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Adolescents With Greater Mental Toughness Show Higher Sleep Efficiency, More Deep Sleep and Fewer Awakenings After Sleep Onset

Published:September 03, 2013DOI:https://doi.org/10.1016/j.jadohealth.2013.07.017

      Abstract

      Purpose

      Mental toughness (MT) is understood as the display of confidence, commitment, challenge, and control. Mental toughness is associated with resilience against stress. However, research has not yet focused on the relation between MT and objective sleep. The aim of the present study was therefore to explore the extent to which greater MT is associated with objectively assessed sleep among adolescents.

      Methods

      A total of 92 adolescents (35% females; mean age, 18.92 years) completed the Mental Toughness Questionnaire. Participants were split into groups of high and low mental toughness. Objective sleep was recorded via sleep electroencephalograms and subjective sleep was assessed via a questionnaire.

      Results

      Compared with participants with low MT, participants with high MT had higher sleep efficiency, a lower number of awakenings after sleep onset, less light sleep, and more deep sleep. They also reported lower daytime sleepiness.

      Conclusions

      Adolescents reporting higher MT also had objectively better sleep, as recorded via sleep electroencephalograms. A bidirectional association between MT and sleep seems likely; therefore, among adolescence, improving sleep should increase MT, and improving MT should increase sleep.

      Keywords

      Implications and Contribution
      Mentally tough adolescents showed objectively assessed improved sleep. Improving adolescents' mental toughness might also improve adolescents' sleep, and vice versa.
      Adolescence is defined as the period of gradual transition between childhood and adulthood, with conceptually distinct physical changes marking puberty and maturation [
      • Spear L.P.
      The adolescent brain and age-related behavioral manifestations.
      ,
      • Pinyerd B.
      • Zipf W.B.
      Puberty—timing is everything!.
      ]. Along with dramatic changes in physiology and neural networks [
      • Giedd J.N.
      • Blumenthal J.
      • Jeffries N.O.
      • et al.
      Brain development during childhood and adolescence: A longitudinal MRI study.
      ,
      • Paus T.
      • Keshavan M.
      • Giedd J.N.
      Why do many psychiatric disorders emerge during adolescence?.
      ], adolescents have to face new challenges and assume responsibility for issues such as their academic and vocational careers; peer and intimate relationships; increased physical, emotional and financial independence from parents and siblings; use of psychoactive substances; extra-curricular employment; and leisure-time activities such as sports participation and music [
      • Spear L.P.
      The adolescent brain and age-related behavioral manifestations.
      ]. Dealing with these issues is potentially stressful; accordingly it is assumed that adolescents with better coping skills will deal more successfully with these challenges [
      • Grant K.E.
      • Compas B.E.
      • Thurm A.E.
      • et al.
      Stressors and child and adolescent psychopathology: Measurement issues and prospective effects.
      ].
      A psychological construct related to favorable stress management is mental toughness (MT). Mental toughness is a relatively new area of academic research [
      ] and a cognitive strength variable known to be associated with good performance both in elite sport [
      • Crust L.
      • Azadi K.
      Mental toughness and athletes' use of psychological strategies.
      ] and, more recently, in non-elite sport [
      • Jones M.I.
      • Parker J.K.
      What is the size of the relationship between global mental toughness and youth experiences?.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ]. Mental toughness has been conceptualized in various ways in the scientific literature [
      ]. In the present study, we used the 4C(+2) (Challenges, Commitment, Control [emotional and control over life], Confidence [interpersonal and in ability]) model of MT, defined as performing well in challenging situations (“Challenges usually bring out the best in me”), commitment (“I don't usually give up under pressure”), control (emotional control: “Even when under considerable pressure I usually remain calm”; and life control: ”I generally feel in control”), and confidence (interpersonal confidence: ”I usually take charge of a situation when I feel it is appropriate”; and confidence in ability: “I am generally confident in my own abilities”) [
      • Clough P.
      • Earle K.
      • Sewell D.
      Mental toughness: The concept and its measurement.
      ]. In previous studies [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ], the authors were able to validate a German version of Mental Toughness Questionnaire–48 (MTQ48) [
      • Clough P.
      • Earle K.
      • Sewell D.
      Mental toughness: The concept and its measurement.
      ] and to show, in a large sample of adolescents and young adults, (1) that the construct of MT is not limited to high-performing elite sportsmen and women [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ]; (2) that MT is associated with increased stress resilience [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ]; and (3) that MT remains stable over time [
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ], which suggests that MT is related to successful stress management and to psychological well-being.
      With regard to this last construct, numerous studies have indicated that independent of age, there is a bidirectional relation between psychological well-being and sleep [
      • Haario P.
      • Rahkonen O.
      • Laaksonen M.
      • et al.
      Bidirectional associations between insomnia symptoms and unhealthy behaviours.
      ]. For adolescents, sleep and sleep regulation play a crucial role in both well-being and development [
      • Owens J.A.
      • Spirito A.
      • McGuinn M.
      • Nobile C.
      Sleep habits and sleep disturbance in elementary school-aged children.
      ,
      • Brand S.
      • Kirov R.
      Sleep and its importance in adolescence and in common adolescent somatic and psychiatric conditions.
      ,
      • Colrain I.M.
      • Baker F.C.
      Changes in sleep as a function of adolescent development.
      ,
      • Jan J.E.
      • Reiter R.J.
      • Bax M.C.
      • et al.
      Long-term sleep disturbances in children: A cause of neuronal loss.
      ]. Lemola et al. [
      • Lemola S.
      • Räikkönen K.
      • Scheier M.F.
      • et al.
      Sleep quantity, quality and optimism in children.
      ,

