Sleep bruxism and its relationship to sleep habits and lifestyle of elementary school children in Japan

Authors


Dr Sachiko Suwa, Department of Craniofacial Growth and Development Dentistry, Kanagawa Dental College, 82 Inaoka-Cho, Yokosuka, Kanagawa 238-8580, Japan. Email: suwa@kdcnet.ac.jp

Abstract

This cross-sectional study was conducted to investigate the influence of sleep habits, sleep problems, and lifestyle on sleep bruxism in Japanese elementary school children. In study 1, to confirm the reliability of replies provided by parents on children's sleep bruxism, we compared the scale of sleep bruxism frequency from questionnaires with actual sleep bruxism tooth wear in 49 elementary school children (22 males and 27 females). In study 2, a total of 1956 children (973 males, 983 females; mean age, 8.8 ± 1.8 years) and parents living in metropolitan Tokyo participated. Parents answered questionnaires on sleep habits, sleep problems, and lifestyle. Participants were classified as severe bruxism group (s-Bx) and non-severe bruxism group (ns-Bx) according to the frequency of sleep bruxism. The six grades were summarized into three categories: 1st/2nd grades (grades 1/2), 3rd/4th grades (grades 3/4), and 5th/6th grades (grades 5/6). The results suggest that the incidence of sleep bruxism in Japanese children living in Tokyo is strongly related to the disturbance of sleep habits and psychological stress, which can be caused by an owl-type lifestyle in children and parents.

INTRODUCTION

Sleep bruxism is an involuntary mandibular movement with tooth grinding and clenching during sleep.1 It is considered to be an oral parafunctional habit,2 and has been reported in 11–20% of children younger than 12 years.3 Sleep bruxism often causes tooth wear, tooth fracture, tooth hypersensitivity, pain in the masticatory muscles, or masseter muscle hypertrophy in a clinical occasion of dentistry.4 Sleep problems in children, including bruxism, can impair development and are often closely related to problems of childhood psychopathology.3,5 LaBerge et al. reported that high anxiety scores were observed in children aged 3–10 years suffering from parasomnias such as restless legs, night terrors, and sleep bruxism.3 In addition, it has been suggested that psychological stress may be involved in the incidence of sleep bruxism.6

In Japan, considering the modern problems of poor sleep environments and variegation in society, children may be under multiple stresses. However, there have been few epidemiological studies regarding sleep bruxism in children and its related risk factors, such as sleep habits, other sleep problems, lifestyle, and the rearing environment.

Therefore, the present cross-sectional study was conducted to explore the influence of sleep habits, sleep problems, and lifestyle on sleep bruxism in Japanese children attending elementary schools. Because sleep bruxism occurs at night and is difficult for parents to observe, prior to this survey, preliminary research was conducted that included an oral examination and evaluation of actual sleep bruxism to confirm the reliability of the questionnaire.

METHODS

The protocol for this study was approved by the Ethics Committee of Kanagawa Dental College, Japan.

Study 1: Evaluation of the questionnaire regarding bruxism in children

Participants

Participants were residents of Kawaguchiko Town in Yamanashi Prefecture. A total of 77 students aged 6–9 years in the first, second, and third grades of elementary school and their parents were enrolled. Informed consent was obtained from all participants, who were assured of confidentiality. A total of 61 eligible children and their parents agreed to participate in this study giving a response rate of 79%.

Sampling and assessment of sleep bruxism

This survey was carried out in late spring 2006; winter and midsummer were avoided, as climatic or other factors can affect behavior patterns. A questionnaire about the frequency of children's bruxism was completed by parents and collected a few days later in the children's elementary school. Parents were instructed to provide information about the previous month. The frequency of sleep bruxism was assessed by the following item: “Child grinds teeth during sleep”, Parents answered according to a four-point ordinal scale: “frequently”, “sometimes”, “occasionally”, and “never”.

