The association between insomnia and cardiovascular risk factors in bus drivers in Iran

1 Introduction

The relationship between psychological parameters and various work conditions has been one of the main concerns of many researchers for decades [1]. One class of people at risky jobs is drivers, especially bus drivers. The potential health risk related to bus drivers is not only important to the health and safety of drivers, but also to the enormous community involved in this kind of transit system [2]. In the public community, there is some concern with general safety and the health of others who could potentially be damaged by aging drivers with decreasing ability [3]. There is a high probability for bus drivers to undergo severe damage and death because of specific bus driving features. It is significant to identify and prevent factors associated with accidents in this population to minimize harm and improve road safety [4]. As the World Health Organization (WHO) reported, mortality rate due to traffic accidents is predicted to increase by 80% by 2020, especially in developing countries [5].

Cardiovascular diseases (CVD) and sleep disturbances of professional drivers are essential issues in this occupational field, which may result in a high risk for accident [5, 6]. Many studies conducted on professional bus drivers have revealed an increased risk of heart disease [6, 7]. However, the underlying etiology for increasing CVD is uncertain. A systematic review of published studies from Iran has estimated the prevalence of hypertension to be 22.1% in the general population [8].

Sleep is an active, cyclic biological phenomenon necessary for survival. Insomnia is another important and common disorder among professional drivers. Sleep patterns are affected by many parameters such as psychological, biological, cultural, and social factors [9–11]. Insomnia affects the nervous system function, which has potentially damaging risk to the cardiovascular system, may alter the blood pressure response, and increases hypertension risk [11]. Compared to normal subjects, insomniac cases cause seven fold more accidents [12]. The prevalence of insomnia in general population in Iran is 59.2% [13].

It has been shown that sleep disorders like obstructive sleep apnea (OSA) can increase the crash risk of motor vehicle drivers by two to seven fold [12], and this may be higher in the heavy vehicle drivers who are over-represented in sleep disorder researches [14]. Excessive daytime sleepiness (EDS) is a major public health problem, leading to impaired cognitive function, reduced alertness, and increased risk of motor vehicle crashes [4]. EDS is one of the most frequent sleep complaints affecting 4% to 12% of the general population. Predictive factors of vehicle accidents could be excessive daytime sleepiness, reduced number of sleep hours, shift work, excessive driving time, and use of alcohol and other drugs [10, 16].

Injuries related to road traffic have only been recently known as a major public health issue in Iran.

Since road accidents constitute the third most common cause of mortality in the general population after the cardiovascular diseases, and more than 25,000 individuals die yearly in road accidents in Iran, the physical health of drivers plays a major role in preventing these accidents and improving road safety [17]. This study was designed to investigate the rate of prevalence of any of the important risk factors (CVD, insomnia) of road accidents among drivers and their probable inter-relationships

2 Methods

This cross-sectional study was carried out among suburban professional bus drivers who were employed full-time by the Ministry of Road and Urban Development in the Sanandaj City, Kurdistan, Iran. Based on the medical examinations system, as part of the drivers’ required annual license renewal process, all of the 1232 drivers, representing 100% of the target population, were included in the study. This study was conducted between May 2011 and October 2012, and data analysis was limited to male drivers because there was no female driver within the study area. We obtained informed written consent from every driver. Two different kinds of data sets were collected: (1) Sleep disorders information using Athens Insomnia Scale (AIS), for which reliability and validity have been globally confirmed [18]. AIS is based on International Classification of Diseases, 10th Revision (ICD-10) criteria and was validated by Soldatos et al., with Cronbach’s α= 0.89. The questionnaire consisted of eight items. Five items estimated the delay in sleep induction, awakening during the night, final awakening earlier than desired, total sleep duration, and overall quality of sleep. Three other items were about the effect of insomnia on the quality of work in the following day (a sense of well-being, functioning, and sleepiness during daytime). Each item was valued from 0 to 3 (no problem, slightly, markedly and very, respectively) based on self-assessment (See Appendix A) [19]. The participants were requested to rate the scores if they experienced any problem in sleep at least three times in a week during the last month. Scores obtained by each responder were calculated; the total score ranged from 0 to 24; those whose score was less than 6 were considered as “without insomnia”, whereas those whose score was equal or more than 6 were considered as “with insomnia” [18, 19]. (2) The subjects were examined by an occupational medicine specialist, a systematic measurement of blood pressure was done, and electrocardiography (ECG), total cholesterol, triglyceride and fasting blood sugar (FBS) were measured using c16000 chemistry analyzer.

