|Year : 2022 | Volume
| Issue : 1 | Page : 34-40
Sleep quality in rheumatoid arthritis
Anil Kumar, Rohini Handa, Sundeep Kumar Upadhyaya, Sirinder Jit Gupta, Deepak Ramchandra Malgutte
Department of Rheumatology, Indraprastha Apollo Hospital, New Delhi, India
|Date of Submission||11-Jul-2021|
|Date of Acceptance||13-Dec-2021|
|Date of Web Publication||27-Jan-2022|
Prof. Rohini Handa
Indraprastha Apollo Hospital, Sarita Vihar, New Delhi - 110 076
Source of Support: None, Conflict of Interest: None
Background: Patients with rheumatoid arthritis (RA) often have sleep dysfunction. This may adversely impact quality of life. Despite being common, sleep dysfunction is uncommonly assessed in the clinic. No data on sleep quality in RA patients is available from India. In this study we examined the sleep quality and factors influencing it in Indian patients with RA.
Methods: This cross-sectional study evaluated sleep quality in 110 patients with RA using the Pittsburgh sleep quality index (PSQI) or its validated Hindi version (PSQI-H). The risk of sleep apnea was assessed by the Berlin questionnaire while the assessment of pain and fatigue was done on a visual analogue scale (VAS). Disease activity was assessed by simplified disease activity index, functional disability was assessed by the health assessment questionnaire disability index-Centre for Rheumatic Diseases version, and quality of life was evaluated by World Health Organization quality of life-BREF (WHOQOL-BREF). Socioeconomic status was assessed by modified Kuppuswamy scale.
Results: Sixty four percent of our patients with RA demonstrated poor sleep quality. The mean global PSQI score was of 6.41 ± 3.07 (range 1–17). A high risk of sleep apnea was present in 26% of the study population. The use of glucocorticoids, high risk of sleep apnea, high disease activity, pain VAS, fatigue VAS, and functional disability were more common in poor sleepers. Multivariate logistic regression analysis revealed that high risk of sleep apnea, fatigue, and poor physical health domain of WHOQOL-BREF were independent risk factors for poor sleep quality.
Conclusions: Patients with RA had a high prevalence of poor sleep quality. High risk of sleep apnea, fatigue and physical health domain of WHOQOL-BREF were identified as independent predictors of poor sleep quality. Clinic evaluation of patients with RA should incorporate the domain of sleep quality.
Keywords: Fatigue, Indian, Pittsburgh sleep quality index, Pittsburgh sleep quality index-Hindi version, quality of life, rheumatoid arthritis, sleep apnea, sleep quality
|How to cite this article:|
Kumar A, Handa R, Upadhyaya SK, Gupta SJ, Malgutte DR. Sleep quality in rheumatoid arthritis. Indian J Rheumatol 2022;17:34-40
| Introduction|| |
Sleep is an important component of health-related quality of life. More than two-thirds of patients with rheumatic diseases report disturbed sleep. Rheumatoid arthritis (RA) is the commonest inflammatory polyarthritis encountered in outpatient practice. Patients with RA frequently exhibit prolonged sleep latency, lower sleep efficiency, an increased number of awakenings and arousals, feelings of nonrestorative sleep and daytime sleepiness.,, Poor sleep quality is associated with fatigue, increased pain perception, reduced daily activity, impaired social relationships, reduced cognitive performance, increased risk of cardiovascular events, and mood disorders resulting in poor quality of life. A number of disease specific factors like disease activity, pain, morning stiffness, and medication may influence sleep quality. The enhanced presence of sleep disorders has also been reported in other rheumatic diseases like lupus, primary antiphospholipid syndrome, Sjogren's syndrome, systemic sclerosis, ankylosing spondylitis, etc.,,,, Lupus, the prototypic connective tissue disease, is accompanied by sleep disorders in more than half of the patients and the sleep disturbances are related to disease activity. Early recognition is important to mitigate the negative impact of sleep disturbances on the patient's quality of life. Apart from autoimmune rheumatic diseases, the interplay between sleep and disease spans the categories of endocrinologic disorders, metabolic/toxic disturbances, renal, cardiovascular, pulmonary, gastrointestinal, infectious diseases, malignancy, and critical illness, as well. Sleep hygiene is defined as a set of behavioral and environmental recommendations intended to promote healthy sleep, and was originally developed for use in the treatment of mild to moderate insomnia. Mounting global public health concern over poor sleep has stimulated tremendous interest in sleep research.