Around the same time of the questionnaire, maxillary impressions were obtained from children. After an examination to determine the stage of tooth eruption, dentists with more than 3 years of clinical experience diagnosed the presence of caries and the relation between the upper and lower jaw. Then we fabricated a device for each child, called the “BruxChecker”, from the maxillary plaster cast. The BruxChecker is a simple device that causes little discomfort and makes it possible to visualize actual tooth wear regions affected by sleep bruxism overnight.7 Parents were instructed on how to use the BruxChecker after brushing teeth and to attach it to the child's upper dentition before going to bed, and to take it off immediately after waking in the morning. The severity of actual facet areas affected on the BruxChecker was scored by using the criteria of Johansson.8 The evaluation was made for six parts of the dental model: bilateral anterior primary teeth, bilateral canine primary teeth, and bilateral molar primary teeth. Each part was visually scored by two dentists with more than 10 years of clinical experience. Parts that were estimated as “0” by the tooth wear scale, that is, parts having no contacts in the enamel, were excluded from this evaluation. Then the sleep bruxism score for each participant was calculated as follows: (total score of ordinal scale for severity of tooth wear by Johansson8)/(number of evaluated parts).

We excluded participants who had many caries, severe gingivitis, and malocclusion that might cause peripheral sleep bruxism. Participants with the most severe score of 4 on Johansson's scale were also excluded because there were no X-ray examinations in this study. In addition, severe dental erosions for whole teeth that were thought to be caused by soft drinks and fruits were differentiated from erosions caused by tooth wear due to sleep bruxism.9 We also excluded participants who were in the later stage of Hellman's dental age III B,10 and undergoing orthodontics treatment. A total of 49 children aged 6–9 years were included in the final analysis (22 males and 27 females). The valid sample rate was 80% (49 of 61). These participants reported no sleep disturbances. The kappa values of the reliability for each tooth wear scale by two dentists ranged from 0.85 to 0.93. We compared the sleep bruxism frequency from questionnaires completed by parents with sleep bruxism scores.

Statistical analysis

Using the three groups classified according to the frequency of sleep bruxism by parental reports, we performed nonparametrical multiple comparison by the Steel–Dwass test for the sleep bruxism scores from tooth wear visualized by the BruxChecker.

Study 2: investigation on the influence of sleep habits, sleep problems, and lifestyle on sleep bruxism

Participants and procedures

Participants were residents of Shinagawa Ward in the Tokyo metropolitan area. A total of 4617 participants studying at 17 elementary schools and their parents were enrolled. All elementary schools were similar in school–home commuting time, school starting/finishing time, and After School Care Programs without parents (ASCP) under the rules of the Shinagawa Ward school committee. All participants and parents obtained informed consent after receiving a complete description of the purpose and procedures of this study; reassurances regarding confidentiality were also provided. A total of 3006 eligible children aged 6–12 and their parents returned the questionnaire regarding sleep habits, sleep problems, and lifestyle 2 weeks after distribution by mail, giving a response rate of 65%. The present survey was conducted in late June to July 2007 to avoid the rainy season, midsummer, and winter, as climatic or other factors can affect behavior patterns.

Measures

A standardized questionnaire on sleep habits, sleep problems, and lifestyle was used in the survey.11,12 Parents were instructed to provide information regarding the usual daily lives of their children over the previous month.

Sleep bruxism.

The frequency of sleep bruxism was assessed by the following item: “Child grinds teeth during sleep”. Parents answered according to a four-point ordinal scale for frequency: “frequently”, “sometimes”, “occasionally”, and “never”.

Sleep habits.

Bedtime and rising time on school nights and non-school nights were used to assess sleep habits. We calculated time in bed (TIB), delay time in bedtime and rising time on weekends, and weekend sleep extensions.

Sleep problems without sleep bruxism.

