Pittsburgh Sleep Quality Index (PSQI)

1. Abstract

The Pittsburgh Sleep Quality Index (PSQI) is a landmark 19-item, self-administered psychometric instrument designed to measure subjective sleep quality and sleep disturbance patterns over a designated 1-month recall interval. Developed in 1989 by Daniel J. Buysse and colleagues at the Western Psychiatric Institute and Clinic of the University of Pittsburgh School of Medicine, the instrument operationalizes sleep quality through a multidimensional architecture composed of seven discrete clinical components: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleep medication, and daytime dysfunction. These seven components are derived from 19 self-rated items (supplemented by five optional bed partner or roommate questions used solely for clinical adjunct evaluation) and are individually weighted on an ordinal scale ranging from 0 to 3. The sum of these component scores yields a singular global score ranging from 0 to 21, wherein a score greater than 5 demonstrates high diagnostic sensitivity and specificity in discriminating between good sleepers and clinically impaired poor sleepers.

Extensive psychometric investigations spanning three decades across diverse psychiatric, medical, occupational, and non-clinical cohorts have consistently demonstrated robust internal consistency (Cronbach’s alpha typically ranging between .70 and .85), high test-retest reliability (Pearson’s r and intraclass correlation coefficients ranging from .82 to .87 across intervals of several weeks), and substantial convergent validity against polysomnography, wrist actigraphy, and validated sleep and psychiatric inventories. While originally postulated as a unidimensional instrument with seven subordinate facets, contemporary structural equation modeling and confirmatory factor analyses frequently reveal that a two-factor (sleep efficiency and sleep quality) or three-factor structure (sleep efficiency, perceived sleep quality, and daytime impairment) provides superior model fit across various demographic groups and clinical entities. Today, the PSQI remains the international gold standard in sleep assessment, widely utilized in psychopharmacological trials, behavioral sleep medicine, neuroepidemiology, and psychosomatic research worldwide.

2. Keywords

Pittsburgh Sleep Quality Index, PSQI, subjective sleep quality, sleep latency, sleep efficiency, sleep disturbances, psychometrics, daytime dysfunction, insomnia assessment, clinical polysomnography, self-report sleep questionnaire, sleep medicine

3. Authors

The Pittsburgh Sleep Quality Index was conceptualized, developed, and empirically validated by a multidisciplinary team of sleep researchers and psychiatric clinicians at the University of Pittsburgh School of Medicine:

• Daniel J. Buysse, M.D. — Professor of Psychiatry and Clinical and Translational Science, Western Psychiatric Institute and Clinic, Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
• Charles F. Reynolds III, M.D. — Professor of Psychiatry, Neurology, and Neuroscience, Western Psychiatric Institute and Clinic, Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
• Timothy H. Monk, Ph.D., D.Sc. — Professor of Psychiatry, Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
• Susan R. Berman, M.S. — Western Psychiatric Institute and Clinic, Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
• David J. Kupfer, M.D. — Thomas Detre Professor of Psychiatry and Professor of Neuroscience, Western Psychiatric Institute and Clinic, Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.

4. Purpose

The Pittsburgh Sleep Quality Index was engineered to address a pervasive diagnostic and methodological void in psychiatric diagnostics and somnology research during the late 1980s. Prior to its dissemination, empirical evaluation of sleep disturbance was heavily polarized between two divergent methodologies: resource-intensive objective laboratory evaluations, such as in-laboratory nocturnal polysomnography (PSG), and simplistic, single-item self-report queries or daily sleep diaries that lacked standardized psychometric validation. Polysomnography, while exceptional for discerning physiological microarchitecture, electroencephalographic anomalies, sleep-disordered breathing, and periodic limb movements, routinely failed to capture the perceived phenotypic experience of clinical distress characteristic of primary insomnia and psychiatric-related sleep pathology. Conversely, unvalidated questionnaires suffered from idiosyncratic scoring rules, ambiguous recall intervals, and poor discriminative accuracy. The PSQI was systematically designed to bridge this chasm by delivering an efficient, standardized, multidimensional self-report instrument capable of generating clinically meaningful, psychometrically rigorous data regarding sleep quality over a standardized 1-month retrospective timeframe.

