The sample consisted of 372 Spanish professional drivers from the transport sector (93.4 % men, 6.6 % women), with an average age of 40.9 (SD= 10.54). They included drivers involved in passenger transportation 33.3 %, freight transport 28.0 %, ambulances drivers 2.4 %, and taxi drivers 36.3 %; the average years of experience was 10.46 (SD=13.05). The average length of time they had been driving professionally was 10.46 (SD= 13.05). Marital status: Married or in a couple (70.8%), single (21.2%), divorced/ separated/ widowed (8.0%). The distribution of education level was as follows: had not finished primary education (20.6%), lower secondary school, professional training-II or primary school (21%), upper secondary school, professional training-I or compulsory secondary education diploma (55.2%), University studies (3.2%).

La Groningen Sleep Scale (GSQS -15; Meijman et al., 1990) was translated into Spanish using translation and “back translation” (Hambleton et al., 2005; Muñiz & Bartram, 2007). This scale evaluates the subjective quality of sleep, covering the following complaints: the general quality of sleep, insufficient sleep, problems falling asleep, problems sleeping, and waking up feeling unrested. The Dutch version consists of 15 items, and its structure is unifactorial (α = 0.89). Six-point answers are applied (1 = Strongly disagree to 6 = Strongly agree), e.g., “1. La pasada noche dormí profundamente [Last night I slept soundly]". The items are grouped so that a higher score on the scale means less subjective sleep quality.

The Trans Driver Stress (TDS-38; Robert-Sentís, 2016) is the Spanish version of the Bus Driver Stress Scale (BDS-59; Dorn et al., 2010). The TDS-38 uses a Likert scale of 6 points and consists of five factors: "F1. Relaxed Driving", refers to the distension or tension that the driver has while driving, as well as before and after driving (7 items, α = 0.7); "F2. Hazard Prevention", indicates the effort made while driving and the possible dangers that may be encountered while driving on the road (8 items, α = 0.77); "F3. Alertness and vigilance", refers to the ease with which the driver can relax while driving or after it (6 items, α = 0.7); "F4. Search for Emotions", refers to the way of driving (5 items and α = 0.8); and "F5. Fatigue and Anxiety", indicates fatigue and the state of anxiety that driving causes in the driver (12 items, α = 0.76).

The Burnout scale (MBI-G; Salanova et al., 2000) assesses burnout and comprises 15 items (3 subscales). The subscale of "Exhaustion (α = 0.87)" comprises 5 items (e.g., "6. Estoy ‘quemado’ por el trabajo [I am 'burned out' by work]"), that of "Cynicism (α = 0.85)" consists of 5 items (e.g., "9. He perdido entusiasmo por mi trabajo [I have lost enthusiasm for my work]") and "Professional efficiency (α = 0.78)" includes 6 items (e.g., "12. I have achieved many valuable things in this position"), with Likert-type responses of 7 points ranging from 0. Never / Never to 6. Always / Every day.

The Irritation Scale (Mohr, 1986; Mohr et al., 2006), in its Spanish version, was adapted by Merino et al. (2006). This scale has 8 items and 2 subscales. The first subscale is called "F1. Emotional irritation" (α = 0.86) and is composed of 5 items (e.g., "6. Me enfado con facilidad [I get angry easily]"); and the second one is called "F2. Cognitive Irritation" (α = 0.87) and is composed of 3 items "(e.g.,"4. Incluso en vacaciones no puedo dejar de pensar a veces en los problemas del trabajo [Even on holidays, I can’t stop thinking about work problems]"). The answers use a Likert scale of 7 points (from 1. Strongly disagree to 6. Strongly agree).

The version of Musculoskeletal Problems (MP-9; Robb & Mansfield, 2007) adapted to Spanish by Robert-Sentís (2016) makes it possible to evaluate musculoskeletal problems and vibrations. It consists of 9 items, and their structure is bifactorial: "F1. Musculoskeletal aspects," refer to the body (e.g., shoulders), (α = 0.72). "F2. Extremities" (e.g., knees), consider the vibrations that indicate the comfort of the seat and the lumbar adjustment (α = 0.7). It is answered with a Likert scale of five points (1 = Never and 5 = Always).

The Trans-18 scale (TRANS-18; Boada-Grau et al., 2012) is for detecting safety behaviours (personal and in the vehicle) as well as psychophysiological disorders and includes 18 items (3 subscales). "F1. Psychophysiological disorders" of the driver (α = 0.81) refer to the occurrence of anxiety, stress, digestive, depressive, musculoskeletal and hypertension disorders (e.g., 11. Mi trabajo me ha producido algún trastorno de depresión [My work has caused me a depression disorder]"). "F2. Personal safety behaviours" (α = 0.80). e.g., not driving after drinking an alcoholic beverage or having eaten copiously, such as not eating and / or drinking while driving; it also refers to not smoking or talking on a mobile phone while driving (e.g., "7. Eludo conducir fumando y no tengo el cigarro, puro, purete,... en la mano [I avoid smoking while I drive and I don’t hold a cigarette or cigar in my hand]"). "F3. Safety behaviour in the vehicle "(α = 0.70) considers aspects such as using gloves to perform work activities, knowing how fire extinguishers work, and being attentive to driving and resting for the established number of hours (e.g., 3. Utilizo los guantes de trabajo cuando manipulo y cargo mercancías, cambio una rueda, etc. [I use work gloves when I handle the cargo, change a wheel, etc.]").

