Personality and insomnia: A systematic review and narrative synthesis

The inherent nature of personality serves as a predisposing, and possible maintaining, factor of insomnia. However, methodological differences limit the ability to draw causal conclusions regarding the specific traits involved in the aetiology of the disorder. This systematic review of the relationship between insomnia and personality provides a narrative synthesis of the literature to date. Here, we identified N = 76 studies meeting the inclusion/exclusion criteria. The outcomes reliably evidenced the experience of insomnia to be associated with personality traits that are typically considered to be negative or maladaptive in nature. More specifically, insomnia was related to neuroticism, introversion, perfectionistic doubts and concerns, elevated personal standards, negative affect, social inhibition and avoidance, hysteria, hypochondriasis, psychasthenia, impulsive behaviour, anger, hostility, and psychopathic tendencies, schizotypal and borderline traits, reduced conscientiousness and self‐directedness, and negatively perceived perception of the self. Several studies examined the role that personality plays in predicting the treatment efficacy and adherence of CBTi. Moving forward, longitudinal research, methodological consistency, the mediating role of treatment outcomes and adherence, and clinical and population representative samples should be prioritised. Methodological strengths and limitations of the literature are discussed alongside the next steps that should be taken to advance our understanding of the literature.


INTRODUCTION
Perhaps the most prevalent sleep disorder, the experience of insomnia, presents significant deficits to key functional domains related to social, occupational, and academic functioning.The prolonged (i.e.chronic) experience of insomnia typically encumbers personal burden in relation to: diminished quality of life (Kyle et al., 2010); impaired neuropsychological function (Fortier-Brochu et al., 2010;Wardle-Pinkston et al., 2019); disturbed mood and risk for depression (Baglioni et al., 2011;Breslau et al., 1996); deficits in self-regulation/perception and socioemotional functioning (Beattie et al., 2015;Baglioni et al., 2014;Ypsilanti et al., 2018); greater absenteeism, physical and mental exhaustion, and impaired productivity (Daley, Morin, LeBlanc, Grégoire, & Savard, 2009).Consequently, it is perhaps no surprise that insomnia presents a public health concern (Morin et al., 2013).Indeed, given that occupational consequences account for more than 90% of insomnia-related costs (Daley, Morin, LeBlanc, Grégoire, Savard, & Baillargeon, 2009), the disorder's most crucial impacts presumably occur within the diurnal, rather than the nocturnal, period (Buysse et al., 2006;Morin, 2003).Despite this, the pre-sleep and nocturnal experience of insomnia remains the focal point of examination.Therefore, to gain a more holistic view, we must look beyond proximal symptoms and contextually consider the psychosocial nature of the individual patient experience (Armstrong et al., 2007;Kyle et al., 2010).Here, re-examining the crucial predisposing, and potentially perpetuating, role of personality may provide a greater understanding of the aetiology of insomnia (Spielman & Glovinsky, 1991).
Personality is consistently evidenced to increase the risk of many psychopathologies (Sellbom et al., 2020), whilst also mediating the extent of treatment outcomes in many psychiatric disorders including insomnia (Ellis et al., 2021;Johann et al., 2023;Quilty et al., 2008;Sellbom et al., 2020).From a stress-diathesis approach, disorder vulnerability may be accentuated by the expression of predisposing traits that interact with significantly stressful life events to precipitate the onset of insomnia (Spielman & Glovinsky, 1991).Whilst a generally supported notion, several studies note that insomnia may predict personality alterations (Akram et al., 2015;Jansson-Fröjmark & Linton;Danielsson et al., 2010).In this work, we systematically review the relevant literature that examines the role of personality in insomnia; more specifically, we evaluate the methodological features and provide a narrative synthesis of the literature base.

METHODS
The current protocol followed the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines for searching and reporting.The initial search and independent screening of all titles/abstracts were performed by UA.The second screening for each of the yielded results was conducted by AJ, SA, MG, and JS, and conflicts were resolved by UA, SA, and MG.The full texts of those meeting the criteria for inclusion were subsequently reviewed for final inclusion by UA and MG.In addition, reference lists of the included studies were screened by UA and JS, with any new studies meeting inclusion criteria being further reviewed.The results were synthesised by UA.

Literature search strategies and data extraction
The following databases were searched for articles from all years until 30 December 2022: Web of Science; PubMed; Scopus; PsychINFO; OR 'Cattell Personality Factor Questionnaire' OR 'International Personality Item Pool' OR 'IPIP').Definitions of key personality traits are provided in Table 1.The outcomes for each database were uploaded to Covidence (Veritas Health Innovation, Melbourne, Australia), a web-based collaboration software platform that streamlines the production of systematic literature reviews.

Study inclusion and exclusion criteria
Where article titles and abstract summaries indicated the assessment between personality and insomnia or insomnia symptoms, the full article was assessed for inclusion.Conference abstracts, case studies, reviews, opinions, pre-prints, and duplicates were omitted.Where studies failed to report enough information, the authors of the study were contacted for clarification.Studies were excluded if the authors did not respond or could not provide a detailed breakdown of sample demographics.The inclusion criteria for studies were: (a) peerreviewed research; (b) samples comprising a non-clinical population where the experience of insomnia symptoms was determined using a validated questionnaire measure; or comparison of good/normal sleepers and individuals meeting a relevant diagnostic criterion for insomnia disorder; (c) the assessment of at least one personality trait (identified through validated questionnaires); and (d) studies sampling an adult population.The exclusion criteria comprised: (i) studies examining sleep quality but not insomnia; (ii) studies examining a sleepdisorder other than insomnia; (iii) studies not in English; (iv) systematic reviews, editorials, opinion pieces, and case reports; and (v) studies of a qualitative design.However, where a mixed-methods approach met the inclusion criteria, the quantitative data were extracted and synthesised.

