Background: Previous research suggests that sleep problems may be an important predictor for chronic widespread pain (CWP). With this study we investigated both sleep problems and fatigue as predictors for the onset of CWP over a 5-year and an 18-year perspective in a population free from CWP at baseline. Methods: To get a more stable classification of CWP, we used a wash-out period, including only individuals who had not reported CWP at baseline (1998) and three years prior baseline (1995). In all, data from 1249 individuals entered the analyses for the 5-year follow-up and 791 entered for the 18-year follow-up. Difficulties initiating sleep, maintaining sleep, early morning awakening, non-restorative sleep and fatigue were investigated as predictors separately and simultaneously in binary logistic regression analyses. Results: The results showed that problems with initiating sleep, maintaining sleep, early awakening and non-restorative sleep predicted the onset of CWP over a 5-year (OR 1.85 to OR 2.27) and 18-year (OR 1.54 to OR 2.25) perspective irrespective of mental health (assessed by SF-36) at baseline. Also fatigue predicted the onset of CWP over the two-time perspectives (OR 3.70 and OR 2.36 respectively) when adjusting for mental health. Overall the effect of the sleep problems and fatigue on new onset CWP (over a 5-year perspective) was somewhat attenuated when adjusting for pain at baseline but remained significant for problems with early awakening, non-restorative sleep and fatigue. Problems with maintaining sleep predicted CWP 18 years later irrespective of mental health and number of pain regions (OR 1.72). Reporting simultaneous problems with all four aspects of sleep was associated with the onset of CWP over a five-year and 18-yearperspective, irrespective of age, gender, socio economy, mental health and pain at baseline. Sleep problems and fatigue predicted the onset of CWP five years later irrespective of each other. Conclusion: Sleep problems and fatigue were both important predictors for the onset of CWP over a five-year perspective. Sleep problems was a stronger predictor in a longer time-perspective. The results highlight the importance of the assessment of sleep quality and fatigue in the clinic.
Keywords: Musculoskeletal pain; Insomnia; CWP; Prospective study; Longitudinal study; Population study
Experiencing musculoskeletal pain is common [[
One way of classifying musculoskeletal pain is by the number and localization of pain-sites. Localized pain is rather uncommon [[
Population-based studies have described a CWP-prevalence of 10–17% among adult men and women [[
Sleep problems are common among individuals reporting musculoskeletal pain. The causal sleep-pain relationship is complex and reciprocal [[
CWP, where pain is present above and below the waist, on the right and left side of the body and the axial skeleton (according to the ACR 1990 criteria for fibromyalgia [[
Chronic fatigue and CWP is known to common co-occur, and the disorders have been suggested to at least partly share pathogenetic pathways [[
Prospective studies of sleep and CWP share some methodological issues to handle. Musculoskeletal pain conditions, including CWP, are recurrent and regardless of using an established definition (e.g. ACR 1990 criteria for fibromyalgia), you will end up with some "border-line-cases" who move in and out of fulfilling criteria for CWP. This migration in and out of CWP has previously been reported, where almost half of those with CWP at baseline no longer reported CWP at one- or three-year follow-up [[
The aim of this population-based study was to investigate if sleep problems and fatigue predicted the onset of CWP over a 5-year and up to 18- year perspective in a cohort who had not reported CWP 3 years prior to baseline.
This study is based on data from the EPIPAIN study, a prospective population study that was initiated in 1995 in order to investigate the prevalence and risk factors for long term musculoskeletal pain in south of Sweden. The target population for EPIPAIN was all of the 70,704 inhabitants aged 20–74, living in two municipalities and healthcare districts on the southern west coast of Sweden. For inclusion to EPIPAIN, a representative sample of subjects was selected from the official computerised population register. The register is categorised by date of birth, and the selection of subjects was made by choosing every 18th man and women respectively for each of the municipalities. A postal questionnaire was sent out in May 1995 to the 3928 selected individuals. After two postal reminders, 2425 subjects had responded to the postal survey in 1995.