Sleep as a component of RA is a less explored entity. Despite the acknowledged importance, no study related to sleep quality in RA patients is available from India. In this study we examined the sleep quality and factors influencing sleep quality in patients with RA.
| Methods|| |
This was a questionnaire based, cross-sectional study conducted from June 2018 to January 2020 in patients with RA attending the outpatient clinic at the Department of Rheumatology, Indraprastha Apollo Hospitals, New Delhi. Nonconsecutive patients with age >18 years and who fulfilled 2010 American College of Rheumatology/European Alliance of Associations for Rheumatology criteria for RA were included in this study. Patients with age <18 years, psychiatric disorders, fibromyalgia, and pregnancy were excluded from study. All the recruited subjects signed an informed consent form before participating in the study. This study was approved by Institutional Ethics Committee-Clinical Studies of Indraprastha Apollo Hospital, New Delhi on June 26, 2018 with approval number of DNB/020/07-18. A detailed history and thorough physical examination were carried out in all the patients. Rheumatological examination included swollen joint count 28, tender joint count 28, and clinical assessment for extraarticular manifestations.
Pittsburgh sleep quality index (PSQI) was used to assess subjective sleep quality over past 30 days. The patients were given a choice amongst the English and the validated Hindi versions of PSQI (PSQI-H)., PSQI consists of 19 items and 5 additional questions. The latter five questions are not included in scoring and are rated by bed partner or roommate. These 19 items are combined to form 7 components of sleep quality: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbance, sleep medication use, and daytime dysfunction. Each component has a score that ranges from 0 to 3. A global PSQI score is obtained by summing the 7 component scores (range = 0–21). Participants were divided into a poor-sleep quality group if the PSQI was ≥5.0 and a good-sleep quality group if the PSQI was <5.0. The risk of sleep apnea was assessed by Berlin questionnaire. Patients were divided into two categories on the basis of risk of sleep apnea; (1) High risk: if there are 2 or more categories where the score is positive and (2) Low risk: if there is only 1 or no categories where the score is positive.
RA disease activity was assessed by simplified disease activity index (SDAI). The assessment of pain and fatigue was done on 100 mm horizontal visual analogue scale (VAS). Functional disability was assessed by health assessment questionnaire disability index (HAQ-DI) Centre for Rheumatic Diseases (CRD) version with either the Hindi or English format. The HAQ-DI CRD version uses 23 questions to assess activity limitation in 8 dimensions of activities of daily living: dressing and grooming, arising, eating, walking, hygiene, reach, grip, and common daily activities. Participants rate degree of difficulty for each activity on a 0–3 scale. Maximum score from each category is to be taken and sum of each item dividing by 8 give disability score. The range of scores is 0–3, with higher levels indicating more disability. On the basis of total score, functional disability is divided into following three groups; (1) mild ≤1, (2) moderate = 1–1.5 and (3) severe ≥1.5.
The quality of life was measured by World Health Organization quality of life-BREF (WHOQOL-BREF). The patients were given a choice amongst the English and the validated Hindi versions of the questionnaire., It measures quality of life in last 2 weeks under four domains, Domain I (physical health), Domain II (psychological), Domain III (social relationships), and Domain IV (environment) using a Likert-type five-point scale to grade the patient's response to the QOL items. The raw score of all four domains was calculated and converted to transformed score of 0–100. Higher the score, better is the quality of life.
The socioeconomic status was measured by Modified Kuppuswamy Socioeconomic scale updated for January 2018. It has three domains; the occupation of head of the family, education of head of the family and family income per month in Rupees. According to the total score of all three domains (3–29), this scale classifies the study populations into high, upper middle, lower middle, upper lower and lower socioeconomic status.
Descriptive statistics were recorded for each variable, with mean (standard deviation), and median (interquartile range). All statistical tests were performed by using Statistical Package for the Social Sciences (SPSS) version 20.0 and a P < 0.05 was considered as significant at 95% confidence interval. Patients were divided in those with good and poor sleep quality according to the PSQI (poor-sleeper if the PSQI was ≥5.0 and a good-sleeper if the PSQI was <5.0.) and these two groups were compared. The differences between groups were calculated with T-tests or Mann–Whitney U-tests, depending on the data distribution. Chi-square tests were used for the differences between categorical variables. The correlations between poor sleep quality and the RA-related variables were investigated with the help of Spearman's correlation coefficients (r). By using a binary logistic regression analysis, a multivariate analysis was performed to detect independent predictors for the poor sleep quality.