From a clinical perspective, the primary objective of the PSQI is to distinguish between individuals experiencing healthy, restorative sleep (“good sleepers”) and those suffering from significant, clinically disruptive sleep architecture deficits (“poor sleepers”). In both outpatient and inpatient clinical workflows, the instrument serves as a critical screening mechanism for diagnosing primary insomnia, co-morbid sleep pathologies associated with major affective and anxiety disorders, chronic pain conditions, fibromyalgia, oncology-related fatigue, and neurodegenerative diseases. By assessing specific functional elements such as habitual sleep efficiency, latency, fragmentation, pharmacotherapeutic dependence, and daytime somnolence, the PSQI enables clinicians to pinpoint the precise behavioral and physiological targets requiring therapeutic intervention, such as Cognitive Behavioral Therapy for Insomnia (CBT-I) or pharmacological adjustment.

In epidemiological, clinical trial, and basic behavioral science contexts, the PSQI provides an indispensable quantitative endpoint. Its operational stability facilitates longitudinal monitoring of therapeutic efficacy across randomized controlled trials evaluating pharmacotherapy, chronobiological interventions, psychotherapy, and lifestyle modifications. Furthermore, because sleep disturbances represent prominent transdiagnostic markers and prospective risk factors for cardiovascular disease, metabolic syndrome, neurocognitive decline, and suicide, the PSQI functions as an essential epidemiological metric for investigating the bidirectional biological links connecting sleep architecture, physiological homeostasis, and psychiatric morbidity.

5. Psychological Construct

Sleep quality represents a complex, multi-tiered neurobiological and phenomenological construct that evades reduction to simple temporal duration. Although physiological variables such as total sleep time (TST) and sleep architecture transitions (e.g., stages N1, N2, N3 slow-wave sleep, and rapid eye movement sleep) provide an objective scaffold, the subjective perception of sleep restorative capacity is modulated by psychological, cognitive, and somatic factors. The PSQI operationalizes this holistic construct across seven distinct, interrelated components:

Component 1: Subjective Sleep Quality

This component reflects the respondent’s overarching cognitive and affective appraisal of their nocturnal sleep experience over the preceding month. Rather than quantifying a specific duration or frequency, it captures the global qualitative sentiment (rated from “very good” to “very bad”) regarding whether the individual feels refreshed, satisfied, and restored by their sleep. This subjective evaluation often correlates strongly with cognitive arousal, nocturnal rumination, and mood states, demonstrating that psychological appraisal frequently overrides objective physiological parameters in shaping patient-reported distress.

Component 2: Sleep Latency

Sleep latency encompasses both the quantitative estimation of the time required to achieve sleep onset (measured continuously in minutes) and the ordinal frequency of experiencing delayed sleep onset exceeding 30 minutes. Psychophysiologically, protracted sleep latency reflects somatic hyperarousal, pre-sleep cognitive worry, elevated sympathetic nervous system activity, or circadian phase delays. In the PSQI scoring paradigm, these two questions are combined and mapped onto a 0–3 metric to index the functional severity of sleep-initiation insomnia.

Component 3: Sleep Duration

This dimension quantifies the absolute number of hours of actual sleep obtained each night, distinct from total time spent in bed. Sleep duration represents a primary indicator of sleep restriction or sleep deprivation. Chronically shortened sleep duration (<6 hours) is empirically linked to neuroendocrine dysregulation, elevated pro-inflammatory cytokines, metabolic pathology, and cognitive impairment, while prolonged duration (>9 hours) can indicate secondary hypersomnia or underlying systemic pathophysiology.

Component 4: Habitual Sleep Efficiency

Habitual sleep efficiency is a mathematical ratio derived from the participant’s reported sleep duration divided by their total time in bed (calculated from reported bedtime to morning waking time), multiplied by 100%. This objective metric provides critical psychophysiological context: poor sleepers frequently spend substantial, frustrating intervals lying awake in bed trying to initiate or regain sleep, leading to a conditioning mechanism where the sleep environment becomes paired with anxiety and arousal rather than relaxation. High sleep efficiency (≥85%) is characteristic of healthy, consolidated sleep.