Follow-up questionnaires were sent out to the subjects in 1998, 2003 and 2016. The cohort included in this study was formed out of the 1922 subjects who responded the survey in 1995 and 1998. Baseline was set to 1998 in this study with a three-year wash-out period between 1995 and 1998. Only individuals who had not reported CWP in 1995 and 1998 were included in the study. The chosen times to follow-up were 5 years (year 2003); and 18 years (year 2016).
Chronic musculoskeletal pain was assessed by an overall key question: Have you experienced pain lasting more than 3 months during the last 12 months? An introduction to the question explained that the pain should be persistent or regularly recurrent in the musculoskeletal system. Pain was considered to be chronic if it had been persistent or recurrent for more than 3 months during the last 12 months.
In addition, if chronic pain was reported, the location and distribution of the pain was reported by a manikin with 18 predefined regions. Head and abdomen were not included amongst the predefined regions [[
A distinction was made between chronic regional pain (CRP) and chronic widespread pain (CWP) according to the ACR 1990 criteria for fibromyalgia [[
Subjects who did not report any chronic pain were labelled as "no chronic pain" (NCP).
Problems related to sleep were assessed by four items adopted from the Uppsala Sleep Inventory (USI), which has been used in several previous epidemiological studies [[
The problems were recorded on a five-point Likert scale: [[
The sleep problems were treated as four separate variables of sleep, 1) Difficulties initiating sleep; 2) Difficulties maintaining sleep; 3) Early morning awakening; and 4) Non-restorative sleep.
Fatigue was estimated by the vitality subscale from the SF-36 questionnaire [[
Mental health [[
Number of (chronic) musculoskeletal pain sites were reported by a manikin, as described above, with eighteen possible regions.
Mental Health was assessed by the subscale MH from SF-36, including five items referring to the last 4 weeks: How much of the time during the last 4 weeks have you... 1) been a very nervous person?; 2) felt so down in the dumps that nothing could cheer you up?; 3) felt calm and peaceful; 4) felt downhearted and blue?; 5) been a happy person? Three levels of mental health (poor, intermediate, good) was constructed by dividing the included cohort's scoring into tertiles. "good mental health" then represent scores between 93 and 100; "intermediate mental health" scores between 84 and 92; and "poor mental health" scores between 0 and 83.
Socio economy as measured by Socio economic groups according to Statistics Sweden was based on self-reported occupation in 1995. Four groups of socio-economic classes were formed; Manual workers, assistant non-manual employees, intermediate/higher non-manual employees and others (including self-employed, farmers, housewives, students).
All study participants signed an informed consent before entering the study. The study was approved by the Regional Ethical Review Board, Faculty of Medicine, University of Lund, Sweden (Dnr LU 389–94; Dnr 2016/132).
The included cohort and the subjects that had been excluded due to reporting CWP in 1995 and/or 1998 were compared with respect to age, gender, the four sleep parameters, fatigue, number of pain regions, mental health and socio economy (based on type of occupation) at baseline. The prevalence of exposed to each sleep problem and fatigue respectively within the different categories of gender, pain regions, mental health and socio economy were compared. Pearson's chi-square test and independent t-test were used in the analyses for difference.
Odds ratios for CWP were calculated using binary logistic regression analysis. The effect of the four different sleep problems and fatigue were tested separately, for each year of follow-up.
Firstly, the four different sleep parameters and fatigue were tested in separate models in order to see if the different sleep problems or fatigue were of different importance. The predictors and the potential confounders were tested in binary logistic regression analysis, adjusting for age and gender. Multivariate analyses were then performed. Due to the complex relationship between pain, sleep and mental ill health [[
Secondly, the independent effect of sleep and fatigue on CWP was tested, and the two variables were included in the same model. To get one sleep variable instead of four separate sleep variables, sleep problems were categorized into having 0–4 of the (
All logistic regression analyses were performed twice, one for CWP at the five-year follow-up, and once for CWP at the 18-year follow-up.
The analyses were performed with the SPSS 21 Statistical Package.