| Results|| |
A total of 141 patients were handed out the questionnaire. Of these, 110 patients completed the questionnaire fully, 26 patients submitted incomplete questionnaires two patients were pregnant and three patients had depression. Only the patients who had submitted fully completed questionnaires were included in the study. Pregnant women and patients with depression were excluded. The demographic and clinical characteristics of study population are listed in [Table 1]. Using the standard cut off for global PSQI score (≥5), 70 patients (63.6%) demonstrated poor sleep quality. The mean global PSQI score in our study subjects was 6.41 ± 3.07 with range of 1-17. Sleep latency and daytime dysfunction were two most affected components of sleep quality, and the use of sleep medication was the least affected component of sleep quality [Table 2]. Thirty two percent patients reported fairly bad to very bad subjective sleep quality, 48% took more than 30 min to fall asleep while 43.6% slept <7 h per night, and 22% had sleep efficiency less than 85%. Only 7.27% of the patients reported use of sleep-inducing medicine during the last month. Daytime dysfunction was reported as a problem, equal or more than two times per week in 44% of the patients. A high risk of sleep apnea was present in about 26% of patients.
|Table 1: Baseline characteristics of rheumatoid arthritis patients, good sleepers and bad sleepers|
Click here to view
|Table 2: Pittsburgh sleep quality index domains and global pittsburgh sleep quality index score in study population (n=110)|
Click here to view
The mean global PSQI score of good sleepers was 3.32 ± 0.88 compared to a score of 8.1 ± 2.39 in poor sleepers (P < 0.001). The prevalence of high risk of sleep apnea was significantly higher in poor sleepers in comparison with good sleepers (36% vs. 10%, P < 0.001). The demographic and clinical variables of good sleepers and poor sleepers have been listed in [Table 1]. There was no significant difference with respect to age, disease duration, gender, body mass index, use of conventional disease modifying antirheumatic drugs and biological agents, comorbidities, extraarticular manifestations, and socioeconomic status. Poor sleepers had significantly higher erythrocyte sedimentation rate (ESR) level, C-reactive protein (CRP) level, prevalence of deformities, high risk of sleep apnea, and use of glucocorticoids in comparison to good sleepers. The mean pain VAS, fatigue VAS, SDAI, DAS28-CRP and HAQ-DI score were also significantly higher (P < 0.001) in poor sleepers compared to good sleepers. Poor sleepers had significantly (P < 0.001) poor quality of life in term of physical health domain, psychological domain and social relationship domain in comparison to good sleepers.
There was a significant positive correlation between ESR, CRP, pain VAS, fatigue VAS, disease activity, HAQ-DI and global PSQI score and a significant negative correlation between all four domains of WHOQOL-BREF and global PSQI score [Table 3]. Univariate analysis [Table 4] revealed that presence of deformities, high risk of sleep apnea, use of glucocorticoids, raised inflammatory markers, pain, fatigue, disease activity, functional disability and poor quality of life in all domains of WHOQOL-BREF except environment domain were associated with poor sleep quality while on multivariate regression analysis [Table 5] high risk of sleep apnea, fatigue and physical health domain of WHOQOL-BREF were independent predictors of poor sleep quality.
|Table 3: Correlation between global pittsburgh sleep quality index score and various variables|
Click here to view
|Table 4: Factor associated with poor sleep quality (univariate analysis)|
Click here to view
|Table 5: Factors associated with poor sleep quality (multivariate analysis)|
Click here to view
| Discussion|| |
RA is multi-dimensional and several psychological, social, and biomedical factors affect health outcomes in RA. Poor sleep quality has been reported in 30-75% of patients with RA.,,, However, despite its commonality, it is seldom ever assessed in routine clinical practice. Sleep is influenced not only by disease related variables but also racial/ethnic differences and socio-economic position.,,
Our study revealed that two-thirds of patients had poor sleep quality. To the best of our knowledge this is the first study on sleep problems in Indian patients with RA. The sleep latency >30 min reported by 48% of our patients is longer than 10–15 min in the normal healthy population. As many as 44.5% of Indian patients had fairly bad to bad daytime dysfunction. This contrasts with data available from Denmark and Canada where only 18% patients reported fairly bad to bad daytime dysfunction., One reason could be the rather limited use of sleep-inducing medications (only 1% of Indian patients used sleep medicine more than once per week as compared to 25.5% and 18% in Denmark and Canada respectively).,
We looked at the influence of different disease variables. ESR, CRP, pain VAS, fatigue VAS, disease activity and HAQ-DI were found to correlate positively with global PSQI score. Many of these variables like fatigue have a bidirectional relationship with poor sleep quality. In patients with RA, fatigue directly or indirectly disturbs sleep quality and disturbance in the sleep quality may also lead to greater fatigue. Pain reduces sleep efficiency and poor sleep exaggerates joint pains leading to a vicious cycle. Some of the disease variables like deformities, pain, raised inflammatory markers that showed association with sleep quality on univariate analysis failed to keep association on multivariate analysis, likely because of their inter related nature.