RESULTS
The initial database search yielded N = 20,923 potentially relevant studies.N = 1750 duplicates were identified and subsequently removed by the system, leaving N = 19,139 for screening.After reading the titles and abstracts, and excluding additional duplicates, N = 135 articles were accessed in full and considered for inclusion.
Here, examination of full texts led to exclusion of N = 73 studies.
Next, an updated and manual review of the final included studies yielded N = 14 additional studies for inclusion resulting subsequently in a final sample of N = 76 studies, which fulfilled the a priori inclusion criteria (see Figure 1).
T A B L E 1 Definitions of key personality traits and relevant subscales and multidimensional facets

Trait Definition
Five Factor Model

Agreeableness
The extent of an individual's inquisitiveness, thoughtfulness, and propensity for intellectually challenging tasks

Conscientiousness
The ability to regulate impulse control and subsequently facilitate goal-directed behaviour.Assesses the tendency to be reliable, well-organised and hardworking Emotional stability (neuroticism) Describes the overall emotional stability of an individual through how they perceive the world.It considers how likely a person is to interpret events as threatening or difficult.Also includes the propensity to experience negative emotions.Previously referred to as neuroticism

Extraversion
The tendency and intensity to which someone seeks interaction with their environment, particularly socially.It encompasses the comfort and assertiveness levels of people in social situations Openness to experience One's willingness to try new things as well as engage in imaginative and intellectual activities.It includes the ability to "think outside of the box" Multidimensional perfectionism Disposition for excessive critical self-evaluation and unrealistically high standards for oneself

Doubts about action
The tendency to doubt one's own performance and actions

Concern over mistakes
The propensity to be concerned over and react negatively to mistakes

Personal standards
The tendency to set and maintain high personal standards

Organisation
The propensity to maintain a high standard of order and organisation

Parental criticism
The perception that one's parents are overly critical towards them

Parental expectations
The perception that one's parents have high expectations of them

Psychasthenia
Examines inability to resist certain thoughts or actions.Considered an outdated term used to describe obsessive-compulsive order tendencies.This scale observes compulsive behaviours, abnormal fears, self-criticisms, difficulties in concentration, anxiety, and guilt feelings Schizophrenia Examines whether a person experiences hallucinations and delusions and is likely to develop schizophrenia.Specifically, it measures whether the patient has bizarre thoughts, peculiar perceptions, poor familial relationships, difficulties in concentration, impulse control, lack of deep interests, questions of self-worth, sexual difficulties, and experiences of social alienation Hypomania Evaluate degrees of excitement, marked by an elated yet unstable mood, psychomotor excitement, such as shaky hands, and a string of never-ending ideas.This dimension looks at both behavioural and cognitive overactivity, grandiosity, impulsivity, rapid speech, irritability, and egocentricity

Social introversion
Examines the extent of social introversion or extraversion of a person.Social introverts may choose to avoid social interactions and prefer to be alone or with a small group of friends

Uncertainty intolerance
Tendency to perceive uncertain situations as aversive and stressful and respond with behavioural inhibition and negative expectations about their possible consequences a These items were not examined in the current review due to the colloquial nature of gender norms and frequent comorbidity between depression and insomnia.

The relationship between personality traits and insomnia
This section provides an overview of the included studies.