In all, 1922 subjects responded to the survey in 1995 and 1998. Missing data on pain questions were found in 49 cases in 1995, and 22 cases in 1998. In all, 1852 subjects had data from both 1995 and 1998 and could thus enter the analysis. Among these, 340 individuals had reported CWP in either 1995 or 1998 and were excluded from the analysis.
The excluded subjects were significantly older, were more likely to be female, to have problems with sleep and differed in socio economy from the included individuals. Further, the excluded individuals generally rated higher fatigue, poorer mental health and more pain severity. See Table 1.
Presenting baseline characteristics of the cohort with baseline stable NCP or CRP and individuals excluded from the study due to reporting CWP in − 95 and/or − 98. Prevalence of baseline exposures and covariates in included and excluded subjects, and history of CWP in excluded individuals. Significance of differences between included and excluded subjects are presented as p -values (tested with t-test and chi 2 -test respectively)
Included cohort (stable NCP or CRP) (n = 1512) Excluded subjects (CWP in 1995 and/or 1998) (n = 340) p-value for diff. Incl. vs excl. Age (1998) < 0.01 Mean (sd) 49 (15) 56 (13) Gender; n (%) <.01 Male 729 (48) 111 (33) Female 783 (52) 229 (67) Initiating sleep; n (%) <.01 No Problem 1211 (81) 175 (53) Problem 282 (19) 158 (47) Maintaining sleep; n (%) <.01 No Problem 1024 (69) 109 (33) Problem 461 (31) 220 (67) Early awakening; n (%) <.01 No Problem 1149 (78) 145 (44) Problem 331 (22) 186 (56) Non restorative sleep; n (%) <.01 No Problem 1075 (73) 123 (37) Problem 404 (27) 207 (63) Fatiguea <.01 Mean (sd) 72 (21) 44 (25) Number of Regions <.01 0 1096 (73) 42 (12) 1–2 204 (13) 10 (3) 3–5 177 (12) 80 (24) 6–11 35 (2) 152 (45) 12–18 0 56 (17) Mental Healthb <.01 Mean (sd) 84 (17) 69 (23) Socio economyc <.01 High non manual work 440 (29) 48 (14) Ass non manual 212 (14) 55 (16) Manual 664 (44) 196 (58) Others 196 (13) 41 (13) CWP history at baseline CWP in −95 only – 103 (30) CWP in −98 only – 101 (30) CWP in both −95 and − 98 – 136 (40)
Out of the 1512 individuals who fulfilled the inclusion criteria, 1249 had responded to the pain items in 2003, and were thus eligible for the analysis referring to the 5-year follow up; and 791 responded to the items in 2016 and were thus eligible for the analysis referring to the 18-year follow-up. See flow-chart in Fig. 1.
Graph: Fig. 1 Flow-chart showing participation in the study
Out of the included subjects, 89 (7%) had CWP in the 5-year follow-up and 103 (13%) had CWP in the 18-year follow-up. There was no difference in proportion of men and women reporting CWP in the 5-year follow up (8% vs 6%, p = 0.14). However, there were significantly larger proportion of females than men reporting CWP in the 18-year follow-up (16% vs 10%, p = 0.01). There was a significantly larger proportion who reported CWP 5- and 18 years later among those with sleeping problems and fatigue compared to those without sleeping problems and fatigue at baseline. See Table 2. The same was seen for differences in number of pain regions and level of mental health. Data not shown.