Disease activity as determined by SDAI was associated with poor sleep quality on univariate analysis but not on multivariate analysis in our study. While some of the earlier studies have showed disease activity as independent predictor of sleep quality,,, others have shown significant association between sleep quality and disease activity only on univariate analysis but not on multivariate analysis.,, This apparent inconsistency may be due to the use of self-reported subjective measurement of sleep quality rather than objective measurement by polysomnography. A recent study that included 95 patients with 57% reporting nonoptimal sleep found no differences in sociodemographic variables, disease duration or activity, inflammatory parameters, or use of biological and corticosteroid therapy. Only more intense pain was shown to be associated with a lower likelihood of optimal sleep. Interestingly, some workers looking at sleep and interleukin 6 (IL-6) receptor inhibition in RA have suggested that aberrant IL-6 regulation could be responsible for sleep disturbances. This pilot study involving 15 patients studies over 6 months showed improvement of sleep quality after tocilizumab treatment in patients with RA although changes in PSQI score over time were not associated with the corresponding changes in DAS28-ESR.
Our patient population exhibited a significant negative correlation between all four domains of WHOQOL-BREF and global PSQI score. On univariate analysis patients with higher physical health, psychological and social relationship domain score had a significantly lower likelihood of having poor sleep quality. Multivariate regression analysis found physical health domain of WHOQOL-BREF as independent predictor of sleep quality indicating that increase of physical health domain score improves sleep quality. Other workers have also reported quality of life as an independent predictor of sleep quality.,
A high risk of sleep apnea was found to be an independent risk factor for poor sleep quality in our study. Sleep apnea in RA is both of obstructive and central type, with the former being much more common. Intermittent hypoxia in obstructive sleep apnea (OSA) leads to activation of NF-kB-dependent inflammatory pathways and increased production of tumor necrosis factor (TNF), IL-6 and others proinflammatory cytokines. Anti-TNF treatment in RA patients with OSA led to significant decrease in daytime sleepiness, sleep latency, sleep disturbance and apnea-hypopnea index. Whether the treatment of sleep apnea improves disease activity in addition sleep quality in RA needs further exploration?
The relationship between the pharmacological treatments and sleep quality in RA is unclear. We could not find any significant association between conventional disease-modifying antirheumatic drug or biologic use and sleep quality. The literature also shows variable results., The small proportion of patients on biological therapy in our study are insufficient to allow any definitive conclusions regarding the influence of biological agents on sleep quality. The use of glucocorticoids was associated with a high likelihood of poor sleep quality on univariate analysis but not on multivariate analysis in our study. High disease activity leading to poor sleep quality could be responsible for the greater use of glucocorticoids in poor sleepers. Chronic glucocorticoids use has been shown to be associated with increased sleep latency, enhanced wake time, increased rapid eye movement sleep and reduced sleep quality.
To the best of our knowledge, our study is the only study in literature that has used a composite scale to measure socioeconomic status, instead of individual parameters of socioeconomic status. Our study failed to demonstrate an association between socioeconomic status and sleep quality in RA. Socioeconomic status influences sleep quality in the normal population., Whether RA patients behave differently cannot be answered with certainty as 61% of our study population belonged to upper middle class reflecting the patient background in a corporate hospital which cannot be generalized to all hospitals.