The frequency of sleep problems without sleep bruxism (other sleep problems) such as sleep start, snoring, and difficulty arising were assessed by the following questions, respectively: “Child suddenly jerks his/her body, legs, or arms”, “Child snores during sleep”, and “Child can get out of bed when it is time to get up.” The frequencies of sleep starts and snoring were assessed with a four-point ordinal scale: “frequently”, “sometimes”, “occasionally”, and “never”. Difficulty arising was assessed by the following responses: “Child can get up easily”, “Child can get up with a little effort”, “Child can get up with considerable effort”, or “Child cannot get up no matter how hard he or she tries”.

Lifestyle, rearing environment, and demographic data.

We also assessed variables related to lifestyle characteristics, including the following: “Skipping breakfast” (no/occasionally/sometimes); “snack after dinner” (no/sometimes/everyday); “cram school or lessons” (no/sometimes/everyday); “television viewing, computer game playing, or Internet use (TV/computer game/Internet)”; (no/<1 h/1–2 h/<3 h/≥3 h); “using ASCP” (no/yes); “employment status of mother” (no job/part-time/flexible work-time/full-time); “frequency of mother or father returning home after 23.00 hours” (<2–3 times per month/≥1–3 times per week).

We excluded 406 participants who had the following disorders under treatment: abnormal blood pressure, cardiac disorder, digestive disorder, breathing disorder, urological diseases, hepatic disease, disorder of the nervous system, disease of the skin, sleep disturbances, autonomic dystonia, and headaches. In addition, participants were also excluded if we had no information regarding age, gender, body height, or weight. A total of 2600 participants had valid data for the present investigation.

Statistical analysis

The six grades were summarized into three categories in consideration of the eruption stage of permanent teeth: 1st/2nd grades (grades 1/2), 3rd/4th grades (grades 3/4), and 5th/6th grades (grades 5/6). During grades 1/2 the eruption stage of anterior permanent teeth occurs, during grades 3/4 the mixed dentition stage occurs, and during grades 5/6 children are right before the permanent dentition stage.10 Each category had a sufficient number of individuals for statistical testing. Children were further classified as severe bruxism group (s-Bx) and non-severe bruxism group (ns-Bx), according to the frequency of sleep bruxism by parental reports. Using these two groups as dependent factors, we performed one-way anova or the student t-test for continuous data on sleep habits and body mass index (BMI). Categorical variables, including gender, the frequency of sleep problems, and lifestyle, were analyzed by Pearson χ2-test or Fisher's exact tests, after the binary conversion of variables.

In order to locate the factors independently associated with the frequency of sleep bruxism, we performed a logistic regression analysis (stepwise forward selection). Independent factors included age and gender, bedtime, and TIB on school nights as representative markers for sleep habits. The frequency of skipping breakfast and snacking after dinner were selected as dietary habits related to regularity of lifestyle. Attending cram school or lessons, TV/computer game/Internet, and using ASCP were selected to be representative of behavior patterns after school. Employment status of the mother and the frequency of a parent returning home after 23.00 hours were selected to be representative of the rearing environment.

Before the analyses, we converted all categorical variables into binary factors: gender (0 = female, 1 = male), frequency of skipping breakfast (0 = no, 1 = occasionally/sometimes), frequency of snacking after dinner (0 = no/sometimes, 1 = everyday), attending cram school or lessons (0 = no/sometimes, 1 = everyday), TV/computer game/Internet (0 ≤ 3 h per day, 1 =≥3 h per day), using ASCP (0 = no, 1 = yes), employment status of mother (0 = no job/part-time/free, 1 = full-time), and frequency of a parent returning home after 23.00 hours (0 ≤ 2–3 times per month, 1 =≥1–3 times per week). Other sleep problems were converted into binary factors using an ordinal scale: sleep starts (high frequency group = never/occasionally, low frequency group = sometimes/often), snoring (high frequency group = never/occasionally, low frequency group = sometimes/often), and difficulty arising (facility group =“Child can get up easily”, “Child can get up with a little effort”; difficulty group =“Child can get up with considerable effort”, “Child cannot get up no matter how hard he/she tries”). Sleep habits such as TIB/bedtime on school nights and other sleep problems such as sleep starts, snoring, and difficulty arising have been adjusted by this logistic model, because they were confounding factors.