Five factor model
One of the most prominent models of personality is the five-factor model (also known as the Big-5: Goldberg, 1993), comprised five traits: extroversion, agreeableness, conscientiousness, emotional stability (or neuroticism), and openness to new experiences (Gosling et al., 2003).
Accordingly, many studies have examined the relationship between the full spectrum of these traits in the context of insomnia both at a disorder and symptoms level.To date, studies evidence at least one trait as being related to the symptoms of insomnia.In particular, the experience of insomnia and/or insomnia symptoms appears to be consistently related to reduced levels of emotional stability, extraversion, and conscientiousness (Akram et al., 2019;Dekker et al., 2017;DeShong & Tucker, 2019;Dørheim et al., 2016;Ellis et al., 2021;Evren et al., 2019;Fabbri et al., 2022;Gurtman et al., 2014;Khazaie et al., 2019;LeBlanc et al., 2007LeBlanc et al., , 2009;;Larsgård & Saksvik-Lehouillier, 2017;Ren et al., 2019;Rojo-Wissar et al., 2021;Sassoon et al., 2014;Sørengaard et al. 2022;Van de Laar et al., 2017;Yuksel et al., 2022).Here, of the N = 19 studies yielded, insomnia symptoms were related to reduced emotional stability in N = 18, reduced conscientiousness in N = 11, and reduced extraversion in N = 10.Whilst less consistent, insomnia and related symptoms are associated with greater reports of openness to new experiences (N = 5: Ellis et al., 2021;Emert et al., 2017;Gurtman et al., 2014), with only one study highlighting a negative relationship (N = 1: Dekker et al., 2017).
Several studies (N = 4) found insomnia symptoms to be related to agreeableness.However, directionality concerning this particular trait remains inconclusive with half of the outcomes indicating a positive relationship (Dekker et al., 2017;Yuksel et al., 2022) and the remainder yielding a   negative relationship (Akram et al., 2019;Sassoon et al., 2014).See Table 2 for full study details.
When sampling clinical outpatients meeting a diagnostic criterion for insomnia, Gurtman et al. (2014) determined insomnia to be characterised by elevated neuroticism and openness, and reduced conscientiousness.
Additional path analysis determined that negative affect, pre-sleep arousal, and dysfunctional sleep-related cognitions mediated the relationship between neuroticism and insomnia severity.Likewise, Van de Laar and colleagues (2014) yielded similar outcomes such that participants reported a respective dose response reduction in conscientiousness, emotional stability, and extraversion when grouped by insomnia severity.
When comparing normal-sleepers and individuals meeting the DSM-5 criteria for acute insomnia, Ellis et al. (2021) determined that acute insomnia is characterised by greater reports of introversion, openness to experience, neuroticism, and reduced conscientiousness relative to normal sleepers.Finally, when comparing post-and perimenopausal women with insomnia disorder, Sassoon et al. (2014) found that those in the perimenopausal period reported reduced conscientiousness, agreeableness, and increased neuroticism.

Multidimensional perfectionism
The experience of perfectionism involves overly critical self-evaluation and the inordinate tendency to set and maintain excessively high self-standards (Frost et al., 1990).From a theoretical perspective, perfectionistic individuals become overly concerned with the daytime consequences (e.g.fatigue, impaired concentration) of acute sleep-loss (Lundh & Broman, 2000).In the pre-sleep period, this may fuel a negative thought cycle comprising worry, rumina- difficulty in sleep initiation and maintenance, facilitating the transition from an acute to chronic sleep disturbance (Lundh & Broman, 2000).
Most studies exclusively examined the role of Frost et al.'s (1990) multidimensional conceptualisation of perfectionism.Here, six dimensions characterise the construct of perfectionism: the propensity to be concerned over and react negatively to mistakes (concern over mistakes); doubt one's own performance and actions (doubts about action); perceive one's parents to have high expectations of them (parental expectations); perceive one's parents as overly critical towards them (parental criticism); maintain a high standard of order and organisation (organisation); set and maintain high personal standards (personal standards).Of these, the experience of insomnia appears consistently related to increased reports of doubts about action, concern over mistakes and parental criticism (Akram et al., 2015(Akram et al., , 2017(Akram et al., , 2020;;Andersson et al., 2005;Brand et al., 2015;Jansson-Fröjmark & Linton, 2007;Lundh et al., 1994;Schmidt et al., 2018;Trudel-Fitzgerald et al., 2017;Vincent & Walker, 2000).
Only two studies have examined the role of Hewitt and Flett's (1991) conceptualisation of perfectionism, both failing to yield any relationships between perfectionism and insomnia (Akram et al., 2015;Vincent & Walker, 2000).
Correlational studies sampling Swiss university students evidence perfectionistic doubts and concerns to be associated with the symptoms of insomnia (Brand et al., 2015;Schmidt et al., 2018).Schmidt et al. ( 2018) also found insomnia symptoms to be associated with organisational behaviour.In contrast, Brand et al. (2015) reported that university students experiencing insomnia symptoms showed increased personal standards and parental evaluation.Likewise, in members of the UK general population, symptoms of insomnia appear to be related to doubts about action, concern over mistakes, personal standards, and parental criticism (Akram et al., 2015(Akram et al., , 2020)).Furthermore, tinnitus patients are evidenced to present relationships between insomnia symptoms and perfectionistic concern over mistakes and parental criticism (Andersson et al., 2005).
To date, only three studies have sampled individuals meeting the diagnostic criteria for insomnia (Akram et al., 2017;Lundh et al., 1994;Vincent & Walker, 2000).Lundh et al. (1994) demonstrated that insomnia patients from a sleep disorders clinic reported greater concern over mistakes, doubts about action, and personal standards relative to members of the Swedish general population.However, it should be noted that only these three dimensions of perfectionism were examined.Nevertheless, these findings have partially been replicated in two studies, each demonstrating that individuals with insomnia report increased concern over mistakes, doubts about action and parental criticism when compared with normal-sleeping controls (Akram et al., 2017;Vincent & Walker, 2000).Several longitudinal studies have sought to address limitations of causality and directionality (Akram et al., 2015;Jansson-Fröjmark & Linton, 2007;Trudel-Fitzgerald et al., 2017).Here, Jansson-Fröjmark and Linton (2007) demonstrated that concern over mistakes appears to be significantly related to pre-existing and future insomnia, characterised by sleep initiation or maintenance difficulties.However, it is relevant to note that the authors only collected data for the two subscales: concern over mistakes and personal standards.Sampling French-Canadian cancer patients, Trudel-Fitzgerald et al. ( 2017) evidenced symptoms of insomnia to predict increased levels of perfectionistic doubts and concerns after 2 months.In contrast, Akram et al. (2015) found that the experience of insomnia symptoms predicted a future increase in perfectionistic doubts about action and parental criticism.Finally, the relationship between perfectionism and insomnia has been reported to be mediated by emotional distress (Jansson-Fröjmark & Linton, 2007), stress perception and emotion regulation (Brand et al., 2015;Molnar et al., 2020), counterfactual thinking (Schmidt et al., 2018), symptoms of anxiety (Akram et al., 2015(Akram et al., , 2017)), and dysfunctional sleep-related cognition (Akram et al., 2020).See Table 3 for full study details.