      Lemola S, Räikkönen K, Gomez V, Allemand M. Optimism and self-esteem are related to sleep: Results from a large community-based sample [published online ahead of print October 4, 2012]. Int J Behav Med.

      ] showed that dispositional optimism was associated with better sleep quality and longer sleep duration among children and adults. In contrast, sleep disturbances have been reported in more than 25% of adolescents worldwide; poor sleep in adolescence has become a significant public mental and physical health problem [
      • Owens J.A.
      • Spirito A.
      • McGuinn M.
      • Nobile C.
      Sleep habits and sleep disturbance in elementary school-aged children.
      ,
      • Laberge L.
      • Tremblay R.E.
      • Vitaro F.
      • Montplaisir J.
      Development of parasomnias from childhood to early adolescence.
      ]. Cross-sectional [
      • Kaneita Y.
      • Ohida T.
      • Osaki Y.
      • et al.
      Association between mental health status and sleep status among adolescents in Japan: A nationwide cross-sectional survey.
      ,
      • Lund H.G.
      • Reider B.D.
      • Whiting A.B.
      • Prichard J.R.
      Sleep patterns and predictors of disturbed sleep in a large population of college students.
      ] and longitudinal studies [
      • Roberts R.E.
      • Roberts C.R.
      • Duong H.T.
      Chronic insomnia and its negative consequences for health and functioning of adolescents: A 12-month prospective study.
      ,
      • Touchette E.
      • Chollet A.
      • Galéra C.
      • et al.
      Prior sleep problems predict internalising problems later in life.
      ] have shown that acute and chronic sleep loss during development persists over time, with negative effects on adolescents' physical and mental health. At the same time, poor psychological well-being may itself negatively affect adolescents' sleep [
      • Gregory A.M.
      • Sadeh A.
      Sleep, emotional and behavioral difficulties in children and adolescents.
      ,
      • Owens J.A.
      • Spirito A.
      • McGuinn M.
      • Nobile C.
      Sleep habits and sleep disturbance in elementary school-aged children.
      ].
      To explain the association between poor sleep and psychological processes, it has been proposed that increased arousal and dysfunctional thoughts are directly involved in psychologically caused sleep disturbances [
      • Harvey A.G.
      Pre-sleep cognitive activity: A comparison of sleep-onset insomniacs and good sleepers.
      ,
      • Harvey A.G.
      A cognitive model of insomnia.
      ,
      • Carney C.E.
      • Edinger J.D.
      Identifying critical beliefs about sleep in primary insomnia.
      ,
      • Riemann D.
      • Spiegelhalder K.
      • Feige B.
      • et al.
      The hyperarousal model of insomnia: A review of the concept and its evidence.
      ], whereas, the absence of stress and worries, for instance, are associated with favorable sleep. This research points to interrelations among low stress, favorable personality traits, and restorative sleep. Consequently, it seems possible that high MT and good quality sleep are closely linked.
      The main goal of the research reported here was to explore the association between MT and objective sleep within a sample of adolescents. The present study may add to the current literature on MT and sleep in an important way by showing a close association between MT (as a marker of psychological well-being) and sleep among non-elite sport adolescents.
      The following hypothesis was formulated. Following previous research [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ,
      • Lemola S.
      • Räikkönen K.
      • Scheier M.F.
      • et al.
      Sleep quantity, quality and optimism in children.
      ,

      Lemola S, Räikkönen K, Gomez V, Allemand M. Optimism and self-esteem are related to sleep: Results from a large community-based sample [published online ahead of print October 4, 2012]. Int J Behav Med.