The adjusted odds ratio (OR) and 95% confidence intervals (CI) of each significant factor are presented to show the association. We conducted the statistical analysis using SPSS 16.0 software (SPSS, Tokyo, Japan). The level of statistical significance was set at P < 0.05.

RESULTS

Study 1: Evaluation of the questionnaire regarding bruxism in children

The frequency of sleep bruxism by parental reports was “frequently” (n= 1, 2%), “sometimes” (n= 5, 10.2%), “occasionally” (n= 13, 26.6%), and “never” (n= 30, 61.2%). We defined “frequently” and “sometimes” as “severe bruxism group (s-Bx)”, “occasionally” as “moderate bruxism group (moderate-Bx)”, and “never” as “non-severe bruxism group (ns-Bx)”. Table 1 shows sleep bruxism scores according to the above sleep bruxism criteria. Sleep bruxism scores were significantly higher in the s-Bx group than in the ns-Bx group (P < 0.05). There were no significant differences between the moderate-Bx group and other groups. Moreover, the moderate-Bx group had various depths of tooth wear compared with the other groups by oral examination with dentists and BruxChecker. This result indicates that the reliability of the questionnaire increased by excluding the moderate-Bx group, which was defined as children whose parents reported that they “occasionally” experienced sleep bruxism. Thus, it was considered that sufficiently reliable data could be obtained for the s-Bx and ns-Bx groups.

Table 1.  Sleep bruxism score according to frequency of children's sleep bruxism by parental report
Sleep bruxism criteriaN(%)Mean
Bruxism score±SD
  • *

    Significant difference by multiple comparison by Steel–Dwass test at P < 0.05. Significant difference was observed between “severe” and “non-severe”. Data include the percentage, means, and SD.

Severe6(12.2)2.19*0.37
Moderate13(26.6)1.770.50
Non-severe30(61.2)1.58*0.34
Total49(100)1.700.43

Study 2: Investigation on the influence of sleep habits, sleep problems, and lifestyle on sleep bruxism

Table 2 shows demographic data for the s-Bx and the ns-Bx groups in each grade category. A total of 1956 (973 males, 983 females; mean age, 8.8 ± 1.8 years) were included in this study. The s-Bx group included 355 participants (191 males, 164 females; mean age, 8.3 ± 1.7 years) and the ns-Bx group included 1601 participants (782 males and 819 females; mean age, 8.9 ± 1.8 years). The 644 participants with occasional bruxism were excluded from this analysis. Table 3 shows the results of one-way anova for bedtime and rising time on school nights and non-school nights, together with calculated TIB, weekend bedtime and rising time delay, and weekend sleep extensions in the s-Bx and the ns-Bx groups. Table 4 shows the frequency of sleep problems and the related factors for the s-Bx and the ns-Bx groups in each grade category.

Table 2.  Demographic characteristics according to the frequency of children's sleep bruxism in each grade category
VariablesGrades 1/2, n= 680Grades 3/4, n= 635Grades 5/6, n= 641
s-Bxns-BxGroup differences-Bxns-BxGroup differences-Bxns-BxGroup difference
n(%)n(%)χ2/F-valuesP-valuesn(%)n(%)χ2/F-valuesP-valuesn(%)n(%)χ2/F-valuesP-values
Total167(24.6)513(75.4)  108(17.0)527(83.0)  80(12.5)561(87.5)  
  1. Data include number, percentage, and P-value of Pearson χ2-test or the Student's t-tests. A total of the row is 100%. BMI, body mass index; ns-Bx, non-severe bruxism group; s-Bx, severe bruxism group.