Presenting prevalence of baseline sleep problems and fatigue. Cross tabulation of baseline sleep problems and CWP at respective follow-up. Differences between no CWP and CWP cases in respective year for follow-up have been tested with chi 2 -test
5-year follow-up(N = 1249) 18-year follow-up (N = 791) No CWP CWP p No CWP CWP p N = 1160 N = 89 N = 688 N = 103 Initiating sleep; n (%) < 0.01 < 0.01 No Problem 950 (95) 53 (5) 580 (89) 70 (11) Problem 198 (85) 34 (15) 103 (76) 32 (24) Maintaining sleep; n (%) < 0.01 < 0.01 No Problem 811 (95) 41 (5) 500 (91) 52 (9) Problem 330 (88) 46 (12) 183 (78) 50 (22) Early awakening; n (%) < 0.01 < 0.01 No Problem 909 (95) 49 (5) 556 (89) 70 (11) Problem 228 (86) 37 (14) 124 (80) 32 (20) Non restorative sleep; n (%) < 0.01 < 0.01 No Problem 856 (95) 42 (5) 525 (90) 56 (10) Problem 283 (87) 44 (13) 159 (78) 45 (22) Fatigue; n (%) < 0.01 < 0.01 Low 455 (97) 14 (3) 285 (91) 27 (9) Intermediate 355 (94) 21 (6) 230 (90) 26 (10) High 346 (87) 53 (13) 173 (78) 50 (22)
One fourth of those who reported all four sleep problems (concurrently) at baseline had CWP 5 years later, and one third had CWP 18 years later. Among those reported none of the sleeping problems at baseline, 4% reported CWP 5 years later and 8% 18 years later.
In all, 769 individuals had data on pain at all three time points. Looking at changes in pain between the 5-year follow-up and the 18-year follow-up, 59% stayed in the same pain group (NCP, CRP or CWP). Out of the 52 who reported CWP in the 5-year follow-up (and who had data also at the 18-year follow-up), 56% (n = 29) reported CWP also in the 18-year follow-up. In all, 100 participants reported CWP in the 18-year follow-up. See Fig. 1.
As presented in Fig. 2, there was a significantly higher proportion of females reporting problems with initiating sleep and problems with maintaining sleep at baseline. No gender differences were seen for problems with early awakening, non-restorative sleep or fatigue.
Graph: Fig. 2 Presenting baseline data. The figure presents the percentage within each category of 1) Gender; 2) Number of pain regions and 3) Mental health, reporting problems with: Initiating sleep, Maintaining sleep, Early awakening, Non-restorative sleep, and Fatigue respectively. * p for difference < 0.05
Graph: Fig. 3 Describing migration between the pain groups over time. The figure describe migration between reporting No Chronic Pain (NCP); Chronic Regional Pain (CRP) and Chronic Widespread Pain (CWP) between the 5-year follow-up and 18-year follow-up. Including the (N = 769) with data on pain in both 5-year follow-up and 18-year follow-up
There were a larger proportion among individuals reporting more number of pain regions who reported problems with sleeping (significant for all sleep parameters) and fatigue. The same trend was seen for mental health, where subjects reporting poor mental health were more likely to also report problems with sleep (all sleep parameters) and fatigue.
After adjusting for age and gender, all sleep parameters, fatigue (SF-36 Vitality), number of pain regions and mental health (SF-36) predicted the onset of new CWP in the 5-year- and 18-year follow-up. Being a manual worker in 1995 predicted the onset of new CWP in the 5-year follow-up. See Table 3.