Type-D personality
Type-D personality, also known as the distressed personality, indicates a joint tendency to experience negative emotions whilst also inhibiting self-expression in social interaction due to a fear of rejection or disproval by others.This personality type is characterised by the two stable traits: negative affectivity (NA) and social inhibition (SI).In the UK general population, Type-D personality has been related to symptoms of insomnia, both as a categorical and dimensional construct (Akram, Allen, et al., 2018;Akram, McCarty, et al., 2018).After with coronary heart disease (Frøjd et al., 2021).Additional research comprehensively examined the relationship between affective temperaments and insomnia symptoms in healthy members of the Polish general population (Oniszczenko et al., 2019).Here, the experience of insomnia was related to increased reports of cyclothymia, irritability, and reduced hyperthymia.However, after accounting for shared variance in the prediction of insomnia symptoms, irritability no longer remained significant (Oniszczenko et al., 2019).Whilst Somma et al.
(2020) failed to evidence differences in negative affect between insomnia outpatients and community dwelling adults, those with insomnia experienced greater levels of disinhibition.Finally, negative affect appears to mediate the relationship between insomnia symptoms and paranoid thinking in patients with non-affective psychotic disorder (Reeve et al., 2018).See Table 4 for full study details.
(2012) determined that social introversion at baseline predicted the onset of insomnia at follow-up (7.5 years).See Table 5 for full study details.
Whilst early studies employing the MMPI largely sampled well screened clinical outpatients and matched controls, sample sizes remained rather small in the range N = 12-44 participants.This might be attributed to the extensive methodological nature of screening (diagnostic interviews, polysomnography, 2-week-sleep diaries), the labourious nature of the N = 550 item MMPI, often a secondary outcome to a larger research question.Furthermore, apart from one study conducted in Switzerland (Schneider-Helmert, 1986), the remaining were geographically limited to North America.

Dark triad and antisocial traits
To date, three studies have examined the relationship between insomnia and the dark triad traits (Akram, Allen, et al., 2018;Akram, McCarty, et al., 2018;Sabouri et al., 2016;Zamani Sani et al., 2023).Sampling young Iranian adults, Sabouri et al. (2016) determined that symptoms of insomnia were independently correlated with greater reports of psychopathy and Machiavellianism, but not narcissism.Whilst these relationships were replicated amongst a sample of the UK general population using a measure of insomnia symptomology, only psychopathy predicted insomnia symptoms when shared variance between dark triad traits was controlled for (Akram, Allen, et al., 2018;Akram, McCarty, et al., 2018).
Recent data sampling Iranian student athletes found psychopathic tendencies, but not Machiavellianism or narcissism, to be associated with symptoms of insomnia after controlling for perceived stress (Zamani Sani et al., 2023).See Table 6 for full study details.
Other studies have independently examined the traits either underlying or related to the dark triad.In a sample of depressed outpatients and healthy controls recruited from a medical hospital in China, the symptoms of insomnia were significantly related to increased reports of passive aggressive and narcissistic, but not to antisocial personality traits (Chen et al., 2021).More crucially, insomnia symptoms mediated the relationships between borderline personality and the experience of passive aggression as predictors of depression.In a community-dwelling sample of older North American adults, Oltmanns et al. (2014) determined that increased symptoms of insomnia were significantly related with antisocial and narcissistic personality traits.Relative to healthy controls, insomnia outpatients from a Chinese hospital displayed increased impulsivity and neurotic anxiety (Wang et al., 2001).However, stratifying those with insomnia patients into those presenting with narcissism in the insomnia group.However, no differences in antisocial behaviour were observed.Finally, several studies using the MMPI evidenced increased psychopathic deviance in those with insomnia when compared with normal-sleepers (Fernandez-Mendoza et al., 2011;Kales et al., 1983;Levin et al., 1984;Schneider-Helmert, 1986;Vgontzas et al., 2001).

Additional traits
Individuals meeting the diagnostic criteria for insomnia appear to report greater levels of harm avoidance relative to normal sleepers.
De Saint Hilaire and colleagues (2005) determined individuals with chronic insomnia presented greater reports of harm avoidance and reduced self-directedness relative to controls.Likewise, relative to normal-sleeping controls, primary insomnia patients reported increased harm avoidance and self-transcendence alongside reduced reward dependence (An et al., 2012).Whilst Lee and colleagues evidenced increased harm avoidance amongst individuals with primary insomnia when compared with controls, this trait was significantly more prominent in those with comorbid and isolated depression (Lee et al., 2012).In contrast, when comparing panic disorder patients with and without comorbid insomnia, Na and colleagues failed to evidence differences in novelty seeking, harm avoidance, reward dependence, persistence, self-directedness, cooperativeness, or self-transcendence.