Component 5: Sleep Disturbances

This composite facet evaluates nine distinct nocturnal disruptions that fragment sleep continuity. These include intrinsic physical discomforts (inability to breathe comfortably, coughing or loud snoring, feeling uncomfortably cold or hot, somatic pain), parasomnias (frequent nocturnal nightmares), early morning or middle-of-the-night awakenings, and the need to get up to use the bathroom (nocturia). Together, these items capture fragmented sleep maintenance and nocturnal physical distress, providing an empirical profile of the physiological and environmental disruptors eroding sleep integrity.

Component 6: Use of Sleep Medication

This component records the frequency of using pharmacotherapeutic aids—including prescription hypnotics, sedatives, benzodiazepines, non-benzodiazepine receptor agonists (“Z-drugs”), and over-the-counter sleep aids (such as antihistamines or melatonin)—to induce sleep. Pharmacotherapy utilization serves as an index of both the behavioral severity of the sleep disorder and the degree of somatic or psychological dependence required to sustain sleep, often indicating chronic, treatment-resistant insomnia.

Component 7: Daytime Dysfunction

Sleep cannot be understood in isolation from diurnal functioning. Component 7 captures the ecological validity of sleep disturbance by assessing its daytime functional consequences. It measures two core domains: involuntary somnolence during awake activities (trouble staying awake while driving, eating meals, or engaging in social activity) and cognitive-motivational deficits (the difficulty encountered in maintaining adequate enthusiasm to complete tasks). This reflects the downstream systemic impact of non-restorative or fragmented sleep on daytime executive functioning, affective stability, and vocational performance.

6. Theoretical Framework

The construction and clinical utility of the Pittsburgh Sleep Quality Index are deeply rooted in contemporary psychophysiological and cognitive-behavioral models of insomnia, most notably Arthur Spielman’s classic 3P Model of Insomnia (Spielman et al., 1987), Charles Morin’s Cognitive Model of Insomnia (Morin, 1993), and Michael Perlis’s Neurocognitive Model of Insomnia (Perlis et al., 1997).

The Spielman 3P Model

Spielman’s diathesis-stress paradigm conceptualizes the development and maintenance of chronic insomnia across three distinct temporal vectors: Predisposing, Precipitating, and Perpetuating factors. Predisposing vulnerabilities (e.g., biological traits such as elevated baseline sympathetic tone, hyper-aroused hypothalamic-pituitary-adrenal [HPA] axis functioning, or trait anxiety) lower the threshold for sleep disruption. Precipitating events (e.g., acute medical illness, interpersonal bereavement, vocational crises) trigger transient acute insomnia. However, as the acute event wanes, maladaptive compensatory behaviors emerge as perpetuating factors. These perpetuating behaviors—such as spending excessive compensatory hours in bed, taking daytime naps, relying heavily on hypnotic pharmacotherapy, and engaging in frustrated clock-watching—permanently decouple the bed from restorative sleep. The PSQI directly interrogates these perpetuating elements, capturing the resulting poor sleep efficiency, high sleep latency, and prolonged daytime functional deficits.

Cognitive and Neurocognitive Frameworks

Morin’s cognitive framework posits that sleep difficulties trigger negative cognitive cascades, catastrophic misappraisal of the consequences of sleep loss, unrealistic sleep expectations, and heightened performance anxiety regarding the sleep process. These dysfunctional beliefs evoke autonomic arousal and emotional distress, establishing a vicious feedback loop. The PSQI measures the behavioral and perceptual manifestations of this loop: subjective sleep quality (Component 1) and daytime dysfunction (Component 7) capture the psychological appraisal and distress of insomnia, while sleep disturbances (Component 5) and latency (Component 2) reflect the resulting hyperarousal. Perlis and colleagues subsequently expanded this framework into a neurocognitive model, demonstrating that individuals with chronic insomnia exhibit conditioned cortical hyperarousal, characterized by elevated fast-frequency (beta and gamma) electroencephalographic activity during NREM sleep. This biological state impairs the typical sensory gating of environmental and somatic stimuli, causing subjective sleep depth to feel shallow and fragmented—an outcome comprehensively documented by the PSQI’s component scoring architecture.

7. Validity

The Pittsburgh Sleep Quality Index has undergone exhaustive empirical validation across a broad spectrum of medical, psychiatric, and cross-cultural cohorts. Its validity profile encompasses robust construct, convergent, discriminant, and criterion-related parameters.