      ], higher scores for MT would be expected to be associated with improved sleep, as assessed by sleep electroencephalogram (EEG) recordings. More specifically, the researchers expected to find higher sleep efficiency, shorter sleep onset latency, more deep sleep, and less light sleep in adolescents with high MT, with compared to adolescents with low MT. Lower daytime sleepiness (DS) was also expected. Findings that higher scores of MT were associated with subjectively increased improved sleep would confirm the hypothesis.

      Methods

      Participants

      A total of 285 adolescents participated in the study (mean age, 18.26 years; standard deviation 4.17 years); preliminary data are presented elsewhere [

      Brand S, Gerber M, Kalak N, et al. In adolescence, greater mental toughness is related to better sleep schedules. Behav Sleep Med. In press.

      ]. All participants completed the MTQ and a questionnaire related to sleep complaints (SC) and daytime sleepiness (DS). To recruit participants, the study was advertised electronically on the homepages of three high schools in the canton of Basel, a district of the German-speaking Northwestern part of Switzerland. Data were collected during spring and summer 2012. Participants were informed about the purpose of the study and about the voluntary basis of their participation. They were also assured of the confidentiality of their responses, and they gave written informed consent. For participants aged <18 years, written informed consent was secured from their parents. The study was approved by the local ethics committee, and the entire study was performed in accordance with the ethical standards in the Declaration of Helsinki.

      Procedure

      After completing the MTQ, a subsample of participants with high MT scores (upper 17% of the total score: ≥141 points) and participants with low MT scores (lower 15.5% of the total score: ≤77 points) were asked to undergo a sleep EEG assessment. Participants undergoing sleep EEG recordings received a voucher of 30 Swiss francs (about $30) for participation.

      Materials

      Participants completed the MTQ48 [
      • Clough P.
      • Earle K.
      • Sewell D.
      Mental toughness: The concept and its measurement.
      ] to assess MT, the Insomnia Severity Index [
      • Bastien C.H.
      • Vallières A.
      • Morin C.M.
      Validation of the Insomnia Severity Index (ISI) as an outcome measure for insomnia research.
      ] to assess SC, and the Epworth Sleepiness Scale [
      • Johns M.W.
      A new method for measuring daytime sleepiness: The Epworth Sleepiness Scale.
      ] to assess DS, and underwent objective sleep EEG monitoring [
      • Rechtschaffen A.
      • Kales A.
      A manual of standardized terminology techniques and scoring system for sleep stages in human subjects. United States Department of Health, Education and Welfare, Public Health Service.
      ].

      Measurement of MT

      Participants (N = 92) were asked to fill out the MTQ48 ([
      • Clough P.
      • Earle K.
      • Sewell D.
      Mental toughness: The concept and its measurement.
      ]; German version: [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ]), which measures overall MT and its six subcomponents: challenge (e.g., “Challenges usually bring out the best in me”), commitment (e.g., I don't usually give up under pressure”), emotional control (e.g., “Even when under considerable pressure I usually remain calm”), life control (e.g., “I generally feel in control”), interpersonal confidence (e.g., “I usually take charge of a situation when I feel it is appropriate”), and confidence in ability (e.g., “I am generally confident in my own abilities”). Answers on the MTQ48 were given on 5-point Likert-type scales ranging from 1 = “strongly disagree” to 5 = “strongly agree”. Items were summed to obtain overall and subscale scores, with higher scores reflecting greater MT (Cronbach α = .89).