Personality and treatment response
Several studies examined the potential influence of personality in predicting patient response to cognitive behavioural therapy for insomnia (CBTi).In the context of temperament and character, high reward dependence has been found to predict greater treatment response amongst insomnia patients receiving CBTi (An et al., 2012) greater sleep disruption relative to controls a year later (Lee et al., 2012).Here, traits such as obsessive-compulsiveness may predict a deterioration in sleep continuity following successful treatment for insomnia, perhaps contributing to an increased risk of relapse (Lee et al., 2012).In contrast, potentially adaptive dimensions of perfectionism (i.e.greater organisation and personal standards) alongside positive response to verbally projected sentiment and reward signals (i.e.reward dependence) predict facilitation of treatment engagement, adherence, and reduced risk of dropout (Lee et al., 2012;Johann et al., 2023).
Early work evidenced that pre-treatment MMPI scores predicted the efficacy (61% prediction accuracy) or failure (77%) of behaviour therapy for insomnia in improving sleep-onset latency and emotional stability (i.e.neuroticism) in patients with acute and chronic insomnia (Shealy et al., 1980).Interestingly, the observation of greater MMPI validity scores in acute versus chronic patients offers a novel insight regarding the developmental course of insomnia.According to the authors, the onset of insomnia activates an initial overconcern and exaggeration of symptom severity, an effect which chronologically declines with acceptance of the problem (Shealy et al., 1980).Conversely, after observing individuals with acute insomnia over the course of a month, recent work failed to show a predictive role of the five-factor traits in determining natural remission or persistence of the disorder (Ellis et al., 2021).Beyond the predisposing role of the Spielman and Glovinsky (1991) model, the latter work questions the relevance of personality once insomnia becomes a chronic condition.
That said, the persistent changes in the duration of symptom presentation required for the diagnosis of acute and chronic insomnia may account for differential outcomes between studies.

DISCUSSION
The current work systematically identified studies that examined the relationship between personality and insomnia from both a diagnostic and a symptom level.The outcomes reliably showed that the experience of insomnia is associated with personality traits which are typically considered to be negative or maladaptive in nature.More specifically, the experience of insomnia appears consistently related with neuroticism, introversion, perfectionistic doubts and concerns, elevated personal standards, negative affect, social inhibition and avoidance, hysteria, hypochondriasis, psychasthenia, impulsive behaviour, anger, hostility and psychopathic tendencies, schizotypal and borderline traits, reduced conscientiousness and self-directedness, and negatively perceived perception of the self.

The role of personality from a theoretical perspective of insomnia
There are two prominent models which may help to further understand the interaction between personality and insomnia.From a stress-diathesis approach, disorder vulnerability may be accentuated by the expression of predisposing traits that interact with significantly stressful life events to precipitate the onset of insomnia (Spielman & Glovinsky, 1991).According to Spielman and colleagues' ( 1987 There is evidence suggesting that personality is a predisposing and possibly an accentuating and perpetuating factor for insomnia.In particular, we observed a clear pattern of results where the experience of insomnia is mainly linked with maladaptive personality variables and traits.Studies deploying the Goldberg (1993) Big Five model consistently reported that insomnia symptoms are associated with reduced emotional stability, extraversion, conscientiousness, and to a lesser extentagreeableness.Studies examining the role of multidimensional perfectionism found that insomnia is associated with doubts about action, concern over mistakes, and parental criticism (Akram et al., 2015(Akram et al., , 2017(Akram et al., , 2020;;Andersson et al., 2005;Brand et al., 2015;Jansson-Fröjmark & Linton, 2007;Lundh et al., 1994;Schmidt et al., 2018;Trudel-Fitzgerald et al., 2017;Vincent & Walker, 2000).Moreover, when investigating individuals with Type-D personality, negative affect emerges as the main predictor of insomnia at the disorder level with social inhibition being also independently related to insomnia symptoms (Akram, Allen, et al., 2018;Akram, McCarty, et al., 2018;Domagalska et al., 2021;Uygur et al., 2023).
Employing the MMPI to examine personality and psychopathology found that insomnia is linked to higher levels of hypochondriasis, psychasthenia, and hysteria.Furthermore, using the dark triad approach yielded mixed evidence regarding the relationship of insomnia with narcissism and antisocial personality traits (Akram, Allen, et al., 2018;Akram, McCarty, et al., 2018;Sabouri et al., 2016;Zamani Sani et al., 2023).Against this background, it can be reasonably argued that maladaptive personality traits are important for understanding the development and maintenance of insomnia.
In relation to the precipitating event and situational variability, maladaptive trait(s) may impair the ability to deploy necessary adaptation and coping skills, where favourable safeguarding approaches include increased sleep effort, extended time in bed, and elevated cognitive activity; each key factor implicated in the development and maintenance of insomnia (Espie et al., 2006;Harvey, 2002).Here, persistence of these behaviours in those with certain traits may increase  (Akram et al., 2020;Schmidt et al., 2018).