Gender
Male96(57.5)247(48.1)χ2(1) = 4.390.03652(48.1)267(50.7)χ2(1) = 0.230.64043(53.8)268(47.8)χ2(1) = 1.000.317
Female71(42.5)266(51.9)  56(51.9)260(49.3)  37(46.2)293(52.2)  
BMI (Mean ± SD)
Male15.59±2.1816.22±5.24F = 1.510.25516.63±2.2816.81±2.49F = 0.520.64217.05±2.1618.13±3.43F = 2.810.047
Female15.09±1.9615.63±2.09F = 0.520.05217.04±9.0016.37±2.66F = 5.910.58517.12±2.2517.65±4.05F = 1.920.436
Table 3.  Sleep habit according to the frequency of children's sleep bruxism in each grade category
VariablesGrades 1/2, n= 680Grades 3/4, n= 635Grades 5/6, n= 641
s-Bxns-BxGroup differencess-Bxns-BxGroup differencess-Bxns-BxGroup differences
Time±SDTime±SDF-valuesP-valuesTime±SDTime±SDF-valuesP-valuesTime±SDTime±SDF-valuesP-values
  1. Data include the mean, SD and P-value of anova. ns-Bx, non-severe bruxism group; s-Bx, severe bruxism group.

School nights                  
 Bedtime21:21±0:3721:21±0:390.000.98321:46±0:4121:47±0:400.100.75522:20±0:5122:08±0:444.860.028
 Rising time6:56±0:226:56±0:230.000.9856:57±0:246:56±0:250.130.7206:58±0:246:57±0:270.160.691
 Time in bed9:35±0:339:35±0:360.010.9729:11±0:339:09±0:370.340.5608:38±0:518:49±0:424.080.044
Non-school nights                  
 Bedtime21:47±0:4921:46±0:450.030.85922:10±0:4722:11±0:440.040.83922:34±0:5122:29±0:470.720.396
 Rising time7:25±0:487:28±0:470.470.4947:39±0:587:34±0:550.730.3937:48±1:057:49±1:040.000.992
 Time in bed9:38±0:459:41±0:490.710.3989:29±0:569:23±0:501.230.2699:15±1:049:20±0:580.490.484
Differences between school nights and non-school nights (min)                  
 Weekend bedtime delay26±0:3325±0:280.090.75824±0:3424±0:300.000.97814±0:2921±0:333.190.075
 Weekend rising time delay29±0:3432±0:390.670.41542±0:5338±0:450.680.41051±0:5952±0:560.040.838
 Weekend sleep extension3±0:467±0:450.850.35717±0:5714±0:480.490.48437±0:5531±0:570.680.411
Table 4.  Frequency of sleep problems and related factors in each grade category
VariablesGrades 1/2, n= 680Grades 3/4, n= 635Grades 5/6, n= 641
s-Bxns-BxGroup differencess-Bxns-BxGroup differencess-Bxns-BxGroup differences
n(%)n(%)χ2P-valuesn(%)n(%)χ2P-valuesn(%)n(%)χ2P-values
  1. Data include the number, percentage, and P-value of Pearson χ2-test or Fisher's exact tests. A total of the column is 100%. ns-Bx, non-severe bruxism group; s-Bx, severe bruxism group.