      Assessment of sleep: objective sleep assessment

      To assess sleep objectively, sleep EEG recordings were performed at home using a three-channel EEG device (Fp2-A1, C3-A2, and C4-A1; electrooculogram; electromyogram; Somnowatch; Randersacker, Germany). Two experienced raters visually analyzed sleep polygraphs according to the standard procedures described and defined by Rechtschaffen and Kales [
      • Rechtschaffen A.
      • Kales A.
      A manual of standardized terminology techniques and scoring system for sleep stages in human subjects. United States Department of Health, Education and Welfare, Public Health Service.
      ] (inter-rater reliability: κ = .88). Raters were completely blinded with respect to participants' group assignments. The device provides assessment of total sleep time, sleep efficiency, sleep onset latency, stages 1–4 (minutes and percentage), light sleep (Stages 1 and 2), slow wave sleep (Stages 3 and 4), rapid eye movement (REM) sleep, and number and times of awakenings after sleep onset.

      Assessment of sleep: SC

      The Insomnia Severity Index [
      • Bastien C.H.
      • Vallières A.
      • Morin C.M.
      Validation of the Insomnia Severity Index (ISI) as an outcome measure for insomnia research.
      ] is a 7-item self-rating questionnaire to assess SC. The items, answered on 5-point rating scales (0 = “not at all,” 4 = “very much”), refer in part to Diagnostic and Statistical Manual of Mental Disorders, 4th Edition criteria for insomnia [
      • American Psychiatric Association
      Diagnostic and statistical manual of mental disorders.
      ] by measuring difficulty in falling asleep, difficulties remaining asleep, early morning awakenings, increased DS, impaired daytime performance, low satisfaction with sleep, and worrying about sleep. The higher the overall score, the more the respondent is assumed to experience insomnia (Cronbach α = .92).

      Measurement of DS

      To assess DS, participants completed the Epworth Sleepiness Scale [
      • Johns M.W.
      A new method for measuring daytime sleepiness: The Epworth Sleepiness Scale.
      ]. Participants were asked about the likelihood of dozing off or falling asleep in specific situations (sitting and reading, watching television, sitting inactive in a public place, being a passenger in a car for an hour without a break, lying down to rest in the afternoon, sitting and talking to someone, and sitting quietly after a lunch). Answers were given on 4-point Likert scales ranging from 0 = “no chance of dozing” to 3 = “high chance of dozing,” with a higher sum score reflecting a greater likelihood of dozing off.

      Statistical analysis

      A series of Student t tests were used to examine difference in target parameters (sleep EEG, SC, and DS) between participants with high MT and those with low MT. Next, Pearson's correlation coefficients were used to calculate the associations between the dimensions of MT and the target parameters across the entire sample.
      The level of significance was set at α ≤ .05. All statistical analyses were calculated with SPSS 19.0 for Windows (IBM Company, New York, NY).

      Results

      All descriptive and inferential statistical information are reported in the tables.

      High and low MT, objective sleep parameters, subjective sleep, and DS

      Table 1 shows the descriptive and statistical comparisons of sleep parameters, SC, and DS scores between participants with high MT and low MT.
      Table 1Descriptive and statistical overview of sleep electroencephalogram data, sleep complaints, and daytime sleepiness separately for participants with high mental toughness (n = 48) and low mental toughness (n = 44)
      Dimension of sleepGroupt
      High mental toughnessLow mental toughness
      Total sleep duration, hours6.85 (1.11)6.98 (1.08).05
      Sleep efficiency, %98.04 (.79)94.96 (1.92)10.19
      p < .001.
      Sleep onset latency, minutes9.54 (7.59)11.26 (5.07).64
      Number of awakenings after sleep onset2.33 (1.62)6.45 (2.77)8.80
      p < .001.
      Sleep 1
       Minutes8.59 (6.55)14.90 (10.53)3.49
      p < .01.
       %2.00 (1.30)3.63 (2.51)3.95
      p < .001.
      Sleep 2
       Minutes186.38 (53.43)200.00 (66.47)1.09
       %43.58 (7.25)46.78 (9.01)1.88
      Sleep 3
       Minutes43.17 (19.41)34.14 (11.32)2.69
      p < .01.
       %10.17 (4.42)8.52 (2.65)2.14
      p < .05.
      Sleep 4
       Minutes95.46 (24.97)82.19 (32.77)2.20
      p < .05.
       %23.51 (7.54)21.61 (9.04)1.10
      Light sleep
       Minutes194.96 (57.61)214.91 (69.42)1.50
       %45.57 (7.96)51.90 (9.79)3.42
      p < .01.
      Deep sleep
       Minutes138.45 (26.97)122.38 (34.84)2.48
      p < .05.
       %33.68 (7.82)30.96 (9.75)1.48
      Rapid eye movement sleep
       Minutes86.54 (27.44)70.64 (19.57)3.18
      p < .01.
       %20.73 (5.72)17.12 (3.09)3.71
      p < .001.
      Sleep complaints6.78 (1.04)12.01 (2.34)9.45
      p < .001.
      Daytime sleepiness2.45 (3.06)10.91 (4.43)10.42
      p < .001.
      Degrees of freedom = always 90. Sleep complaints were assessed with the Insomnia Severity Index; daytime sleepiness was assessed with the Epworth Sleepiness Scale.
      p < .05.
      ∗∗ p < .01.
      ∗∗∗ p < .001.
      Relative to participants with low MT, participants with high MT had higher sleep efficiency, more deep sleep, more REM sleep, less light sleep, fewer awakenings after sleep onset, and a lower score of subjective SC and DS. No statistically significant differences were found for sleep duration, sleep onset latency, and Stage 2 sleep.

