Background
Sleep bruxism (SB) is defined as a 'stereotyped movement disorder characterized by grinding or clenching of the teeth during sleep usually associated with sleep arousal' [
1]. It might lead to abrasive tooth wear, hypermobility of teeth, tooth hypersensitivity, hypertrophy of the masticatory muscles and pain in the masticatory muscles [
2]. There are no gender differences in SB. SB can also be found in children [
3,
4], but the age distribution reveals a larger incidence in individuals between 20 to 45 years of age [
1,
5].
Presently, the etiology of SB is not well defined. Different etiological factors have been investigated, e.g. occlusal interferences [
6,
7], transient sleep arousal episodes [
8‐
10], a side imbalance in striatal D2 receptor binding [
11,
12], personality traits [
5,
13‐
16], psychosocial factors [
17‐
19] and psychological stress [
17,
18,
20‐
22]. At the same time, the multifactorial nature of SB is widely accepted [
5,
12,
23‐
27].
Several authors have investigated stress as one of the causal agents of SB. In 1975
Rugh and Solberg reported that SB seemed to appear after days which were exhausting and stressful [
28]. In an epidemiological study on British, German and Italian population samples, self-reported SB was also positively associated with a highly stressful lifestyle [
21]. In another research on 1339 employees of a Finnish broadcasting company, frequent bruxism was significantly associated with severely stressful situations at work. Furthermore, frequent bruxism was significantly positively associated with the number of occupational health care and dental visits. It was concluded that bruxism may reveal ongoing stress in normal work life [
22]. In a follow-up study of 30-50 year-old employees (n = 211) of the Finnish broadcasting company it could be reconfirmed that psychosocial factors and perceived stress should not be ignored [
17]. In the same study it could also be shown that smoking was significantly positively associated with frequent bruxism. It was concluded that tobacco use may both amplify the patient's pain response and provoke bruxism [
17]. Another epidemiological study examined the relationship between psychosocial job stress and SB in a Japanese population of 1944 male and 736 female factory workers. The study found that SB was weakly associated with some aspects of job stress in men among the Japanese working population [
18].
Two further studies demonstrated an association between SB and an overtly ambitious character or behavior (Type A), which in turn is related to a stressful life [
14,
15]. A psychometric study found a significantly higher stress perception in bruxers compared to healthy controls [
29]. Studies on urinary catecholamines in bruxers, indicating stressful states, detected a significant association of urinary epinephrine and dopamine with bruxism in children [
30], as well as a positive relationship between increased urinary epinephrine and high levels of sleep masseter muscle activity [
31]. Animal experiments with rats concerning the relationship between emotional stress and brux-like activity of their masseter muscles have also suggested a positive correlation [
32].
Furthermore
Schneider et al. investigated stress-coping strategies in patients with SB compared to non-bruxing controls. They observed a significant difference in positive coping strategies, which are capable of reducing stress, between the two groups. SB-patients reported significantly less positive coping strategies, like 'reaction control' and 'positive self-instructions'. Based on the above, a deficit of functional coping strategies in SB-patients could be demonstrated [
33].
In contrast to these findings, two other studies showed that there was no relationship between the degree of SB and self-reported stress [
34,
35]. Another study, found no significant differences between bruxers and controls with respect to perceived stress during the previous year [
36].
Overall, the majority of studies suggest an association between stress and SB, although increased SB as a direct consequence of diurnal stress could not be proved. It remains debatable as to which specific stress-factors correlate with SB.
According to the transactional model of stress and coping [
37], stress depends on the impact of an external stressor, which is mediated by the appraisal of the stressor (primary appraisal) on the one hand, and the appraisal of the individual's capabilities to handle the situation (secondary appraisal) on the other hand. Coping represents the actual strategy of an individual to deal with the stressor. Dispositional coping styles are generalized ways of behaving in stressful situations, stable across time and circumstances. However, investigations regarding the correlation between stress-coping and SB are rare.
Therefore, the aim of the present study was to investigate whether specific stress related factors and coping strategies, from different areas of life, correlate with SB. A reliable and concurrently practicable instrument should be used, in order to measure the degree of SB. This allows for a high number of participants to be easily examined.
The literature states numerous methods with different validity and practicability for the assessment of SB-activity: laboratory polysomnographic recordings as the gold standard [
9,
10,
12,
27,
38], portable electromyographic (EMG) recordings [
28,
31,
39], accelerometer systems [
40], a force-based bruxism detection system [
41], questionnaires for self-evaluation of bruxism [
14,
21,
22], dental examinations [
30,
42], examination of stone casts [
43] as well as measuring abrasion on a diagnostic plate, the Bruxcore Bruxism-Monitoring Device (BBMD, Bruxcore, USA) [
44]. Recently, it has been shown that the BBMD, in combination with a newly computer-based analyzing-method, is a reliable and feasible instrument to quantify abrasion as an indication for current SB [
45].
The hypothesis used in this study was that high levels of specific stress related factors are associated with a high level of SB-activity. Additionally, it was hypothesized that handling stress in a non-effective way may lead to high SB-activity.