Our study has some limitations-it was a hospital based, single centre study with a cross-sectional design that included nonconsecutive patients which may have introduced a selection bias. The lack of controls is another shortcoming. Many RA patients get treated in clinics and may not visit hospitals to seek treatment. Multi-site, pan India studies including clinics and hospitals with a prospective longitudinal follow up are warranted to reveal the causal relationship between sleep quality and various disease variables in RA. Polysomnographic evaluation, while challenging in terms of logistics, could complement the questionnaire design to provide a more accurate assessment of sleep.
| Conclusions|| |
We report here the first study from India to assess sleep quality in patients with RA. We used a validated Hindi version of PSQI in addition to the English version, facilitating questionnaire administration to non-English speaking patients. Nearly two-thirds of our patients with RA were found to have poor sleep quality. Assessment of sleep health should be incorporated in the routine management of patients with RA.
The authors thank Dr. DJ Buysse for giving permission to use the PSQI in the present study.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Boeselt T, Koczulla R, Nell C, Beutel B, Guenter K, Cassel W, et al.
Sleep and rheumatic diseases. Best Pract Res Clin Rheumatol 2019;33:101434.
Sariyildiz MA, Batmaz I, Bozkurt M, Bez Y, Cetincakmak MG, Yazmalar L, et al.
Sleep quality in rheumatoid arthritis: Relationship between the disease severity, depression, functional status and the quality of life. J Clin Med Res 2014;6:44-52.
Mahowald MW, Mahowald ML, Bundlie SR, Ytterberg SR. Sleep fragmentation in rheumatoid arthritis. Arthritis Rheum 1989;32:974-83.
Grewal GS, Amlani A, Hazlewood G. Sleep in rheumatoid arthritis: A systematic review and meta-analysis of objective sleep parameters. Sleep 2018;41:A340.
Coskun Benlidayi I. Sleep impairment: An obstacle to achieve optimal quality of life in rheumatoid arthritis. Rheumatol Int 2018;38:2183-92.
Palagini L, Tani C, Mauri M, Carli L, Vagnani S, Bombardieri S, et al.
Sleep disorders and systemic lupus erythematosus. Lupus 2014;23:115-23.
de Oliveira LV, Sinicato NA, Appenzeller S, Rodrigues CEM, de Carvalho JF. Sleep disorders in primary antiphospholipid syndrome. Clin Rheumatol 2018;37:3345-9.
Priori R, Minniti A, Antonazzo B, Fusconi M, Valesini G, Curcio G. Sleep quality in patients with primary Sjögren's syndrome. Clin Exp Rheumatol 2016;34:373-9.
Leverment S, Clarke E, Wadeley A, Sengupta R. Prevalence and factors associated with disturbed sleep in patients with ankylosing spondylitis and non-radiographic axial spondyloarthritis: A systematic review. Rheumatol Int 2017;37:257-71.
Figueiredo FP, Aires GD, Nisihara R, Skare TL. Sleep disturbance in scleroderma. J Clin Rheumatol 2021;27:S242-5.
Davis EM, Ramani C, Quigg M. Neurologic manifestations of systemic disease: Sleep disorders. Curr Treat Options Neurol 2020;22:30.
Hauri P. Sleep hygiene. In: Hauri P, editor. Current Concepts: The Sleep Disorders. Kalamazoo, MI: The Upjohn Company; 1977. p. 21-35.
Aletaha D, Neogi T, Silman AJ, Funovits J, Felson DT, Bingham CO 3rd
, et al.
2010 Rheumatoid arthritis classification criteria: An American College of Rheumatology/European league against rheumatism collaborative initiative. Arthritis Rheum 2010;62:2569-81.
Buysse DJ, Reynolds CF 3rd
, Monk TH, Berman SR, Kupfer DJ. The Pittsburgh sleep quality index: A new instrument for psychiatric practice and research. Psychiatry Res 1989;28:193-213.
Kumar A, Handa R, Upadhyaya SK, Gupta SJ. Validation of Hindi version of the Pittsburg sleep quality index. J Assoc Physicians India 2021;69:11-2.
Netzer NC, Stoohs RA, Netzer CM, Clark K, Strohl KP. Using the Berlin questionnaire to identify patients at risk for the sleep apnea syndrome. Ann Intern Med 1999;131:485-91.
Smolen JS, Breedveld FC, Schiff MH, Kalden JR, Emery P, Eberl G, et al.
A simplified disease activity index for rheumatoid arthritis for use in clinical practice. Rheumatology (Oxford) 2003;42:244-57.
Chopra A, Saluja M. Validation and usefulness of Indian version (CRD Pune) health assessment questionnaire: Drug trials, community practice and COPCORD Bhigwan population study (1994-2012). Indian J Rheumatol 2012;7:74-82. [Full text]
Development of the World Health Organization WHOQOL-BREF quality of life assessment. The WHOQOL Group. Psychol Med 1998;28:551-8.
