crossover studies, and cluster randomized trials were all eligible for this meta-analysis. No restrictions in terms of language and countries were applied.

Types of participants
Studies that included women (aged ≥18 years) with or without sleep problems were eligible. No restrictions on the ethnicity and comorbidity of participants were applied.

Types of interventions
No restrictions regarding yoga type, form, structure, frequency, duration or length of intervention programs were applied. Studies on cointerventions that included yoga as a part of multicomponent interventions were excluded because it would be difficult to distinguish the effects of yoga from additional modalities. Studies in control interventions that compared yoga treatments with nontreatment, usual care, wait-lists, and education without active physical exercise programs were all eligible.

Types of outcome measures
The primary outcome of this study was sleep quality. To be included in this review, studies had to assess at least one of the sleep quality measures by using standardized instruments and provide outcomes both at the baseline and follow-up for primary outcomes. In particular, instruments in question included subjective measurements, such as the Pittsburgh Sleep Quality Index (PSQI) and Insomnia Severity Index (ISI), or objective measurements, such as polysomnography (PSG) and actigraphy. The PSQI and ISI scores have been recommended as reliable and valid instruments to measure sleep quality. A global PSQI score of 5 or higher is indicative of poor sleep quality [15]. A global ISI score of 8 or higher is indicative of some degree of insomnia [16].
PSG or actigraphy report the most complete and precise information on the construction and distribution of sleep periods, such as total sleep time (TST), sleep efficiency (SE), and percentage of slow-wave sleep (SWS) [17].
Secondary outcomes: The secondary outcome included in this study was the safety of the intervention, which was assessed as number of patients with adverse events (AEs), including serious adverse events or nonserious events. Serious adverse events referred to those events that caused death, life-threatening situations, hospitalization, disability or permanent damage, congenital anomaly/birth defect, or the need for medical or surgical intervention to prevent any of the aforementioned outcomes [18]. All other adverse events were regarded as nonserious.

Search methods
The search strategy comprised four electronic databases from their inception through June 01, 2019: Medline/PubMed, Clincalkey, ScienceDirect, Embase, PsycINFO, and the Cochrane Library. The literature search was constructed around search terms for "yoga," "women," and "sleep" and adapted for each database as necessary. The complete search strategy for PubMed was as follows: ("yoga" OR "asana" OR "pranayama" OR "dhyana" ) AND ( "women" OR "female" ) AND ("sleep" OR "sleep quality" OR "sleep disturbance" OR "insomnia"). Additional reference lists of identified original articles or reviews, the tables of the contents of the Journal of Yoga and Physical Therapy, and Journal 6 of National Taiwan Sports University were searched manually.
Retrieved articles were scanned independently to verify their eligibility, and the entire text was assessed by two reviewers. A conflict of reviewers' opinions on inclusion or exclusion of any article was discussed with a third reviewer to reach a consensus.

Data extraction and management
Two reviewers independently extracted data on design (e.g., article setting, author/year, country of studies, and sampling strategy), participants (e.g., age, body max index, clinical characteristics, comorbid condition, and overall sample size), interventions (e.g., yoga type, frequency of sessions per week, duration of yoga intervention, and total length of intervention time), control interventions (e.g., type, frequency, length, and duration), and outcomes (e.g., outcome measures with sleep quality and safety-related events). A conflict of reviewers' opinions were discussed with a third reviewer until consensus was reached.

Risk of bias in individual studies
Two reviewers independently assessed the risk of bias on the following seven domains: selection bias (random sequence generation and allocation concealment), performance bias (blinding of participants and personnel), detection bias (blinding of outcome assessment), attrition bias (incomplete outcome data), reporting bias (selective reporting), and other bias using the Cochrane risk of bias tool [14]. All domains were scored as low risk, high risk, or unclear risk of bias and assessed individually. A risk of bias table was completed for each included study. A conflict of reviewers' opinions were discussed with a third reviewer until consensus was reached.