      Associations between dimensions of MT and objective sleep dimensions SC and DS

      Table 2 shows the correlation matrix and descriptive statistics between the dimensions of MT and objective sleep, SC, and DS across the entire sample (N = 92).
      Table 2Correlations (Pearson correlation coefficients) between mental toughness and dimensions of objective sleep, sleep complaints, and daytime sleepiness (N = 92)
      Dimension of sleepMental toughnessDescriptive statistics, mean (standard deviation)
      ChallengeCommitmentControl emotionLifeConfidence interpersonalAbilitiesMental toughness total score
      Total sleep duration, hours.14.02.10–.05.09.00.056.95 (1.38)
      Sleep efficiency, %.75
      p < .001.
      .70
      p < .001.
      .83
      p < .001.
      .63
      p < .001.
      .65
      p < .001.
      .72
      p < .001.
      .74
      p < .001.
      95.04 (2.62)
      Sleep onset latency, minutes−.10−.08−.10−.09−.13−.06−.0910.40 (9.78)
      Number of awakenings after sleep onset−.57
      p < .001.
      −.67
      p < .001.
      −.69
      p < .001.
      −.70
      p < .001.
      −.60
      p < .001.
      −.72
      p < .001.
      −.69
      p < .001.
      4.30 (3.05)
      Sleep 1
       Minutes−.20−.21−.27
      p < .05.
      −.14−.09−.23
      p < .05.
      −.2111.61 (9.20)
       %−.27
      p < .05.
      −.22
      p < .05.
      −.32
      p < .01.
      −.14−.12−.24
      p < .05.
      −.24
      p < .05.
      2.78 (2.12)
      Sleep 2
       Minutes−.04−.18−.06−.19−.15−.19−.14192.89 (60.08)
       %−.23
      p < .05.
      −.31
      p < .01.
      −.22
      p < .05.
      −.29
      p < .01.
      −.28
      p < .01.
      −.29
      p < .01.
      −.28
      p < .01.
      45.10 (8.25)
      Sleep 3
       Minutes.13.09.14.09.22
      p < .05.
      .17.1438.85 (16.60)
       %−.02.03.03.06.10.10.059.38 (3.76)
      Sleep 4
       Minutes.27
      p < .05.
      .28
      p < .01.
      .26
      p < .05.
      .11.18.19.24
      p < .05.
      89.11 (29.56)
       %.14.22
      p < .05.
      .19.12.17.19.1822.60 (8.30)
      Light sleep
       Minutes−.07−.20−.09−.20−.15−.21−.16204.50 (63.96)
       %−.30
      p < .01.
      −.40
      p < .001.
      −.32
      p < .01.
      −.33
      p < .01.
      −.37
      p < .001.
      −.39
      p < .001.
      −.37
      p < .001.
      48.60 (9.38)
      Deep sleep
       Minutes.33
      p < .01.
      .36
      p < .01.
      .33
      p < .01.
      .22
      p < .01.
      .35
      p < .01.
      .31
      p < .01.
      .34
      p < .01.
      130.77 (31.85)
       %.17.26
      p < .05.
      .18.18.20.22
      p < .05.
      .22
      p < .05.
      32.38 (8.85)
      Rapid eye movement sleep
       Minutes.31
      p < .01.
      .25
      p < .05.
      .32
      p < .01.
      .23
      p < .05.
      .32
      p < .01.
      .28
      p < .01.
      .29
      p < .01.
      78.94 (25.17)
       %.27
      p < .05.
      .31
      p < .01.
      .30
      p < .01.
      .30
      p < .01.
      .35
      p < .01.
      .35
      p < .01.
      .32
      p < .01.
      19.00 (4.97)
      Sleep complaints−.78
      p < .001.
      −.69
      p < .001.
      −.74
      p < .001.
      −.87
      p < .001.
      −.79
      p < .001.
      −.69
      p < .001.
      −.73
      p < .001.
      9.92 (1.93)
      Daytime sleepiness−.86
      p < .001.
      −.90
      p < .001.
      −.87
      p < .001.
      −.64
      p < .001.
      −.81
      p < .001.
      −.88
      p < .001.
      −.88
      p < .001.
      6.68 (4.69)
      Sleep complaints were assessed with the Insomnia Severity Index; daytime sleepiness was assessed with the Epworth Sleepiness Scale.
      p < .05.
      ∗∗ p < .01.
      ∗∗∗ p < .001.
      The overall pattern of results shows that any dimension of MT was positively and statistically significantly correlated with sleep efficiency, deep sleep, and REM sleep, and negatively and statistically significantly correlated with the number of awakenings after sleep onset, light sleep, SC, and DS. Interestingly, no significant correlations were found for sleep duration and sleep onset latency.