Other sleep problems
 Sleep starts
  Low frequency136(81.9)470(92.0)χ2(1) = 13.480.00183(76.9)487(92.4)χ2(1) = 23.61<0.00164(80.0)507(91.0)χ2(1) = 9.150.002
  High frequency30(18.1)41(8.0)  25(23.1)40(7.6)  16(20.0)50(9.0)  
 Snoring
  Low frequency124(74.3)463(90.3)χ2(1) = 27.33<0.00174(68.5)481(91.3)χ2(1) = 42.14<0.00159(73.8)495(88.2)χ2(1) = 12.520.001
  High frequency43(25.7)50(9.7)  34(31.5)46(8.7)  21(26.2)66(11.8)  
 Difficulty arising
  Facility136(81.9)470(92.0)χ2(1) = 7.020.01497(90.7)492(93.7)χ2(1) = 1.310.25273(91.2)519(92.5)χ2(1) = 0.160.654
  Difficulty30(18.1)41(8.0)  10(9.3)33(6.3)  7(8.8)42(7.5)  
Dietary habits
 Skipping breakfast
  No164(98.2)489(95.5)χ2(1) = 2.490.16297(90.7)489(93.1)χ2(1) = 0.820.36673(91.2)517(92.8)χ2(1) = 0.250.616
  Occasionally/sometimes3(1.8)23(4.5)  10(9.3)36(6.9)  7(8.8)40(7.2)  
 Snacking after dinner
  No/sometimes137(82.0)457(89.0)χ2(1) = 6.000.01494(87.9)469(89.0)χ2(1) = 0.120.73262(77.5)508(90.7)χ2(1) = 12.55<0.001
  Everyday30(18.0)55(10.7)  13(12.1)58(11.0)  18(22.5)52(9.3)  
Behavior pattern after school
 Cram school or lessons
  No/sometimes165(99.4)503(98.4)χ2(1) = 0.890.696100(92.6)512(97.2)χ2(1) = 5.340.04169(86.2)539(96.2)χ2(1) = 14.740.001
  Everyday1(0.6)8(1.6)  8(7.4)15(2.8)  11(13.8)21(3.8)  
 TV/computer game/Internet
  <3 h/day139(83.2)469(92.3)χ2(1) =11.610.00190(83.3)455(86.8)χ2(1) = 0.920.33765(81.2)471(84.3)χ2(1) =0.470.468
  ≥3 h/day28(16.8)39(7.7)  18(16.7)69(13.2)  15(18.8)88(15.7)  
 Use of After School Care Program
  No73(48.0)305(64.1)χ2(1) = 12.39<0.00184(84.8)419(85.7)χ2(1) =0.470.82965(87.8)459(90.7)χ2(1) = 0.610.434
  Yes79(52.0)171(35.9)  15(15.2)70(14.3)  9(12.2)47(9.3)  
Rearing environment
 Employment status of mother
  No job/part-time/free118(73.8)369(75.2)χ2(1) = 0.130.72366(62.9)393(78.6)χ2(1) = 11.750.00154(69.2)400(75.3)χ2(1) = 1.330.248
  Full-time42(26.2)122(24.8)  39(37.1)107(21.4)  24(30.8)131(24.7)  
 A parent returning home after 23.00 hours
  <2–3/month80(47.9)282(55.3)χ2(1) = 2.760.09754(50.0)330(62.7)χ2(1) = 6.090.01447(58.8)365(65.3)χ2(1) = 1.310.253
  ≥1–3/week87(52.1)228(44.7)  54(50.0)196(37.3)  33(41.2)194(34.7)  

The logistic regression analyses were performed for eight independent variables, several of which exhibited significant associations with severe bruxism after adjustment for confounding factors. Independent risk factors for severe bruxism varied among the three grade categories. Table 5 shows the adjusted OR and 95% CI of each significant risk factor.

Table 5.  Factors associated with severe bruxism determined by multivariate logistic regression analysis
Risk factorsAdjusted odds ratio95% confidence intervalP-values
Grades 1/2, n= 577
  1. Data include the adjusted odds ratio, 95% confidence interval, and P-value.