Presenting results from univariate model (adjusted for age and gender). Odds ratio (OR), 95% confidence intervals (95% CI) and p-value (p) for reporting CWP in the 5-year and 18-year follow-up respectively. Number of individuals per category of exposure at baseline are presented as N for the 5-year follow-up and 18-year follow-up respectively
N CWP 5-year OR (95% CI) p N CWP 18-year OR (95% CI) p Gender Male 586 1 345 1 Female 663 1.44 (0.92–2.24) .107 446 1.77 (1.14–2.75) .011 Initiating sleepa No problem 1003 1 650 1 Problem 232 2.82 (1.77–4.50) <.001 135 2.49 (1.55–4.00) <.001 Maintaining sleepa No problem 852 1 552 1 Problem 376 2.53 (1.61–3.98) <.001 233 2.74 (1.75–4.28) <.001 Early awakeninga No problem 958 1 626 1 Problem 265 2.83 (1.79–4.46) <.001 156 1.97 (1.23–3.15) .005 Non-restorativea No problem 898 1 581 1 Problem 327 3.15 (2.02–4.92) <.001 204 2.62 (1.70–4.04) <.001 Fatigueb Low 469 1 312 1 Intermediate 376 1.99 (1.00–3.99) .051 256 1.20 (0.68–2.11) .533 High 399 4.93 (2.69–9.05) <.001 223 2.96 (1.78–4.92) <.001 Pain regions 0 904 1 571 1 1–2 167 1.61 (0.78–3.32) .202 113 3.03 (1.72–5.31) <.001 3–5 146 7.40 (4.40–12.42) <.001 89 5.33 (3.05–9.31) <.001 6–11 32 12.40 (5.50–28.02) <.001 18 13.38 (4.99–35.89) <.001 Mental healthc Good 365 1 234 1 Intermediate 487 1.89 (0.96–3.73) .065 326 1.37 (0.77–2.45) .288 Poor 388 3.87 (2.05–7.34) <.001 229 2.70 (1.53–4.75) .001 Socio economyd High non manual 384 1 285 1 Ass non manual 178 1.98 (0.93–5.21) .076 107 1.17 (0.59–2.96) .660 Manual 537 2.74 (1.51–4.94) .001 321 1.45 (0.89–2.36) .136 Others 150 1.45 (0.61–3.55) .395 78 1.25 (0.58–2.68) .574
Over a five-year perspective, all sleep parameters and fatigue predicted the onset of CWP, irrespective of age, gender, mental health and socio-economy (model 1). When adjusting for number of pain sites at baseline instead of mental health (model 2), fatigue and all sleep parameters except maintaining sleep predicted the onset of CWP. Finally, adjusting for age, gender, socio-economy, mental health and number of pain sites (model 3), none of the sleep parameters or fatigue significantly predicted the onset of CWP 5 years later.
Over the 18-year perspective, fatigue and all sleep parameters except early awakening significantly predicted CWP when adjusting for age, gender, socio-economy and mental health. However, when including number of pain sites at baseline in the model separately (model 2) and simultaneously with mental health (model 3), maintaining sleep was the only parameter that remained significant in the 18-year follow-up.
Results are presented in Table 4.
Results from multivariate analysis. Presenting the effect of difficulties initiating sleep, difficulties maintaining sleep, early morning awakening, non-restorative sleep and fatigue on the odds ratio (OR) for reporting CWP in the 5-year and 18-year follow-up respectively. The OR, 95% confidence intervals (95% CI) and p-value (p) are presented in model 1–3. N = 1249 entered the 5-year follow-up analyses; N = 791 entered in the 18-year follow-up analyses
Adjusted for age gender, mental healthc, socio-economyd Adjusted for age, gender, socio-economydand number of pain sites Adjusted for age, gender, socio-economyd, mental healthc and number of pain sites CWP 5-year CWP 18-year CWP 5-year CWP 18-year CWP 5-year CWP 18-year OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI) Initiating sleepa No Problem 1 1 1 1 1 1 Problem 1.91 (1.16–3.14)* 1.93 (1.18–3.18)* 1.71 (1.03–2.85)* 1.68 (1.00–2.81) 1.37 (0.81–2.34) 1.51 (0.89–2.57) Maintaining Sleepa No problem 1 1 1 1 1 Problem 1.85 (1.14–3.01)* 2.25 (1.40–3.61)* 1.60 (0.98–2.63) 1.88 (1.16–3.05)* 1.32 (0.79-2.21) 1.72 (1.05–2.84)* Early awakeninga No problem 1 1 1 Problem 2.00 (1.23–3.27)* 1.54 (0.94–2.54) 1.71 (1.04–2.83)* 1.32 (0.79–2.21) 1.45 (0.86–2.43) 1.18 (0.69–2.01) Non-restorativea No problem 1 1 1 1 Problem 2.27 (1.37–3.75)* 2.04 (1.26–3.29)* 1.90 (1.15–3.13)* 1.57 (0.97–2.56) 1.51 (0.88–2.58) 1.39 (0.83–2.34) Fatigueb Low 1 1 1 1 1 1 Intermediate 1.75 (0.83–3.66) 1.09 (0.60–2.00) 1.71 (0.84–3.45) 1.05 (0.59–1.89) 1.48 (0.70–3.15) 0.97 (0.52–1.79) High 3.70 (1.76–7.84)* 2.36 (1.24–4.50)* 2.59 (1.34–3.47)* 1.62 (0.92–2.85) 1.93 (0.87–4.26) 1.34 (0.69–2.68)
In all, 785 individuals did not report any of the sleeping problems at baseline (fatigue not included), 268 reported one of the problems, 167 two, 128 three and 117 subjects reported to have all four sleep problems.