      Discussion

      The key findings of the present study are that in a sample of adolescents, greater MT was related to better objective and subjective sleep and lower DS.
      To perform the study, following previous research [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ,
      • Lemola S.
      • Räikkönen K.
      • Scheier M.F.
      • et al.
      Sleep quantity, quality and optimism in children.
      ,

      Lemola S, Räikkönen K, Gomez V, Allemand M. Optimism and self-esteem are related to sleep: Results from a large community-based sample [published online ahead of print October 4, 2012]. Int J Behav Med.

      ], the authors expected that higher scores for MT would be associated with improved sleep, as assessed by sleep EEG recordings; data fully confirmed this assumption: Adolescents with high MT, relative to adolescents with low MT, had more favorable sleep dimensions such as an increased sleep efficiency, a lower number of awakenings after sleep onset, or more deep sleep. Subjectively, as in a previous study [

      Brand S, Gerber M, Kalak N, et al. In adolescence, greater mental toughness is related to better sleep schedules. Behav Sleep Med. In press.

      ], adolescents reported lower SC and a lower score of DS. The present data agree with the many studies showing an association between restoring sleep and psychological well-being [
      • Gregory A.M.
      • Sadeh A.
      Sleep, emotional and behavioral difficulties in children and adolescents.
      ]. The present findings mirror previous research in that favorable psychological concepts such as optimism [
      • Lemola S.
      • Räikkönen K.
      • Scheier M.F.
      • et al.
      Sleep quantity, quality and optimism in children.
      ,

      Lemola S, Räikkönen K, Gomez V, Allemand M. Optimism and self-esteem are related to sleep: Results from a large community-based sample [published online ahead of print October 4, 2012]. Int J Behav Med.