Construct and Structural Validity

The original psychometric validation by Buysse et al. (1989) demonstrated that the PSQI possesses exceptional construct validity. The seven component scores demonstrated strong positive correlations with the global score (ranging from r = .55 to r = .83), indicating that each subcomponent contributes substantial, distinct variance toward the overarching construct of sleep pathology. Factorial construct validity has been extensively confirmed through structural equation models demonstrating coherent latent factors mapping onto sleep efficiency, sleep quality, and daytime impairment.

Convergent and Concurrent Validity

Convergent validity has been established through comparisons with both objective somnographic measurements and subjective self-report instruments:

• Polysomnography and Actigraphy: While the PSQI measures a broader perceptual construct than acute laboratory recordings, Buysse et al. (1989) demonstrated significant correlations between the PSQI global score and laboratory-derived polysomnographic measures, including polysomnographic sleep latency (r = .43), total sleep time (r = -.38), and wake after sleep onset (r = .39). Similarly, actigraphy studies over 7- to 14-day intervals consistently show significant convergent alignments between the PSQI habitual sleep efficiency and objective actigraphic sleep efficiency (typically r ≈ .40–.55).
• Self-Report Sleep Inventories: The PSQI shows exceptionally strong positive correlations with the Insomnia Severity Index (ISI; r = .70 to .82) and the Epworth Sleepiness Scale (ESS; r = .40 to .55), validating its ability to reflect subjective severity and excessive diurnal sleepiness.
• Psychiatric Scales: Given the high comorbidity between sleep disturbance and psychiatric disorders, the PSQI global score exhibits strong convergent relationships with the Beck Depression Inventory (BDI; r = .50 to .65) and the State-Trait Anxiety Inventory (STAI; r = .45 to .60).

Discriminant and Criterion-Related Validity

The clinical utility of the PSQI is primarily founded upon its criterion-related discriminative power. In the seminal 1989 validation trial, a global PSQI cut-off score of >5 accurately identified 89.6% of patients with clinical sleep disorders (sensitivity) while correctly identifying 86.5% of healthy control participants (specificity). Receiver operating characteristic (ROC) curves across subsequent international validation studies have consistently yielded area under the curve (AUC) values exceeding .85 to .93. The instrument reliably discriminates clinical insomniacs, patients with major depressive disorder, generalized anxiety disorder, fibromyalgia, and chronic renal disease from matched healthy reference cohorts, confirming its role as an effective clinical screening boundary.

8. Reliability

The PSQI possesses highly stable psychometric reliability indices across multiple clinical populations and non-clinical control groups.

Internal Consistency

In the original validation study by Buysse et al. (1989), the internal consistency of the seven component scores yielded an overall Cronbach’s alpha of .83, reflecting a harmonious, homogeneous psychometric instrument. Subsequent large-scale meta-analyses and systematic reviews (e.g., Mollayeva et al., 2016; Carpenter & Andrykowski, 1998) across diverse medical and linguistic groups have corroborated this finding, with Cronbach’s alpha coefficients consistently reported between .70 and .85. Component-to-total score correlations remain reliably elevated, confirming that each clinical component shares common variance with the overarching construct of sleep disturbance.

Test-Retest Reliability

Because the PSQI utilizes a 1-month recall window, test-retest reliability assessments must balance instrument stability with true clinical fluctuation. Studies evaluating test-retest reliability across intervals ranging from 2 days to 6 weeks in clinically stable populations report Pearson correlation coefficients (r) and intraclass correlation coefficients (ICC) ranging between .82 and .87 for the global score. In a comprehensive study by Backhaus et al. (2002) evaluating primary insomniacs, the test-retest reliability for the global score across a 2- to 4-week interval was .87, with individual component test-retest correlations ranging from .65 to .84, demonstrating excellent temporal stability in the absence of targeted therapeutic interventions.

9. Factor Analysis

Although the PSQI was originally conceived as a unidimensional instrument that sums seven clinical components into a singular global index, extensive psychometric scrutiny utilizing Exploratory Factor Analysis (EFA) and Confirmatory Factor Analysis (CFA) has sparked ongoing discussion regarding its optimal underlying latent structure.