Discussion
The present results emphasize the assumption that individuals with high SB-activity seem to feel more stressed in their daily life and at work. This is in accordance with an epidemiological study on British, German, and Italian population samples, where a highly stressful life was positively associated with self-reported SB [
21]. Similar results are also provided in a previous study on 1339 employees of a Finnish broadcasting company, which demonstrated that frequent bruxers, regardless of work category, reported more stress. In the same study, frequent bruxism was both significantly positively associated with severe stress experience and with the number of occupational health care and dental visits [
22]. In a follow-up study of 30-50 year-old employees (n = 211) of the Finnish broadcasting company it could be reconfirmed that psychosocial factors and perceived stress should not be ignored [
17]. The assumption of
Ehlert, who describes a correlation between stress in daily life ('daily hassle') and the individuals' health status, concerns a similar theory [
53]. Similarly, in the present study, it may be assumed that stress could influence the subjects' physical state. This is expressed in the statistically significant values for the subscales ‚fatigue’ and ‚physical problems’ of the EBF-24 A/3. In this context the follow-up study mentioned above could play an important role: it could be shown that, among others, smoking was significantly positively associated with frequent bruxism. It was concluded that tobacco use may both amplify the patient's pain response and provoke bruxism [
17]. The factor 'smoking' was however, not controlled for in the present study, so that a possible influence cannot be excluded.
In addition, this study revealed that subjects with high pixel scores do not seem to be able to deal with stress in an adequate way. They seem to prefer negative coping strategies like ‚escape’. This, in general, increases the feeling of stress, instead of looking at the stressor in a positive way.
Schneider et al. who investigated maladaptive coping strategies in individuals with SB compared to non-bruxing controls observed less positive coping strategies in SB-patients and could therefore also demonstrate a deficit of functional coping strategies for the SB-patients [
33].
Similar findings were seen in patients with craniomandibular dysfunction, which demonstrated that these individuals rather daydream, push problems away and do not instruct themselves positively [
54]. Other psychosocial factors like trait anxiety [
36] coincide with their ineffective reactions like resignation and flight.
The stress questionnaires used in the present study represent valid instruments for the acquisition of stress-parameters of a German population [
46‐
49]. As all subjects were German native speakers, it can be assumed that small semantic differences were also understood.
Due to the fact that all questionnaires were used only for the inquiry of subjective current stress-parameters, meaningful results concerning the correlation between two current parameters - stress and current SB - could be expected. The presence of SB was diagnosed clinically according to the AASM [
55] and was done only by one trained dentist, in order to minimize variance. This, however, could be questioned, because clinical criteria like abnormal tooth wear or shiny spots on restorations do not verify the presence of current SB. It could be supposed that the tooth wear had been caused many years ago. Nevertheless, criteria employed in the study, like sleeping partner's report of grinding sounds in combination with the self-report of muscle fatigue or pain on awakening give a clear hint towards current SB-activity. Furthermore, a previous investigation has verified the ability of the BBMD combined with the computer-based analyzing method, used in this investigation, to record current SB-activity [
45].
In summary, several stress-parameters described here, concerning specific stress-factors as well as stress-coping strategies correlated significant with SB. Essentially, the stress-factor 'daily problems' seems to play an important role for increasing SB-activity, expressed by a high significance-level and moderate correlation coefficient r, according to the scale of
Zöfel[
52]. Although causal relationships between stress and SB cannot be concluded due to the descriptive character of the statistical analysis, the present results may be interpreted as a promising hint about an existing relationship between stress and SB. However, since, to date, the etiology of SB remains unclear, a moderate correlation between factors like abrasion on a plate, verified as current SB by the computer-based analyzing method, and stress gives enough ground for future investigations in this field in order to identify possible causal relationships between stress, stress-coping and SB.
Concluding from the above, it is worthwhile to further examine stress as one possible etiological factor for the development of SB in experimental trials, preferably in a sleep laboratory. Of interest would be a study designed to measure stress-parameters, stress-reaction and the SB-activity of different subjects, who are exposed to the same stressor, regularly for a longer, defined observation period. Such a study design would allow identification of the different types of daily life events and factors at work that are believed to be stressful. Furthermore, it would be interesting to investigate the reasons due to which individuals in a similar stress situation feel stressed at times and not at others. In order to understand the parameter 'coping' more precisely, further research needs to be carried out into the following: what leads to the development of positive or negative coping strategies?, to what extent does stress influence and modify the type of coping strategies?, and whether positive coping of daily problems reduces SB-activity. In the same context, it should additionally be examined, whether SB itself could lead to stress and/or maladaptive coping strategies as a result of the chronic dental and/or myofascial disturbance and a possible subsequent helplessness felt by the patient.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
MAO, CS and RS conceived the study design, MG fabricated the BBMDs, performed its analysis as well as the statistical data analysis and wrote the manuscript. CS conducted the psychological part of this study, MAO executed the dental part of this study, PS made the necessary language correction in the manuscript, MF and WHMR participated in the early preparation of the manuscript and contributed to writing the article. All authors have read and approved the final manuscript.