Data assessment of overall effect size
A meta-analysis was conducted with Review Manager 5 software (Version 5.3, The Nordic Cochrane Centre, Copenhagen) using a random effects model if at least 2 studies assessing this specific outcome were obtainable. For continuous outcomes, standardized mean differences (SMDs) with 95% 7 confidence intervals (CIs) were calculated as the difference in means between groups divided by the pooled standard deviation.
Values of P < 0.05 were regarded as indicating statistical significance. For studies that did not report data with standard deviations, we calculated these values from standard errors, confidence intervals, or t-values. If adequate information was available, we planned to perform subgroup analysis.
A negative SMD was provided a definition to display the beneficial effects of yoga intervention compared with the control intervention for sleep quality outcomes. Cohen's categories were used to assess the significance of the overall effect size, with SMD = 0.2-0.5: small effect size; SMD = 0.5-0.8: medium effect size; and SMD > 0.8: large effect size [19].

Assessment of heterogeneity
Statistical heterogeneity between studies was analyzed using the I 2 statistics, the Cochrane chisquare. The variance between studies was measured using the tau-square (Tau 2 ). The level of heterogeneity was classified as I 2 = 0%-24%: low heterogeneity; I 2 = 25%-49%: moderate heterogeneity; I 2 = 50%-74%: substantial heterogeneity; and I 2 = 75%-100%: considerable heterogeneity. Given the low power of this test when only few studies or studies with a low sample size are included in a meta-analysis, a P value of ≤0.1 for the chi-square test was regarded as indicating significant heterogeneity [14].

Risk of publication bias
Risk of publication bias was evaluated for each meta-analysis that included at least 10 studies. Funnel plots generated using Review Manager 5 software was estimated from individual studies against each study's standard error. Publication bias was evaluated through visual analysis, in which roughly symmetrical funnel plots generally defined a low risk of publication bias and asymmetrical funnel plots generally defined a high risk of publication bias [20].

Literature search
The results of the literature search and screening process are summarized in Fig. 1. The literature search totaled 1338 records; one additional record was retrieved from the Journal of National Taiwan Sports University in the Chinese language database [21]. In all, 1295 records were excluded because they did not meet all predefined inclusion criteria or were duplicated. Forty-four full-text articles were assessed for eligibility. Twenty-five were excluded because they were not randomized [21,22], did not include relevant outcomes [23][24][25][26][27][28][29], did not include only female participants [30][31][32][33][34][35][36][37][38], included yoga as a part of a multimodal intervention (or combined with other intervention) [39-42], lacked adequate control [43], and did not include a form of yoga intervention [44,45]. Nineteen full-text articles with 1832 participants were included in the qualitative synthesis. One RCT had unique measurements of sleep quality that could not be compared with other RCTs in the meta-analysis [46]. Finally, 18 fulltext articles with 1646 participants met the inclusion criteria and were included in the meta-analysis.
All articles were published in English.

Study characteristics
A total of 19 studies were considered eligible for systematic reviews. Information regarding the characteristics of the sample, yoga or control group interventions, outcome measures, and results are listed in Tables 1 and 2.
Overall, the 19 RCTs included were conducted in the United States [10, 46, 49, 50, 52-58, 61, 62], Brazil [51,60], India [48,63], Iran [59], and China [47]. Study participants were recruited from hospitals [47, 53-57, 59, 63], outpatient clinics [51,52] and schools [48]. The process of recruitment also included using purchased lists and health-plan enrollment files [46,49] and multiple other mechanisms, including flyers, newspaper advertisements, web-based announcements, brochures, public health departments, tumor registry systems, and doctor referrals [10,50,58,61,62]. One study did not reveal the source from which participants were recruited [60]. Nineteen studies included in the systematic review displayed a baseline of PSQI higher than 5 or ISI higher than 8, indicating poor sleep quality. The only exceptions were two studies, with individual control groups in each study displaying a baseline of PSQI lower than 5 [48] or ISI lower than 8 [57]. The sample size ranged from 20 to 249 with a median of 96. Participant's mean age ranged from 29.8 to 71.9 years, with a median of 50.1 years. All participants were women.