The Unidimensional vs. Multidimensional Debate

The original singular latent construct proposed by Buysse et al. (1989) assumes that all seven components load uniformly onto a single overarching factor: General Sleep Quality. While adequate for basic clinical screening, subsequent psychometric investigations have frequently found that the unidimensional model exhibits poor or mediocre goodness-of-fit indices (e.g., Root Mean Square Error of Approximation [RMSEA] > .08, Comparative Fit Index [CFI] < .90) in complex clinical cohorts.

The Two-Factor Model

Investigators such as Buysee and colleagues themselves, alongside European validation cohorts, evaluated a two-factor structure. This model typically divides the instrument into:

• Factor 1 (Sleep Efficiency): Comprising sleep duration and habitual sleep efficiency.
• Factor 2 (Sleep Quality / Impairment): Comprising subjective sleep quality, sleep latency, sleep disturbances, sleep medication, and daytime dysfunction.

The Three-Factor Model (Cole et al., 2006)

The most widely accepted and replicated structural model is the three-factor model proposed by Cole et al. (2006) in an extensive psychometric evaluation of older adults, later validated in cancer patients, university students, and psychiatric outpatients. The three correlated latent factors are defined as follows:

• Factor 1: Sleep Efficiency — Defined by Component 3 (Sleep Duration) and Component 4 (Habitual Sleep Efficiency), capturing the physiological, temporal conservation of sleep.
• Factor 2: Perceived Sleep Quality — Defined by Component 1 (Subjective Sleep Quality), Component 2 (Sleep Latency), and Component 6 (Use of Sleep Medication), capturing the cognitive, qualitative appraisal of initiating and securing sleep.
• Factor 3: Daily Disturbances / Daytime Impairment — Defined by Component 5 (Sleep Disturbances) and Component 7 (Daytime Dysfunction), capturing fragmented sleep ecology and diurnal consequences.

Confirmatory factor analyses testing Cole’s three-factor model routinely yield superior fit statistics (e.g., CFI ≥ .95, Tucker-Lewis Index [TLI] ≥ .94, and RMSEA ≤ .05), suggesting that reporting the individual three factor indices alongside the global score provides enhanced clinical granularity, particularly when parsing the differential effects of targeted somatic or behavioral sleep therapies.

10. Instrument / Measurement Tool

• Test Type: Self-report questionnaire / psychometric assessment tool.
• Format: Pen-and-paper self-administered survey, digital clinical form, or computer-assisted interview.
• Item Count: 19 self-rated questions (which generate the 7 composite scores). An additional 5 questions are rated by a bed partner or roommate (if applicable), but these 5 items are solely used for supplementary clinical information and are NOT factored into the 7 components or the global score.
• Recall Window: The past 1 month (30 days).
• Administration Time: Approximately 5 to 10 minutes.
• Response Scale / Formats:
  • Items 1–4: Open-ended time and duration responses (e.g., typical bedtime, minutes to sleep onset, morning rising time, actual hours slept).
  • Items 5a–5j, 7, 8: 4-point Likert-type frequency scale (0 = Not during the past month, 1 = Less than once a week, 2 = Once or twice a week, 3 = Three or more times a week).
  • Item 6: 4-point Likert-type rating (0 = Very good, 1 = Fairly good, 2 = Fairly bad, 3 = Very bad).
  • Item 9: 4-point Likert-type severity rating (0 = No problem at all, 1 = Only a very slight problem, 2 = Somewhat of a problem, 3 = A very big problem).
• Component Scoring Rules: The 19 items are mapped onto 7 component scores, each scored from 0 (no impairment) to 3 (severe impairment):
  • Component 1: Subjective Sleep Quality: Equivalent to the score of Item 6 (rated 0–3).
  • Component 2: Sleep Latency: Calculated from Item 2 (≤15 min = 0; 16–30 min = 1; 31–60 min = 2; >60 min = 3) added to the score of Item 5a (0–3). The sum (0–6) is recoded: 0 = 0; 1–2 = 1; 3–4 = 2; 5–6 = 3.
  • Component 3: Sleep Duration: Calculated from Item 4 (>7 hours = 0; 6–7 hours = 1; 5–6 hours = 2; <5 hours = 3).
  • Component 4: Habitual Sleep Efficiency: Computed as: [Total Sleep Hours (Item 4) / Total Hours in Bed (calculated from Item 1 to Item 3)] × 100%. Scored as: ≥85% = 0; 75–84% = 1; 65–74% = 2; <65% = 3.
  • Component 5: Sleep Disturbances: Items 5b through 5j are summed (range 0–27). The total sum is recoded: 0 = 0; 1–9 = 1; 10–18 = 2; 19–27 = 3.
  • Component 6: Use of Sleeping Medication: Equivalent to the score of Item 7 (rated 0–3).
  • Component 7: Daytime Dysfunction: Item 8 score (0–3) is summed with Item 9 score (0–3) yielding a range of 0–6. Recoded as: 0 = 0; 1–2 = 1; 3–4 = 2; 5–6 = 3.
• Global Score Calculation: Sum of Component 1 + Component 2 + Component 3 + Component 4 + Component 5 + Component 6 + Component 7.
• Score Range: 0 to 21, where higher scores indicate worse sleep quality and greater sleep pathology.
• Diagnostic Cut-off: A global score > 5 indicates poor sleep quality, differentiating good from poor sleepers with high diagnostic sensitivity (≥89.6%) and specificity (≥86.5%).