Based on the new data on lifetime risk of hypertension and the significant increase in the risk of cardiovascular disorders accompanied by levels of BP previously rendered as normal, the Seventh Report of the Joint National Committee (JNC-7) on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure has provided a new taxonomy including the term “prehypertension” for those with BPs with a range of 120–139 mmHg systolic and/or 80–89 mmHg diastolic. The aim of the new classification is identifying individuals in whom early treatment by adoption of healthy lifestyles could decrease BP, reduce the amount of progression of BP to hypertensive levels with increasing age, or prevent hypertension completely. This designation is based on the mean of two or more appropriately measured, seated, BP readings on each of two or more office visits [20, 21].

A short-term 12-lead electrocardiogram was obtained and assessed based on the Minnesota code. According to the criteria, the ECG results were categorized into major and minor abnormalities [22]. Electrocardiograms were measured at a paper speed of 25 mm/s, at a gain of 10 mm/mV, using electrocardiographic tools available at the institutes (FCP130-A9, FCP145-M4, and FCP270-M5, Fukuda Denshi, Tokyo, Japan). Hypercholesterolemia and hypertriglyceridemia were defined as ≤00 mg/dl and ≤150 mg/dl respectively, according to standard laboratory procedures.

3 Results

A total of 1232 bus drivers with sufficient data qualified for inclusion in the analysis. All participants were male, with a mean age of 42.75 (±10.95) years in persons with insomnia and 40.95 (±10.4) in non-insomniac persons, BMI of 26.5 (±4.1) and 26.3 (±4.1) Kg/m² and work experience of 18.1 (±10.8) and 16 (±9.4) respectively (Table 1). Based on the criteria stipulated by the JNC, 7163 drivers (13.3%) were hypertensives considering systolic blood pressure and 169 (13.8%) of the drivers had diastolic hypertension and 63.5% (n = 782) had systolic pre-hypertension (Table 2). Mean cholesterol was 173 mg/dL, with 222 subjects (18%) exhibiting levels above 200 mg/dL. Mean triglyceride level was 155 mg/dL, with 482 subjects (39%) exhibiting levels above 150 mg/dL. We found that the mean FBS was 104 mg/dL that 123 subjects had level about 126 mg/dL, and 31 drivers (2.5%) had abnormal ECG (Table 3).

Our study demonstrated that the prevalence of insomnia in professional bus drivers was 8 percent. A high percentage of drivers (35.4%) complained of delaying in sleep induction, as intense, markedly, or slightly. Moreover, it was found that 25.5% drivers complained of awakening during the night. Meanwhile, 201 (16.3%) drivers complained of sleepiness during daytime, whereas 11.4% of individuals reported symptoms suggesting dissatisfaction from overall quality of sleep (Table 4).

There were significant correlations between age, diastolic hypertension and work experience with final awakening earlier than desired (P < 0.05), the rate of the triglyceride level and total sleep duration (p = 0.02), and, BMI and sleepiness during the day (P < 0.05), although these associations were not linear. Except for the above variables, we did not find any significant correlation between sleep-related symptoms and cardiac risk factors.

4 Discussion

There are many parameters included work history, physical and mental health, and healthcare access which affect on the driver’s health [23]. The aim of the present study was to characterize a population of Iranian professional bus drivers with respect to demographic, lipid profile, the presence of cardiovascular risk factors, and insomnia.

In the present study, systolic blood pressure values revealed that more than 25% of the bus drivers were characterized as normal subjects and the prevalence of hypertension in the whole sample was 13%, and 63.5% (n = 782) had systolic pre-hypertension. We also noticed that the greater the BMIs, the greater was the proportion of hypertensive subjects and the correlation between the two was completely significant (P < 0.01) (Fig. 1).

Although some studies have identified a trend towards higher blood pressure among shift workers in some occupations, the present study and the results of other studies showed no statistically significant differences in both the mean systolic and diastolic blood pressures [24–28].

Several studies reported that the systemic hypertension has been raised in professional drivers [24, 26]. For instance, Cavagioni et al. conducted a study in Brazil on the drivers who transport shipments. In fact, they found that the prevalence of SBP (i.e. ≥130 mmHg) or diastolic blood pressure (DBP) (≥85 mmHg) was 59%, and 63.5% (n = 782) had systolic pre-hypertension [29].