In a multivariate model (Model A) including age, gender, mental health, socio-economy and number of sleep problems, reporting four sleep problems, reporting poor mental health and the socio-economy-parameter manual work were all associated with the onset of CWP 5 years later. Adding fatigue to the model showed that sleep problems, fatigue and manual work predicted CWP independently from each other, however mental health did not remain significant in the model. When, instead, including number of pain sites to Model A, reporting four sleep problems, reporting at least three pain-sites and manual work predicted the onset of CWP. Reporting at least three pain sites at baseline and manual work were the only parameters that remained significant when adding also fatigue to the model. See Table 5.
Presenting odds ratios (OR) and 95% confidence intervals (95% CI) for CWP 5 years later. Model A includes age, gender, mental health, socio economy and number of sleep problems
Model A Model A + fatigue Model A + number of pain regions Model A + number of pain regions and fatigue OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI) No of sleep problems 0 1 1 1 1 1 1.13 (0.57–2.27) 1.00 (0.50–2.03) 0.93 (0.45–1.89) 0.88 (0.43–1.80) 2 1.01 (0.44–2.32) 0.84 (0.36–1.96) 0.73 (0.31–1.72) 0.68 (0.29–1.62) 3 1.71 (0.77–3.81) 1.39 (0.62–3.15) 1.14 (0.49–2.63) 1.05 (0.45–2.45) 4 4.00 (2.03–7.91)** 3.06 (1.51–6.21)** 2.18 (1.04–4.56)* 1.99 (0.93–4.24) Fatiguea Low 1 1 Intermediate 1.62 (0.76–3.45) 1.47 (0.69–3.16) High 2.68 (1.21–5.96)* 1.59 (0.68–3.71) Pain regions at baseline 0 1 1 1–2 1.39 (0.64–3.02) 1.34 (0.62–2.91) 3–5 4.89 (2.73–8.76)** 4.53 (2.47–8.30)** > 6 8.24 (3.37–20.15)** 7.78 (3.13–19.26)** Mental healthb Good 1 1 1 1 Intermediate 1.60 (0.79–3.23) 1.26 (0.60–2.66) 1.42 (0.70–2.90) 1.23 (0.58–2.62) Poor 2.41 (1.17–4.93)* 1.42 (0.62–3.24) 1.93 (0.93–4.02) 1.54 (0.67–3.56) Socio economy (work)c High non-manual 1 1 1 1 Ass non-manual 1.69 (0.75–3.82) 1.69 (0.75–3.84) 1.87 (0.81–4.32) 1.86 (0.80–4.28) Manual 2.75 (1.47–5.13)** 2.73 (1.46–5.12)** 2.34 (1.23–4.44)* 2.34 (1.23–4.46)* Other 1.40 (0.56–3.49) 1.35 (0.54–3.39) 1.27 (0.50–3.27) 1.26 (0.49–3.23)
Over an 18-year perspective, sleep problems were stronger independent predictor for CWP. Out of the included parameters, only sleep problems and number of pain regions at baseline significantly and independently, predicted CWP. See Table 6.