11. Permissions & Fee and Test Year

The Pittsburgh Sleep Quality Index was initially developed and published in 1989 by Daniel J. Buysse, Charles F. Reynolds III, Timothy H. Monk, Susan R. Berman, and David J. Kupfer at the University of Pittsburgh. The scale is copyrighted by the University of Pittsburgh.

For standard non-commercial academic research, non-sponsored clinical investigations, and individual medical practice, the PSQI is broadly accessible and may be used without royalty fees, provided proper academic attribution and bibliographic citation are maintained. However, for-profit entities, funded pharmaceutical clinical trials, commercial diagnostic platforms, software applications, or healthcare organizations seeking to integrate the PSQI into commercial electronic medical record interfaces or proprietary diagnostic software must obtain formal licensing permission and pay corresponding licensing fees through the University of Pittsburgh’s Office of Technology Management (OTM) or authorized licensing distributors.

12. References

• Backhaus, J., Junghanns, K., Broocks, A., Riemann, D., & Hohagen, F. (2002). Test-retest reliability and validity of the Pittsburgh Sleep Quality Index in primary insomnia. Journal of Psychosomatic Research, 53(3), 737–740. https://doi.org/10.1016/s0022-3999(02)00330-6
• Buysse, D. J., Reynolds, C. F., Monk, T. H., Berman, S. R., & Kupfer, D. J. (1989). The Pittsburgh Sleep Quality Index: A new instrument for psychiatric practice and research. Psychiatry Research, 28(2), 193–213. https://doi.org/10.1016/0165-1781(89)90047-4
• Carpenter, J. S., & Andrykowski, M. A. (1998). Psychometric evaluation of the Pittsburgh Sleep Quality Index. Journal of Psychosomatic Research, 45(1), 5–13. https://doi.org/10.1016/s0022-3999(97)00298-5
• Cole, J. C., Motivala, S. J., Buysse, D. J., Monga, M. N., & Irwin, M. R. (2006). Validation of a 3-factor scoring model for the Pittsburgh Sleep Quality Index in older adults. Sleep, 29(1), 112–116. https://doi.org/10.1093/sleep/29.1.112
• Mollayeva, T., Thurairajah, P., Burton, K., Mollayeva, S., Shapiro, C. M., & Colantonio, A. (2016). The Pittsburgh Sleep Quality Index as a screening tool for sleep dysfunction in clinical and non-clinical samples: A systematic review and meta-analysis. Sleep Medicine Reviews, 25, 52–73. https://doi.org/10.1016/j.smrv.2015.01.009
• Morin, C. M. (1993). Insomnia: Psychological assessment and management. Guilford Press.
• Perlis, M. L., Giles, D. E., Mendelson, W. B., Bootzin, R. R., & Wyatt, J. K. (1997). Psychophysiological insomnia: The behavioural model and a neurocognitive perspective. Journal of Sleep Research, 6(3), 179–188. https://doi.org/10.1111/j.1365-2869.1997.00179.x
• Spielman, A. J., Caruso, L. S., & Glovinsky, P. B. (1987). A behavioral perspective on insomnia treatment. Psychiatric Clinics of North America, 10(4), 541–553. https://doi.org/10.1016/s0193-953x(18)30532-x

13. Items of the Scale (Questionnaire)