Age (years)0.660.50–0.880.047
Male1.491.01–2.210.04
Snack after dinner everyday1.801.06–3.060.029
TV/computer game/Internet: ≥3 h/day2.781.54–5.020.001
Use of After School Care Program1.981.34–2.930.001
 Grades 3/4, n= 529
Attending cram school or lessons every day3.151.24–8.050.016
Mother is full-time worker2.661.65–4.30<0.001
A parent returning home after 23.00 hours: ≥1–3/week1.741.10–2.740.018
 Grades 5/6, n= 540
Snack after dinner everyday3.351.76–6.38<0.001
Attending cram school or lessons every day7.302.93–18.18<0.001

DISCUSSION

Reliability of questionnaire for bruxism

It is difficult to conduct a cross-sectional investigation of sleep bruxism in children using polysomnography. It is easier to evaluate the condition via a questionnaire; however, since sleep bruxism occurs at night, the reliability of the report is questionable. To confirm the reliability of the replies provided by the parents, we compared the sleep bruxism frequency from questionnaires completed by parents with sleep bruxism scores from the actual tooth wear visualized by the BruxChecker.7 The group who reported they experienced bruxism “occasionally” did not show significant differences in sleep bruxism scores from other groups, and showed various depths of tooth wear. This group was thus excluded from the present analysis. Based on these findings from study 1, it was considered that sufficiently reliable data could be obtained from the questionnaire filled in by parents.

Sleep bruxism prevalence

The percentage of s-Bx among elementary school children was 12.2% (six of 49 students) in study 1 and 13.7% (355 of 2600 participants) in study 2. The ratios were similar to previous research reporting that 11–20% of school-age children younger than 12 years in other countries have sleep bruxism.3 Severe bruxism was more prevalent in younger children (OR, 0.66; 95% CI, 0.50–0.88) and in males (OR, 1.49; 95% CI, 1.01–2.21); this was consistent with the results of previous reports.3,13

Sleep habits

There was no significant difference on sleep habits between the s-BX and the ns-BX groups in grades 1/2 and 3/4. However, in grades 5/6, bedtimes on school nights were delayed and TIB was shortened in the s-Bx group. It is possible that poor sleep habits might promote severe sleep bruxism in children in higher grades of elementary school. A survey of elementary school children aged 6–12 years in Hong Kong showed a high incidence of bruxism, with 20.5% of students grinding their teeth.13 The report indicated that the sleep duration of children in Hong Kong was much shorter than that of children in the United States.14

Other sleep problems

For all grade categories, children in the s-Bx group had a high frequency of “sleep starts” and “snoring”. A recent study showed evidence that sleep bruxism episodes are shown mostly in stages 1 and 2 of non-rapid eye movement sleep (NREM) sleep, about 10% occur during REM sleep, and the highest frequency is seen in transition periods preceding REM sleep.15 The International Classification of Sleep Disorders indicates that sleep starts represent sleep–wake transition disorders and mostly occur in NREM sleep stage 1.1 Because sleep bruxism and sleep starts are often observed in children during the light NREM sleep stages, there is a possibility that they are involved in the hyperactivity of similar motoneuron systems. However, we cannot clarify the relationship between sleep starts and high frequent bruxism based on this study. An extensive European study has shown that snoring and obstructive sleep apnea syndrome (OSAS) are risk factors for sleep bruxism in participants older than 15 years.6 Sleep bruxism and habitual snoring are closely related in children, 13,16 which was confirmed in the current study; children in the s-Bx group showed a higher frequency of snoring than those in the ns-Bx group. Because no clear relationship with BMI index was observed in the s-Bx group, it is suggested that obesity does not affect the occurrence of snoring and sleep bruxism in children.

Children in the s-Bx group in grades 1/2 had significantly more difficulty arising in the morning. It has been reported that micro-arousals occur just before the onset of sleep bruxism.17,18 In addition, children-targeted research (age, 5–18 years) reveals higher arousal indices among a sleep bruxism group than a control group.19 However, except for arousal indices, no significant difference in sleep architecture between children with bruxism and controls has been reported.19 In the present study, there was no significant difference in sleep duration between the s-Bx and ns-Bx groups in grades 1/2. Thus it was suggested that severe bruxism might have a detrimental effect on sleep quality, and become a risk factor for difficulty arising, especially in the lower grades. Further investigation is needed to confirm this relationship by means of polysomnography.