      ] were positively related to sleep. The present results expand previous findings in that this was the first study to prove that among non-elite sport adolescents, high scores of MT were associated with improved objective sleep. The authors also think that the present data add to the literature in that the construct of MT is still poorly investigated, but it has the potential to cover a broad variety of favorable motivational, emotional, cognitive, and stress-buffering processes. As this study showed, higher MT scores were related to stress resilience and increased physical activity also over time [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ]. Indeed, research has established links between MT and hardiness, which has previously been found to be associated with stress resilience, which is in accord with the idea that resilience does not evolve from avoidance of adversity, but from successful dealing with negative stimuli [
      • Masten A.S.
      Regulatory processes, risk, and resilience in adolescent development.
      ]. Importantly, stress has been proven to increase the risk for maladjustment and psychopathology during adolescence and early adulthood [
      • Grant K.E.
      • Compas B.E.
      • Thurm A.E.
      • et al.
      Stressors and child and adolescent psychopathology: Measurement issues and prospective effects.
      ]. Although highly speculative and not provable with the present data, the authors assume that MT influences sleep positively via reduced stress [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ], reduced hyperarousal [
      • Riemann D.
      • Spiegelhalder K.
      • Feige B.
      • et al.
      The hyperarousal model of insomnia: A review of the concept and its evidence.
      ], and reduced dysfunctional thoughts [
      • Harvey A.G.
      Pre-sleep cognitive activity: A comparison of sleep-onset insomniacs and good sleepers.
      ,
      • Harvey A.G.
      A cognitive model of insomnia.
      ,
      • Carney C.E.
      • Edinger J.D.
      Identifying critical beliefs about sleep in primary insomnia.
      ], because it is highly plausible that dysfunctional thoughts and maladaptive behavior are incompatible with the dispositions toward appraisal of high control, challenge, commitment and confidence that characterize an MT person. Here, this overall concept of MT is closely related to objectively and subjectively assessed favorable sleep, and lower DS among a sample of non-elite sport adolescents.
      Surprisingly, MT was related neither to sleep duration nor sleep onset latency (SOL). Whereas the relative importance of sleep duration as a key factor for psychological well-being is a matter of debate, there is no explanation for why SOL was unrelated to MT.
      Difficulty falling asleep is one of the core symptoms of sleep disturbance. It is assumed that dysfunctional thoughts (such as the fear of not being able to fall asleep, fear of not feeling restored the next day; and difficulties with coping with stress) increase psychophysiologic arousal, which in turn hinders falling asleep [
      • Harvey A.G.
      Pre-sleep cognitive activity: A comparison of sleep-onset insomniacs and good sleepers.
      ,
      • Harvey A.G.
      A cognitive model of insomnia.
      ,
      • Carney C.E.
      • Edinger J.D.
      Identifying critical beliefs about sleep in primary insomnia.
      ,
      • Riemann D.
      • Spiegelhalder K.
      • Feige B.
      • et al.
      The hyperarousal model of insomnia: A review of the concept and its evidence.
      ]. In this view, it is proven [
      • Riemann D.
      • Spiegelhalder K.
      • Feige B.
      • et al.
      The hyperarousal model of insomnia: A review of the concept and its evidence.
      ] that stress as a dysfunctional concept prolongs SOL. Moreover, MT and stress resilience are closely associated [
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Adolescents' exercise and physical activity are associated with mental toughness.
      ,
      • Gerber M.
      • Kalak N.
      • Lemola S.
      • et al.
      Are adolescents with high mental toughness levels more resilient against stress?.
      ,
      • Gerber M.
      • Brand S.
      • Feldmeth A.K.
      • et al.
      Adolescents with high mental toughness adapt better to perceived stress: A longitudinal study with Swiss vocational students.
      ]; as a consequence, it would have been expected that MT and shortened SOL were associated; however, the current data did not confirm this expectation.
      Despite the clarity of the findings, several considerations caution against overgeneralization. First, data were derived from a selected subsample, and participants were recruited only from high schools; therefore, a selection bias cannot be excluded. Second, no thorough psychiatric or medical examination was made; therefore, the pattern of results might result from further latent although unassessed variables such as eating disorders, bipolar disorders, early-stage romantic love, substance abuse, stress at school, and stressful relationships. Third, only participants willing and able complete the questionnaires and to undergo sleep EEG recordings volunteered to take part in the study; therefore, a positive selection bias cannot be ruled out. Finally, in the present model, it was assumed that MT as a mental construct affects objective sleep, although the authors are aware of the bidirectionality between psychological functioning and sleep; that is, favorable or poor sleep also has an impact on cognitive and emotional processing and behavior [
      • Haario P.
      • Rahkonen O.
      • Laaksonen M.
      • et al.
      Bidirectional associations between insomnia symptoms and unhealthy behaviours.
      ]. Therefore, it is also conceivable that adolescents sleeping particularly well have developed a particularly high MT.
      Among a sample of participants in their late adolescence, greater MT was associated with objectively assessed better sleep and low DS. Future research should apply longitudinal and interventional designs to investigate the causal directions between MT and sleep, and to investigate to what extent improving adolescents' MT may improve also adolescents' sleep, and vice versa.

      Acknowledgments

      The authors thank Nick Emler (Surrey, UK) for proofreading the manuscript. Moreover, they thank Marielle Koenig and Vladimir Djurdjevic from the Sleep Laboratory of the Center for Affective, Stress, and Sleep Disorders, for sleep EEG recordings and analysis.

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