Intervention characteristics
Of the 19 included studies, three revealed that Iyengar Yoga [10,50,62] was used; two revealed that Hatha Yoga [58,61] was used; two revealed that Tibetan Yoga [51,54] was used; two revealed that Restorative Yoga [56,57] was used; one revealed that Vini Yoga [46] was used; one revealed that Yoga Relaxation with MindSound Resonance Technique [48] was used; one revealed that yoga breathing exercise in warm water [60] was used; and only seven RCTs revealed yoga programs with postures, breathing, relaxation or mediation, without defining the specific style of yoga [47,49,52,53,55,59,63]. All RCTs included yoga postures in their yoga intervention; 16 RCTs included yoga breathing; 15 RCTs included yoga relaxation; 12 RCTs included meditation; and 7 RCTs included all contents with postures, breathing, relaxation, and meditation for the yoga intervention group [47,49,53,55,57,59,63]. The duration of yoga interventions ranged from 1 week to 24 weeks, with a median of 10 weeks; the frequency of yoga interventions ranged from one to five weekly sessions of 45 to 120 min. Sixteen studies compared the yoga group with waitlist control groups with no specific treatment; three studies compared the yoga group with the control group, including two studies for education groups [10,62] and one study for social support groups [52].

Outcome measures
All studies evaluated outcomes directly at the end of interventions. All studies assessed the subjective or objective measurements of sleep quality: 16 RCTs used the PSQI; three RCTs used the ISI [49,51,57]; two RCTs used PSG [46,51]; and one RCT used actigraphy [54]. Safety-related events were reported in two RCTs only [49,58].

Risk of bias in individual assessments
Graphical representation of the risk-of-bias assessment is represented in Fig. 2. All studies had a high or unclear risk of bias in at least one domain. All studies claimed to be randomized; however, three studies did not reveal their content and method of random sequence [51,56,59]. Twelve studies did not report methods applied to perform adequate allocation [46-48, 50-56, 59, 63]. Most studies offered no data material on blinding. Three studies definitely revealed that participants and personnel were blinded [10,52,61]. Four studies definitely revealed that researchers and outcome assessments were blinded [10,52,58,61]. Six studies had insufficient data on attrition rates [49-51, 59, 60, 62].
Twelve studies had a low risk of selection reporting; only two studies had a high risk of selective reporting due to several reported outcome parameters not being revealed in study protocol or duplicate publications reporting different results of the same trial [49,51]. Six studies had a high risk of other potential sources of bias due to poor participant compliance, intervention length, sample size or baseline differences [50,57,[60][61][62][63].

Publication bias
The meta-analysis of the effect of yoga on the sleep quality of women that involved yoga groups compared with control groups included 16 studies. The asymmetrical shape of the funnel plot indicated that publication bias was detected (Fig. 3).

Primary outcomes
The meta-analysis revealed the effects of yoga compared with the control group on the sleep quality of women using the PSQI or ISI, as displayed in Fig. 4. Sixteen RCTs revealed evidence for effects of yoga compared with the control group in improving sleep quality in women using the PSQI (SMD = −0.42; 95% CI = −0.76 to −0.09; P = 0.01). However, three RCTs revealed no effects of yoga compared with the control group in improving sleep quality in women using ISI (SMD = −0.13; 95% CI = −0.74 to 0.48; P = 0.69)

Secondary outcomes (safety)
Only two studies reported safety-related events. Two events revealed in one study could potentially be attributed to yoga intervention: two women reported the recurrence of chronic back and/or shoulder problems [58]. In another study, adverse events reported did not differ between the yoga intervention group and the control group (P = 0.41). These adverse events included muscle aches and strains (6.7%, yoga group; 10.3%, control group), low back pain (4.2%, yoga group; 3.1%, control group), or changes in strength or sensation in arms or legs (5.5% yoga group; 8.9% control group).
Dropouts were not regarded as being adverse events because it was not explicitly telling as the reason for dropout in the original study. No serious adverse effects were reported in the included studies.

Subgroup analyses
Participants were divided into two separate subgroups. Meta-analyses revealed the effects of yoga

Sensitivity analyses
In the included studies with low risk of selection bias, reporting bias, and other bias, the effect of yoga group compared to control group on women sleep PSQI did not change substantially, including

Summary of evidence
In this systematic review of 19 studies for yoga's effect on improving women's sleep quality, 16 RCTs revealed evidence for yoga improving sleep quality in women. However, seven RCTs revealed no 13 evidence for yoga improving sleep quality in women with breast cancer. Four RCTs revealed no evidence for improving PSQI in peri-or postmenopausal women. Two RCTs revealed no evidence for improving ISI in peri-or postmenopausal women.
However, heterogeneity of effects were high across studies. Overall, the application of yoga was not associated with deteriorating sleep problems or increased adverse effects. Only two studies explicitly assessed safety-related nonserious adverse events. Yoga is most likely a comparatively safe intervention in this population. However, future RCTs should take more measures to ensure stricter reporting of adverse events and reasons for dropouts.