Based on the studies conducted in Iran, the prevalence of hypertension in Iran is reported to be 42.7% which is higher than the rate reported in our study [8]. One reason may be due to the fact that the drivers in Iran are selected from among the healthy individuals based on the protocols set by the responsible organizations. In fact, not all drivers can enter this profession. Another important reason is related to the method of measurement and assessment of BP which was JNC-7 in this study. Although the prevalence of hypertension was reported to be 13% in this study, 63.5% of drivers are at the pre-hypertension stage suggesting that there has been an increase in hypertension rate rather than a decrease. The sample in the present study had high frequencies of hypertriglyceridemia (39%) and hypercholesterolemia (18%). A triglyceride level of 150 mg/dL is one of the five accepted criteria for the definition of the individual risk of cardiovascular disease [30, 31]. Previous studies reported different findings; for example, prevalence rates of hypercholesterolemia and hypertriglyceridemia among drivers carrying goods were found to be 33 and 38%, respectively [32], whereas these values for drivers carrying passengers were 34.0 and 69.4%, respectively [2].

Insomnia is a common sleep disorder in the world, affecting at least 8.4–11.8% of the adult population in Iran [33]. Our study demonstrated that the prevalence of insomnia in professional bus drivers was 8 percent.

A number of studies conducted in Sweden [34], the USA [35], have revealed high prevalence of insomnia, while some other researches in France [36], England, Germany, Italy [37, 38], and Spain [39] were among those suggesting low prevalence of insomnia. Our study revealed a comparatively low incidence of insomnia among the bus drivers (8%) which is consistent with other studies.

It seems that two parameters have played a significant role in decreasing the prevalence of Insomnia: a) the pre-employment medical and physical examinations and acquiring the required qualifications on the part of bus drivers b) other preventive measures as the cyclical examinations of drivers as part of the drivers’ required annual license renewal process.

As various methodologies and evaluation instruments have been applied in different investigations, the prevalence of SDD has also varied within both population and drivers (40). In Australia, Johns and Hocking did an evaluation of SDD among 507 workers by the Epworth Sleepiness Scale (ESS) where they have found 10.9% prevalence. However, there was no correlation between SDD and age (22–59 years), sex, obesity, or drug abuse, but there was a correlation with reduced number of sleeping hours and insomnia [41]. In addition, Souza et al. reported a 14% prevalence of SDD in Brazilian population [42]. Here in this study, we have found a 16.3% prevalence of SDD that seems to be different with regard to the general population (27.9%) [33]. SDD has been reported to be one of the factors that have highest risk for road accidents based on the Stoohs et al. study in 90 distant commercial drivers aged between 20–64 [43].

Another factor that may cause road accidents is reduction of sleep duration. Mc Cartt et al., moreover, studied long-distance truck drivers in the United States, and reported that reduction of sleep duration has been a predictive factor of falling asleep at the steering wheel [44]. Häkkänen and Summala reported that 21% of short-haul studied drivers in Finland had problem with staying conscious. Again, reduction of sleep duration was a predicting factor [45]. Here in this study, decreased sleep time was 17.7% (n = 218) and the correlation was significant (p < 0.05).

There was no statistically significant correlation between AIS and any of the cardiovascular parameters. However, it is important to note that diastolic pressure and triglyceride level were the only cardiovascular parameters which showed a significant correlation with some of sleep complaints in the questionnaire. This finding was consistent with some findings of the previous studies.

It also seems that the following parameters affected the obtained results greatly: limitations of the study as the inclusion of only healthy drivers in the study, excluding individuals with insomnia and cardiovascular disorders as drivers, ignoring obstructive sleep apnea (OSA) as one important differential diagnosis of insomnia, and its correlation with cardiovascular diseases. It is recommended that a follow-up study of these individuals in the form of a cohort study be carried out as a complementary research to approve or reject the findings of this study.

5 Conclusion

Generally, these results are consistent with previous studies. The clinical characterization of bus drivers’ population revealed a high frequency of cardiovascular risk factors, such as BMI, diastolic hypertension and hyperlipidemia.

Although there was no statistically significant correlation between cardiovascular diseases and AIS, our study suggests that elevated serum triglycerides level and increased diastolic blood pressure may be associated with some sleep complaints in drivers.

Conflict of interest

Authors are not declaring any conflict of interest in this study.