Does personality influence treatment response?
To date, outcomes concerning the insomnia-personality relationship provide important evidence that personality plays a role in the treatment of insomnia in terms of efficacy and long-term success.A randomised controlled trial showed that patients receiving CBTi presented with increased levels of perfectionistic organisation and personal standards after completion of therapy (Johann et al., 2023).
Alterations of this nature may reflect the very structured nature of CBT-I where patients are instructed to strictly adhere to the agreed bedtimes and methods of stimulus control whilst receiving weekly feedback on their progress.Accordingly, future studies should examine whether such changes (i.e.elevated levels of perfectionistic organisation and personal standards) are temporary side effects of CBT-I.
In the context of temperament and character, high reward dependence (i.e.positive response to verbally projected sentiment and reward signals) have also been found to predict greater treatment response amongst insomnia patients receiving CBTi (An et al., 2012).

Methodological considerations
Nevertheless, the findings of this review identified several key limitations of the literature to date.Of the studies reviewed in the current work, very few examined possible longitudinal relationships between personality and insomnia (Akram et al., 2015;Jansson-Fröjmark & Linton, 2007;Lee et al., 2012;Trudel-Fitzgerald et al., 2017;Singareddy et al., 2012), This disproportionate reliance on crosssectional data inherently limits any tangible conclusion to be drawn in relation to cause-and-effect.Numerous studies have failed to identify the required sample size using a power analysis calculation to ensure reliability of results.In addition, substantial variation between sample populations and inclusion criteria limits the extent that each study outcome may be extrapolated to clinical or wider populations.Indeed, many studies drew upon members of the general population completing self-report measures of insomnia symptom severity.While the relationship between insomnia symptoms and personality traits may certainly provide valuable insights, generalisability to insomnia patients remains limited.Finally, age, ethnicity, and gender have not been sufficiently investigated yet.As subtle changes in personality traits are observed across the lifespan and, additional work should examine the possible mediating role age which may influence insomnia differently in various stages of life (Costa Jr et al., 2019;Lucas & Donnellan, 2011;McCrae et al., 1999).
Moving forward, we offer suggestions for future research which Between 50% and 93% of individuals with a personality disorder present at least one key feature of insomnia (Kamphuis et al., 2013;Selby, 2013;Semiz et al., 2008), whereas approximately 50% of individuals with insomnia also display key features of at least one personality disorder (Somma et al., 2018).In the former scenario, individuals seeking cognitive behavioural treatment for their personality disorder may benefit from a CBTi session.Certainly, by providing individuals with the correct information about sleep we may prevent acute sleep difficulty from transitioning into a long-term problem by preventing dysfunctional cognitions which facilitate increased behavioural efforts to sleep.Indeed, as evidenced recently, this may be achieved through a single session of CBT-I supplemented with a self-help guide (Ellis et al. 2015).Against the backdrop that personality appears to play a substantial role in insomnia and its treatment, future studies should also examine how CBT-I might be tailored to a patient's personality.
More tailor-made approaches to CBT-I might help to better reach the target population, increase treatment adherence, and reduce dropout.
This way it might be possible to increase treatment success and prevent relapse.

Strengths and limitations of current review
A number of strengths and limitations of the current work should be noted.To the best of the authors knowledge, this is the first review concerning the relationship between personality and insomnia since the diagnostic paradigm shift away from primary, secondary, and additional subtypes of insomnia in favour of a more