Risk factors for severe bruxism

In grades 1/2 and 5/6, eating a snack after dinner everyday was found to be a significant risk factor for severe bruxism. This finding may indicate that sleep bruxism is independently associated with dietary factors. However, in the present study, the proportion of children who skip breakfast was extremely low and was not related to bruxism in any grades. Again, additional research is needed to confirm any connection between dietary habits and bruxism.

“TV/computer game/Internet ≥3 h/day” was a significant risk factor in the s-Bx group in grades 1/2. Previous studies have reported an association between an increase in television viewing, computer game playing, or Internet use, and delay in bedtime.20,21 However, there was no difference in the bedtime between the s-Bx and the ns-Bx groups in grades 1/2. There was a significant relationship between using ASCP and the s-Bx group in grades 1/2. All the elementary schools that participated in the study conducted ASCPs as requested by the parents, regardless of the employment status of mother. Children with mothers who were full-time workers or whose mother or father frequently returned home after 23.00 hours had a higher incidence of s-Bx in grades 3/4. These rearing environments might decrease the time for close family communication between parents and children, resulting in an aggravation of sleep habits in children. We framed a hypothesis that the s-Bx in grades 1/2 and 3/4 were related to deprivation of communication at home. In grades 3/4 and 5/6, “attending cram school or lessons everyday” was a significant risk factor for s-Bx. In the participating areas, many children among the higher grades attend cram school and take an exam for entering junior high school. A survey of children aged 8–12 years in Hong-Kong reported that homework time each day and signs of anxiety or depression were closely related.22 To date, sleep bruxism and stress also appear to be closely related.3,6 For children in higher grades, increased opportunities for attending cram school and worsening sleep habits may increase psychological stress. This may be related to the higher incidence of severe bruxism.

Limitations

In study 1, sleep bruxism evaluation was conducted by oral examination by dentists and the BruxChecker. Polysomnographic confirmation of its reliability will be needed. Additionally, the number of children (49) included in the study 1 analysis meant that the investigation had slightly weak statistical power.

In study 2, the present community-based cross-sectional study has been carried out only in a metropolitan area. It did not target the general population, and did not use randomized sampling. Since there have been no previous reports concerning sleep bruxism and lifestyle in Japanese children, we have no information about the sampling bias based on geographic area. Further investigation of sleep bruxism frequency and lifestyle is needed in order to compare city and rural areas.

Conclusion

The present study revealed that children in the s-Bx group had some specific characteristics in regards to sleep habits, sleep problems, and lifestyle, including rearing environment, which varied across the elementary school grades. At the present time, Tokyo is a relatively sleepless and restless city. Moreover, it is rather common for both parents to work. As a result, it is natural that parents return home late and the lifestyle becomes owl-type. Capaldi et al. reported that children and adolescents aged 10–17 years who slept less and had poor quality sleep showed greater cortisol responses to stress. 23 Recently it has been reported that children with bruxism have a higher arousal index at night-time, and a higher score on the child behavior checklist.19,24 This study suggests that the incidence of sleep bruxism among elementary school children is strongly related to the disturbance of sleep habits resulting from a nocturnal lifestyle, and to deprivation of communication at home. If the incidence of severe sleep bruxism increases with psychological stress, our results suggest that the following factors were associated with the stress in children: (i) TV/computer game/Internet ≥3 h/day and using ASCP in grades 1/2; (ii) having a mother who works full-time and a parent who frequently returns home after 23.00 hours in grades 3/4; and (iii) attending cram school or lessons everyday in grades 5/6.

Future investigations into sleep bruxism will be focused on differences in geographic areas or changes with longitudinal study. There is also a need for intervention studies concerning improvement in sleep habits for children and parents. Furthermore, physiological investigations regarding sleep bruxism and coping with stress are needed.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Education, Culture, Sports, Science and Technology in Japan and the Ebara Association of School Dentists in Tokyo.

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