The Dickman's Impulsivity Inventory scale (DII; Dickman, 1990), in its Spanish version (Chico et al., 2003), consists of 23 items and 2 subscales, and the response format is dichotomous (1 = true / 0 = false): "F1. Functional Impulsivity" evaluates the impulsiveness that favours and helps us to adapt to unexpected situations that require a quick response. It consists of 11 items (α = 0.77) (e.g., "5. La mayor parte del tiempo puedo concentrarme en mis trabajos de forma rápida [Most of the time I can concentrate on my work quickly]"). And "F2. Dysfunctional impulsivity" refers to that impulsivity that can cause problems and is formed by 12 items (α = 0.76), e.g., "2. Frecuentemente digo lo primero que se me ocurre sin pensar mucho antes [I often say the first thing that comes to mind without thinking much before]".

Other data were also collected, such as age, seat comfort, seat suspension, adjustable lumbar support for the driver's seat, and the number of driving hours.

We used non-probabilistic sampling (Hernández et al., 2004), also called random-accidental sampling (Kerlinger & Lee, 2004), to obtain the sample.

To collect the data, we made telephone contact with the directors of several transport companies and agreed on the best moment to meet the drivers. We produced a booklet that included all the questionnaires to be used and instructions on how to complete them. A psychologist was responsible for collecting all the data at a particular company and ensuring that no questionnaire was missing any data. Several ethical guidelines were taken into account (Declaration of Helsinki, the Belmont Report, and the Council for International Organizations of Medical Sciences Guidelines), and all participants provided informed consent. The response rate was approximately 80%.

The AMOS Program (Analysis of Moment Structures) (version 5.0) makes it possible to analyse latent structures and create structural equations (SEM). This program makes it easier to specify, estimate, evaluate and present a model in an intuitive diagram that shows the possible relationships between the variables. Therefore, the validity of the model can be tested and confirmed. A step diagram can be elaborated, and modelling can obtain an immediate result concerning the 15 items applied in the program. The SPSS program (version 23.0) was used to calculate internal consistency (Cronbach’s alpha) and evidence of convergent validity (Pearson correlations). Indications of validity were calculated using Pearson correlation coefficients between the subfactors of the scales (TDS-38, MBI-GS, Irritation, MP-9, and Trans-18) and the Groningen Sleep Scale (GSQS -15).

The results of the Bartlett sphericity test (Chi-square, df =20 = 55.123; p = 0.00) show that the AFE is appropriate (Kaiser, 1974). The sample adequacy is .936. Parallel analysis (Lattin et al., 2003; Timmerman & Lorenzo-Seva, 2011) and the “minimum partial partial” criterion of Velicer (1976) confirmed that the eight-item unifactorial solution is adequate. The Promin rotation method was used to establish the EFA (Lorenzo-Seva, 1999). The variance explained was 44.90% (Table 1).

Table 1

(a) Reverse items, (b) Definitive items of the scale derived from the AFC.

Table 2 shows the mean, the standard deviation, the reliability coefficients, and the confidence intervals of the GSQS-8. The reliability of the scale is 0.90 with a 95 % confidence interval of 88 to 0.92. Therefore, the values were higher than the recommended value of 0.70 (Nunnally & Bernstein, 1994).

Table 2 Table 2 GSQS-8: Descriptive statistics, reliability, confidence intervals

Validity indicators were calculated using Pearson correlation coefficients between the subfactors of the scales (TDS-38, MBI-GS, Irritation, MP-9, DII and Trans-18) and the Groningen Sleep Scale (GSQS-8). External correlates were also used. Table 3 shows the correlations between the GSQS-8 factors and the different correlates and instruments. The external correlates used are associated with several factors, both directly and inversely.

Thus, GSQS-8 is positively associated with 13 correlates (days that you have felt tired or without energy during the past week, times you have felt tired or without energy for more than 3 hours on any day during the past week, times you have felt so tired that you had to force yourself to do things during the past week, days that you have gone to work sick, search emotions, fatigue and anxiety, exhaustion, cynicism, emotional and cognitive irritation, musculoskeletal problems in the body and extremities and psychophysiological disorders), and negatively with 7 correlates (the comfort of the seat, the suspension of the seat, the adjustable lumbar support of the seat, relaxed driving, prevention of hazards, safety behaviours in the vehicle and functional impulsivity) (see Table 3).

Table 3

**p < 0.01; *p < 0.05. Bold remarks significant p values.