CONCLUSION
The inherent nature of personality serves as a predisposing, and possible maintaining factor of insomnia.When exploring the outcomes of N = 76 reviewed studies, the literature evidenced insomnia and/or the experience of insomnia symptoms to be reliably associated with negatively oriented and maladaptive personality traits.However, methodological differences limit the ability to draw causal conclusions regarding the specific traits involved in the aetiology of insomnia.In addition, our findings highlight meth- Medline; and ScienceDirect.The following Boolean terms were used for searching titles and abstracts: ('poor sleep' OR 'insomnia') AND ('personality' OR 'big five' OR 'five factor model' OR 'extraversion' OR 'introversion' OR 'neuroticism' OR 'emotional stability' OR 'openness' OR 'agreeableness' OR 'conscientiousness' OR 'type-d' OR 'social inhibition' OR 'negative affect' OR 'dark triads' OR 'psychopathy' OR 'narcissism' OR 'Machiavellianism' OR 'schizotypal' OR 'borderline' OR 'perfectionism' OR 'temperament' OR 'internalisation' OR 'internalisation' OR 'MMPI' OR 'MMPI-2' OR 'Minnesota Multiphasic Personality Inventory' OR 'Neuroticism Extraversion Openness Five Factor Inventory' OR 'NEO FFI' OR 'Karolinska Scales of Personality' OR 'KSP' OR 'Eysenck personality inventory' OR 'EPI' Number of records idenঞfied based on iniঞal database searching (30 th December 2022) N = 20923 -N = 1750 duplicates removed… …leaving N = 19173 studies screened Retrieval of full-text arঞcles considered relevant a[er reviewing ঞtle and abstract: N = 135 Studies that fulfilled the eligibility criteria and were included in the review: N = 62 Exclusion of irrelevant arঞcles and duplicates based on ঞtle and abstract review: N = 19038 Exclusion of full-text arঞcles (N = 73) that did not meet eligibility criteria, with reasons: • Measure of sleep quality and not insomnia (N = 13) • Studies missing variables of interest (N = 27) • Wrong paঞent populaঞon (N = 10) • Inadequate insomnia/symptom assessment (N = 5) • No comparison or control group (N = 9) • Pediatric populaঞon (N = 2) • Review arঞcle (N = 2) • Arঞcle retracted by Editor (N = 1) • Conference abstract (N = 1) • Duplicate (N = 2) • No record of an exisঞng full text (N = 1) Addiঞonal manual search (June 2023) yielded N = 14 addiঞonal studies meeঞng the eligibility criteria for inclusion: Final Inclusion: N = 76 F I G U R E 1 Flowchart presenting the literature search and selection process.[Color figure can be viewed at wileyonlinelibrary.com]T A B L E 2 Outcomes and characteristics from reviewed studies examining insomnia and the five-factor model of personality negatively related to increased levels of agreeableness, contentiousness, and extraversion, and reduced emotional stability.When accounting for shared variance and controlling for age and sex, regression analysis determines only conscientiousness and emotional stability to remain significant predictors of insomnia Symptoms of insomnia were positively associated with neuroticism, and negatively associated with conscientiousness and extraversion.When accounting for shared variance between these three traits, neuroticism remained the only significant predictor of insomnia(Continues) Abbreviations: ASI-3, anxiety sensitivity scale; BFI-10, Brief Big Five personality inventory; DASS-21, depression, anxiety and stress scale; ISI, insomnia severity index; MINI-IPIP, Mini Five factor personality scale; NEO-FFI, Neo Five factor inventory; RSQ, relationships scale questionnaire; SCID-II, structured clinical interview for DSM-IV Axis II personality disorders; TIPI, Ten item personality inventory; WHIRS, women's health initiative insomnia rating scale; ZKPQ, Zuckerman-Kuhlman personality questionnaire.
accounting for comorbid sleep disorder symptoms,Akram and colleagues (2018)  observed increased insomnia severity amongst Type-D relative to non-Type D individuals.Whilst negative affect and social inhibition were independently related to insomnia symptoms, negative affect emerged to be the most prominent predictor of insomnia when accounting for shared variance.These outcomes are partially replicated amongst Turkish university students(Uygur et al., 2023) and Polish high-school teachers(Domagalska et al., 2021) where increased reports of insomnia symptoms were observed in those being characterised as Type-D individuals.Likewise, North American students meeting the diagnostic criteria for insomnia demonstrated increased negative affect and social avoidance compared with normal sleeping controls(Yuksel et al., 2022).While the relative roles of NA and SI remained unexamined, behavioural sleep effort and vulnerability to stress-related sleep disturbances were associated with the experience of Type-D personality(Uygur et al., 2023).In survivors of colorectal cancer, individuals characterised as Type-D presented a twofold increase in symptoms of insomnia relative to non-Type-D controls after controlling for sociodemographic factors, time since diagnosis, chemotherapy, and depression(Husson et al., 2015).In particular, those displaying negative affect, but not social inhibition, reported the highest level of insomnia symptomatology when compared with controls and Type-D individuals.Other studies have independently examined the traits which underlie Type-D personality.In a sample of UK university students, Scott et al. (2017) determined symptoms of sleep-onset insomnia and sleep maintenance difficulties to be associated with increased reports of negative affect.In other work, after identifying subtypes of insomnia based T A B L E 3 Outcomes and characteristics from reviewed studies examining insomnia and dimensions of perfectionism over mistakes were related to pre-existing insomnia at baseline whilst also predicting the future onset of insomnia Akram et al., , HF-MPS At baseline, insomnia symptoms were associated with doubts about action and parental criticism.Longitudinal analysis determined baseline symptoms of insomnia predicted future doubts about action and parental criticism.This latter outcome was partially mediated by anxiety Brand et al., , HF-MPS Compared with normal-sleepers, individuals with insomnia presented increased greater concern over mistakes, doubts about action, and parental criticism.These differences were partiality mediated by symptoms of anxiety, Insomnia symptoms were related to greater reports of doubts about action and concern over mistakes, but reduced organisation on affective traits and increased distress, negative affect appeared to increase in a dose response manner (Blanken et al., 2019).Oltmanns et al. (2014) evidenced a significant relationship between symptoms of insomnia and social avoidance amongst a sample of the US general population.Furthermore, when accounting for sociodemographic factors, physical activity, body mass index and dietary health, the experience of Type-D personality appears to predict the onset of insomnia in patients Abbreviations: DBAS, dysfunctional beliefs about sleep questionnaire; F-MPS, Frost multidimensional perfectionism scale; HADS, hospital anxiety and depression scale; HF-MPS, Hewitt & Flett multidimensional perfectionism scale; ISI, insomnia severity index; NSQ, Nordic sleep questionnaire; USI, Uppsala sleep inventory.