Saxena S, Chandiramani K, Bhargava R. WHOQOL-Hindi: A questionnaire for assessing quality of life in health care settings in India. World Health Organization Quality of Life. Natl Med J India 1998;11:160-5.
Saleem S. Modified Kuppuswamy scale updated for Year 2018. PARIPEX – Indian J Res 2018;7:6-7.
Guo G, Fu T, Yin R, Zhang L, Zhang Q, Xia Y, et al.
Sleep quality in Chinese patients with rheumatoid arthritis: Contributing factors and effects on health-related quality of life. Health Qual Life Outcomes 2016;14:151.
Løppenthin K, Esbensen BA, Jennum P, Østergaard M, Tolver A, Thomsen T, et al.
Sleep quality and correlates of poor sleep in patients with rheumatoid arthritis. Clin Rheumatol 2015;34:2029-39.
Kontodimopoulos N, Stamatopoulou E, Kletsas G, Kandili A. Disease activity and sleep quality in rheumatoid arthritis: A deeper look into the relationship. Expert Rev Pharmacoecon Outcomes Res 2020;20:595-602.
Knutson KL. Sociodemographic and cultural determinants of sleep deficiency: Implications for cardiometabolic disease risk. Soc Sci Med 2013;79:7-15.
Grandner MA, Patel NP, Gehrman PR, Xie D, Sha D, Weaver T, et al.
Who gets the best sleep? Ethnic and socioeconomic factors related to sleep complaints. Sleep Med 2010;11:470-8.
Jeon M, Dimitriou D, Halstead EJ. A Systematic review on cross-cultural comparative studies of sleep in young populations: The roles of cultural factors. Int J Environ Res Public Health 2021;18:2005.
Hirshkowitz M. Normal human sleep: An overview. Med Clin North Am 2004;88:551-65.
Taylor-Gjevre RM, Gjevre JA, Nair B, Skomro R, Lim HJ. Components of sleep quality and sleep fragmentation in rheumatoid arthritis and osteoarthritis. Musculoskeletal Care 2011;9:152-9.
Katz P. Causes and consequences of fatigue in rheumatoid arthritis. Curr Opin Rheumatol 2017;29:269-76.
Westhovens R, Van der Elst K, Matthys A, Tran M, Gilloteau I. Sleep problems in patients with rheumatoid arthritis. J Rheumatol 2014;41:31-40.
Pehlivan S, Karadakovan A, Pehlivan Y, Onat AM. Sleep quality and factors affecting sleep in elderly patients with rheumatoid arthritis in Turkey. Turkish J Med Sci 2016;46:1114-21.
Grabovac I, Haider S, Berner C, Lamprecht T, Fenzl KH, Erlacher L, et al.
Sleep quality in patients with rheumatoid arthritis and associations with pain, disability, disease duration, and activity. J Clin Med 2018;7:336.
Fragiadaki K, Tektonidou MG, Konsta M, Chrousos GP, Sfikakis PP. Sleep disturbances and interleukin 6 receptor inhibition in rheumatoid arthritis. J Rheumatol 2012;39:60-2.
Abd Elazeem MI, Salem MN. Can rheumatoid arthritis affect sleep in Egyptian patients? Egypt Rheumatol 2018;40:11-6.
Taylor-Gjevre RM, Nair BV, Gjevre JA. Obstructive sleep apnoea in relation to rheumatic disease. Rheumatology (Oxford) 2013;52:15-21.
Taylor-Gjevre RM, Gjevre JA, Nair BV, Skomro RP, Lim HJ. Improved sleep efficiency after anti-tumor necrosis factor α therapy in rheumatoid arthritis patients. Ther Adv Musculoskelet Dis 2011;3:227-33.
Balbo M, Leproult R, Van Cauter E. Impact of sleep and its disturbances on hypothalamo-pituitary-adrenal axis activity. Int J Endocrinol 2010;2010:759234.
Anders MP, Breckenkamp J, Blettner M, Schlehofer B, Berg-Beckhoff G. Association between socioeconomic factors and sleep quality in an urban population-based sample in Germany. Eur J Public Health 2014;24:968-73.
Marco CA, Wolfson AR, Sparling M, Azuaje A. Family socioeconomic status and sleep patterns of young adolescents. Behav Sleep Med 2011;10:70-80.
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]