Presenting odds ratios (OR) and 95% confidence intervals (95% CI) for CWP 18 years later. Model A includes age, gender, mental health, socio economy and number of sleep problems
Model A Model A + fatigue Model A + number of pain regions Model A + number of pain regions and fatigue OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI) No of sleep problems 0 1 1 1 1 1 1.82 (1.00–3.34) 1.73 (0.93–3.20) 1.57 (0.84–2.95) 1.57 (0.83–2.96) 2 1.38 (0.65–2.91) 1.27 (0.59–2.71) 1.03 (0.48–2.22) 1.02 (0.47–2.23) 3 2.44 (1.19–5.00)* 2.17 (1.03–4.56)* 1.78 (0.84–3.80) 1.75 (0.81–3.82) 4 3.95 (1.90–8.20)** 3.33 (1.55–7.13)** 2.36 (1.06–5.23)* 2.29 (1.01–5.18)* Fatiguea Low 1 1 Intermediate 0.96 (0.51–1.79) 0.89 (0.47–1.67) High 1.79 (0.89–3.60) 1.13 (0.54–2.37) Pain regions at baseline 0 1 1 1–2 2.83 (1.57–5.12)** 2.76 (1.52–5.02)** 3–5 4.03 (2.20–7.37)** 3.84 (2.05–7.20)** > 6 8.63 (3.05–24.43)** 8.24 (2.88–23.63)** Mental healthb Good 1 1 1 1 Intermediate 1.22 (0.67–2.25) 1.15 (0.61–2.18) 1.21 (0.65–2.24) 1.21 (0.63–2.31) Poor 1.75 (0.92–3.13) 1.25 (0.59–2.66) 1.36 (0.70–2.63) 1.27 (0.59–2.72) Socio economy (work)c High non-manual 1 1 1 1 Ass non-manual 1.22 (0.61–2.47) 1.25 (0.62–2.54) 1.21 (0.58–2.49) 1.22 (0.59–2.53) Manual 1.49 (0.90–2.48) 1.50 (0.90–2.51) 1.40 (0.83–2.38) 1.41 (0.83–2.38) other 1.24 (0.56–2.77) 1.26 (0.56–2.83) 1.29 (0.56–3.00) 1.31 (0.56–3.04)
The aim of this study was to investigate if sleep problems and fatigue predict the onset of CWP five- and 18 years later. The results from this study indicate that in a cohort free from CWP at baseline, difficulties initiating sleep, maintaining sleep, early morning awakening, non-restorative sleep and fatigue respectively predicted the onset of CWP in a 5-year perspective irrespective of age, gender, socio-economy and mental health. This was true also over an 18-year perspective, except for the sleep parameter early morning awakening. When adding number of pain sites in the model none of the sleep parameters or fatigue predicted the onset of CWP in the 5-year follow-up. However, problems with maintaining sleep consistently predicted the onset of CWP 18 years later in all models.
When adding all sleep problems together, receiving a parameter measuring 0–4 sleep problems, sleep problems predicted the onset of CWP 5 years later irrespective of age, gender, mental health and socio-economy. Further, sleep problems and fatigue predicted the onset of CWP 5 years later irrespective of each other. However, in a full model including the parameters mentioned above plus sleep problems, fatigue and number of pain sites, only ≥3 pain sites at baseline and having manual work at baseline significantly predicted the onset of CWP 5 years later. Over an 18-year perspective however, reporting 4 sleep problems at baseline and reporting pain at baseline (at least 1–2 pain sites) were the only predictors that remained significant when included in a full multivariate model.