Comparison with prior reviews
No systematic review explicitly focusing on yoga for improving sleep quality in a specific gender was accessible. Ours is the first and largest systematic review and meta-analysis with 18 RCTs that examined the effects of yoga on women with sleep problems. A previous review published until February 2019 included subgroup analysis of yoga on mind-body therapies on insomnia [64]. This recent review illustrated that yoga had beneficial effects on subjective sleep quality in participants in all gender groups. Our meta-analysis with 16 RCTs uncovered evidence for the effects of yoga on the sleep quality in women. Only six RCTs were found to have overlapped with this previous review. Our meta-analysis also examined the potential effect on specific subgroups, such as breast cancer and menopausal subgroups, with these subgroups serving as potential factors in sleep quality effects (although the result did not show any clear difference). Results from the peri-or postmenopausal subgroup of women in our systematic review also agreed with previous published reports that suggested that yoga had no significant effect on the severity of insomnia in middle-aged women [65].
There were baseline differences between participants based on intervention assignment in PSQI scores [49,62]. This may have contributed to results displaying no significant effect in sleep quality in the peri-or postmenopausal subgroup of women.

External and internal validity
Major threats to external validity included the specificity of variables of sampled participants and multiple yoga types or styles. The majority of RCTs included participants from North America, South America, and Asia. The lack of studies from Europe and Africa could be seen as a geographical limitation. There were several other limitations in this review: the wide variety of diagnoses included; the inclusion of only certain types of people or professions, such as nurses, teachers, and peri-or postmenopausal women; and patients with breast cancer, type 2 diabetes mellitus, fibromyalgia syndrome, osteoarthritis of the knee, restless leg syndrome, and primary dysfunctional uterine bleeding. The heterogeneity of interventions with different types or styles of yoga (postures, breathing, relaxation, or mediation), and potential bias were included in this systematic review.
Other threat to internal validity was the study bias. Only few effects were robust against all potential bias. All of our studies claimed to have applied randomization methods; however, three RCTs failed to provide the design protocol of randomization. Some of the included studies may not have been truly randomized. Erroneous allocation concealment has been empirically revealed to be a significant source of bias in RCTs [66]. Our included studies only had a low risk or an unclear risk of detection bias without high risk detection bias, and the results of meta-analysis did no convert when studies with a high risk of bias or an unclear risk of bias were excluded. The internal validity of the review appeared to be limited but acceptable.

Strengths and weaknesses
This is the first and latest systematic review and meta-analysis available on yoga for sleep quality in women. A large number of RCTs on female population-related physiological and physiological comorbidities and risk factors in insomnia could be included. Subgroup analyses were performed to evaluate the effects on different participant groups. There were five primary limitations of this review.
First, the participant characteristics included in the review were heterogeneous; subgroups were included to analyze the effectiveness of different participant groups; and the small number of RCTs limited data presentation. Second, the severity of the sleep complaints and health status of participants was not considered or individually listed in each study. Baseline differences in PSQI 15 scores were found between intervention and control groups in three studies [49,58,62]. This may have led to heterogeneity. Third, the intensity, frequency, and duration of yoga interventions were heterogeneous; short term applications of less than 1 month yoga intervention were found in some studies [48,54,60]. Only four reviews reported long-term follow up effects, ranging from 3 months to 12 months [53][54][55]58]. The fourth limitation was the lack of objective measures and safety issues in each study. The interpretation of findings was limited due to insufficient data on research methodology. Lastly, publication bias revealed in this review may have been due to selective reporting bias, which means that articles with negative findings may have not been published or poor methodological quality of including articles.

Implications for further research
This systematic review and meta-analysis was limited by the low methodological quality of included studies. Further RCTs should ensure rigorous methodology and reporting, which would mean adequate sample size, adequate randomization, allocation concealment, intention-to-treat analysis, and blinding of at least outcome assessors [67]. Researchers for study interventions may need to apply a standard protocol, such as at least three sessions of yoga programs with unified yoga component programs per week for at least 3 months would helpful for the further interference.
Adequate reporting with safety issue should adhere to future yoga trials, although yoga for a long time has been promoted as being harmless. Evidence was limited because few studies report safety-

Ethics approval and consent to participate
Not applicable. Tables Table 1 Characteristics of included