Abbreviations: AIS, Athens insomnia scale; ATQ, adult temperament questionnaire; DASS-21, depression, anxiety and stress scale; EORTC QLQ-C30, European Organisation for Research and Treatment of Cancer: A Quality of Life Instrument; GPTS-B, Part B of Green Paranoid Thoughts Scale; GTCI, Global Talent Competitiveness Index; ISI, insomnia severity index; ITQ, insomnia type questionnaire; MAPP, multi source assessment of personality pathology; PSQI, Pittsburgh sleep quality index; SCIDP-IV, structured clinical interview for DSM-IV axis 1 disorder; SLEEP-50, SLEEP-50 Questionnaire; SPEQ, psychotic experiences questionnaire; TCQI-R, thought control questionnaire insomnia revised; TDS, Type D scale; TEMPS-A, temperament evaluation of Memphis, Pisa, Paris and Santiago questionnaire.
. Relatedly, in patients receiving CBTi, Johann et al. (2023) observed elevated levels of perfectionistic organisation and personal standards on completion of treatment when compared with baseline.In a sample of patients with hypnotic dependant insomnia, Petrov and colleagues (2018) examined the role of obsessive-compulsive features in predicting the response to behavioural therapy for insomnia.Here, obsessive-compulsive features (based on cluster C personality disorders) failed to predict treatment outcomes based on subjective reports of symptoms severity following treatment and 1-year follow-up.Nevertheless, following examination of objectively observed sleep using polysomnography, those initially indicating obsessive-compulsive traits evidenced shorter sleep duration and ) 3-Pmodel, a series of predisposing, precipitating, and perpetuating factors influence the course of insomnia, personality traits such as perfectionism and neuroticism are understood to be predisposing factors that make the onset of insomnia more likely.Also underscoring the role of personal traits in their contribution to the development and maintenance of the disorder, the Lundh and Broman (2000) model integrates sleep-interfering and sleep-interpreting processes.According to the authors, physiological, emotional, and cognitive arousal, a key component of insomnia, is influenced by arousability and interpersonal relations.Here, arousal is influenced by emotional sensitivity and the (slow) pace of habituation, whereas interpersonal are affected by conflicts and emotional involvements.
the possibility of an acute bout of sleep-disturbance transitioning into insomnia.Furthermore, the nature of personality may predict the likelihood of emerging cognitive and/or behavioural factors.With that in mind, those experiencing perfectionistic doubts and concerns, and an unrealistic level of personal standards may alter behavioural strategies to compensate for their sleep deficit and consequently aim to perfect sleep through increased sleep-efforts (i.e.increased time in bed through napping or attempting to sleep earlier than normal).This maladaptive behaviour may cause individuals to experience sleep loss during the night and deficits during daytime, which helps to fuel the vicious circle of insomnia, especially when paired with possible mediating factors including dysfunctional sleep-related cognition and presleep arousal may improve and expand upon the literature to date.Addressing the fundamental limitations discussed (i.e.longitudinal data, cross cultural research, adequate control variables, consideration of age and gender) should remain the priority.Whilst the current picture confirms negatively oriented traits to be typically associated with insomnia, given the lack of prospective research, the precise and most prominently predictive traits related to the onset of insomnia remains unclear.Likewise, it remains unclear whether the experience of insomnia eventually accentuates the presentation of otherwise stable traits.As such, future work should seek to provide a welldefined portrayal of directional causality.This knowledge would allow for crucial questions to be explored: could therapeutic work in relation to the most prominent traits function to curb associated behaviours which perpetuate insomnia in the face of an acute sleep disturbance?These may include traits characteristic of social withdrawal and isolation (e.g.social introversion, social inhibition, negative affect), those related to reduced structure and punctuality (e.g.conscientiousness, perfectionistic organisation, and personal standards), and related difficulties in emotion regulation and self-critical emphasis (e.g.self-disgust:Ypsilanti et al., 2018).
practical and causal free classification of insomnia disorder.Indeed, prior reviews of the current topic largely examined the role of personality in the identification of insomnia subtypes (Van deLaar et al., 2010).Here, target assessments for inclusion (i.e.NEO FFI, MMPI, MMPI-II, KSP, EPI, TCI, ZKPQ, F-MPS) yielded substantial degree trait overlap for greater reliability of subtype identification.Expanding upon this approach, the current work used more comprehensive search terms related to conceptualisations of personality, individual traits, and subdimensions.Several studies were excluded from the final analysis due to the use of non-validated measures or with an exclusive focus on sleep quality rather than the experience of insomnia specific symptoms.The ongoing and problematic inclusion of sleep quality serves to convolute outcomes of reviews that specifically pertain to the experience of insomnia.Likewise, single item or bespoke in-house scales to fail to adequately capture the true extent of symptom experience, often leading to exaugurated prevalence statistics and misrepresentation of the captured problem.Future work should aim to consistently use validated measures directly related to the research question.Next, whilst a comprehensive search was undertaken, the results may not be fully representative of the actual relation due to missing manuscripts.Finally, the review included only studies published in English language, and thus studies in other languages may provide different results to those reported here.
odological factors related to study design, sample population, and discrepant use of personality measures which may influence outcome variation.Moreover, given the absence of longitudinal data, we cannot infer causal influence on the development and maintenance of insomnia.Nevertheless, personality traits should be considered when assessing insomnia in research and in a clinical context and additional work should clarify the role of personality in predicting treatment outcomes and adherence.AUTHOR CONTRIBUTIONS Umair Akram: Conceptualization; methodology; data curation; investigation; writingoriginal draft; writingreview and editing; project administration; resources.Jodie Stevenson C: Investigation; data curation.Maria Gardani: Investigation; data curation.Sarah Allen: Investigation; data curation.Anna Johann F: Conceptualization; investigation; writingoriginal draft; writingreview and editing; methodology; project administration; data curation; resources.