The over-all results, that sleep is a predictor for CWP, is in line with what previous studies have found [[
The mechanistic relationship between sleep problems and chronic pain is not yet clear. One suggested mechanism is that insufficient sleep alters the processes of pain habituation and sensitization, and increase vulnerability to chronic pain [[
Using the wash-out period, allowing only those who had not reported CWP both at baseline and 3 years prior to baseline, we attended to capture new onset CWP. This is however more likely to be true for the results found in the 5-year follow-up than the 18-year follow-up. One of the reasons for why sleep problems predict CWP also over longer time periods may be that sleep problems [[
In this study we found that difficulties maintaining sleep, and possibly difficulties initiating sleep, seems to be stronger independent predictors for CWP in a long-term perspective (18 years) than in a 5-year perspective. The opposite trend was seen for early morning awakening, non-restorative sleep and fatigue. Other studies of sleep and CWP that has specified single component of sleep in their analysis [[
We chose to include mental health and number of pain regions in separate models in the analysis. Pain, sleep problems and mental health are known to commonly co-occur. It was not under the scope of this study to unravel the different pathways through which sleep, pain, fatigue and mental health interact. However, knowing that the relationship is complex, we wanted to be as transparent as possible when exploring the effect of sleep and fatigue on CWP. A recent review [[
Having had pain previously is an important predictor for reporting more pain sites at follow-up [[
Previous population-based studies have established a higher CWP-prevalence among women than men [[
There are some methodological issues to be considered when interpreting the results from this study. The results rely upon self-reported data from questionnaires. Individuals classified as CWP in this study may have differed if they were diagnosed by a physician. By the criterion used in this study, subjects could have reported up to 11 sites without fulfilling the criteria for CWP. One could fulfil the CWP criteria if reporting pain from only three sites, thus the individuals classified as CWP may be heterogeneous. This approach for classifying CWP [[
The parameter indicating socio economy may have some problems. The classification was made based on occupation according to a classification system by Statistics Sweden, 1982. In 1998, when data was assessed from which the socio-economy parameter was classified in this study, this classification was considered accurate to use. We chose to use this parameter in this study to get an idea of the impact of socio economy and/or type of work, although we are aware it is not a perfect estimation.
Another issue worth mentioning is the repeated analysis, due to our decision to perform separate analysis in models including mental health and number of pain sites, separately and simultaneously. The multiple analysis increases the risk for false positive associations. In the age-gender adjusted analysis (presented as crude analysis) of the predictors, the p-values were very low (below 0.001). This imply that there is a very small risk for false positive results in the "crude" relationship between the investigated predictors and CWP. However, as moving on and analysing the predictors in several models, with less convincing p-values, the risk for false positive results increase. This is a limitation with the study.
Another issue is the potential nonparticipation bias. In the five-year follow-up, the response rate was 90% and in the 18-year follow-up, the response rate was 63%. Especially the response rate of the latter follow-up could potentially have caused bias. However, in comparison to other studies with this long-term follow-up, the response rate is within a range one would expect.
This prospective populations study showed that all the investigated sleep problems, (initiating sleep, maintaining sleep, early awakening and non-restorative sleep) as well as fatigue are important predictors for the future development of CWP both over a 5-year, and 18-year perspective. Problems with maintaining sleep was a weaker predictor in the five-year follow-up but was the only sleep parameter that predicted the onset of CWP over an 18-year perspective irrespective of both mental health and number of pain sites.
Reporting all four sleep problems simultaneously was a strong predictor for CWP 5 years- and 18-years later. Fatigue predicted the onset of CWP 5 years later irrespective of sleep problems, age, gender, socio-economy and mental health, but not independently from pain sites. The results from this study are in line with what previous prospective studies have shown, and with this study we add knowledge of the importance of fatigue as predictor over a shorter perspective. This highlights the importance of the assessment of sleep quality and fatigue in the clinic. This study suggests that the fatigue-CWP association is explained by number of pain sites (and possibly disturbed pain systems) rather than mental health or sleep problems. This study also suggests that sleep problems may indicate a vulnerability to chronic pain at an earlier stage.
KA and SB have taken the lead of the work with the study and has shared the responsibility for the integrity of the study as a whole and for taking the progress forward with analysing data and writing the manuscript. MA, AB, EH and IL has actively contributed to the work of formulating the conception and design, data assessments, analysing and interpretation of data and critical review of the manuscript. All authors have approved the final version of the article.
All study participants signed an informed consent before entering the study. The study was approved by the Regional Ethical Review Board, Faculty of Medicine, University of Lund, Sweden (Dnr LU 389–94; Dnr 2016/132).
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The authors declare that they have no competing interests.
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The Epipain study was supported from a grant from Swedish AFA insurance.
The datasets used and analysed during the current study is available from the corresponding author on reasonable request.
By Katarina Aili; Maria Andersson; Ann Bremander; Emma Haglund; Ingrid Larsson and Stefan Bergman