Elsevier

Safety Science

Volume 126, June 2020, 104664
Safety Science

Associations between high caffeine consumption, driving safety indicators, sleep and health behaviours in truck drivers

https://doi.org/10.1016/j.ssci.2020.104664Get rights and content

Highlights

  • 1,653 low and 1,354 high caffeine consuming truck drivers were compared.

  • High caffeine consumers felt sleepier and reported shorter sleep duration.

  • High caffeine consumers exhibited poorer health behaviours e.g. smoking, poor diet.

  • Caution is needed if recommending caffeine to manage driver sleepiness.

  • High caffeine use is associated with poorer health and does not reduce crashes.

Abstract

Background

Truck drivers are vulnerable to driver sleepiness due to pressures of shift work and prolonged driving hours; this is a safety concern as sleepiness more than doubles crash risk. Caffeine is a commonly used countermeasure to driver sleepiness. Previous research with truck drivers has demonstrated a positive impact of caffeine consumption on crash risk. However, habitual use of caffeine has the potential to impair night time sleep resulting in increased sleepiness and safety risk. One group that may be particularly vulnerable to the potential negative impacts of high caffeine consumption are those truck drivers who consume ≥ 5 caffeinated drinks per day (≥90th percentile American average daily caffeine consumption).

Method

1,653 self-reported low (1 caffeinated drink per day) and 1,354 high (≥5 caffeinated drinks per day) caffeine consumers were compared across a range of driving safety indicators, health and sleep variables. All drivers completed an initial background questionnaire including the Epworth Sleepiness Scale (ESS), Berlin Questionnaire (BQ) and Dula Dangerous Driving Index (DDDI). Crashes and driving violations were monitored for up to 3 years (depending on enrolment date). Medical Examination Reports were available for 83.0% of the 3,007 participants.

Results

High caffeine consumers were more likely to report poor sleep outcomes: shorter average sleep time, (7.08 h compared to 7.41 h), higher prevalence of excessive daytime sleepiness (7.5% compared to 5.7%) and higher proportion at high risk of obstructive sleep apnoea (OSA) on the BQ (13.1% compared with 9.4%). The higher caffeine consumers were more likely to report negative health behaviours: smoking, alcohol consumption, poor diet and infrequent exercise. Poorer driving safety indicators (negative emotions and aggressive driving on the DDDI) and previous crashes (27.8% compared to 21.6%) were also more common in high than low caffeine consumers.

Conclusion

Caffeine is an effective countermeasure to driver sleepiness, however, high caffeine consumers do not exhibit any benefit to driving safety indicators. In fact, there is some evidence for safety concern because high caffeine consumers self-report more crashes and worse driving safety indicators than low caffeine consumers. High caffeine consumption was associated with poor health behaviours. Further research is needed to fully understand the implications of high caffeine use. Interventions aiming to reduce truck driver sleepiness should be cautious in promotion of caffeine in isolation; a wholistic approach to improve driver health would likely be more effective than focusing on sleep health alone.

Introduction

The trucking industry is vital for transporting the freight goods on which society depends. This is particularly true in the USA where road transport is the major method of freight transport. It is estimated that more than 10 billion tons of freight per year are moved on US roads, equating to approximately 70% of all freight transportation in the country (American Trucking Association, 2018). To meet this demand truck drivers are exposed to increasing pressures, such as long work hours, reduced sleep periods, expectation to drive at night and over multiple consecutive days, all of which have been associated with increased sleepiness (McCartt et al., 2000). This is concerning as the risk of crashing more than doubles when drivers are sleepy (Bioulac et al., 2017). One countermeasure to increase alertness and combat driver sleepiness is caffeine consumption.

Caffeine is a chemical compound found in many products. Over the last 10 years there has been an increase in caffeine consumed in drinks (Mitchell et al., 2014). This stimulant acts on the brain, but is widely available and subject to minimal regulations. Many everyday drinks (including coffee, tea, energy drinks and sodas) contain caffeine. Consumption of caffeine gives the user a feeling of alertness because caffeine is a neuro inhibitor, that is, it acts on the brain to supress adenosine receptors (Fredholm et al., 1999, Magkos and Kavouras, 2005). Adenosine levels are known to increase in the brain with duration since last sleeping. These increasing adenosine levels exert an increase in “sleep pressure” that creates a feeling of sleepiness. By blocking the adenosine receptors feelings of sleepiness will be reduced. However, adenosine continues to accumulate in the brain even though the receptors are blocked (Fredholm et al., 1999). This means that when the caffeine is broken down and removed by the body there is an influx of adenosine joining to the now vacant receptors which can result in extreme increase of sleepiness, sometimes colloquially referred to as a “caffeine crash” (Walker M., 2017).

The positive impact of caffeine in reducing sleepiness and improving task performance are well documented (e.g. Reyner and Horne, 2000, Mets et al., 2011, Smith, 2002). Furthermore, the wealth of evidence supporting caffeine use to improve alertness has led to its status as a commonly recommended countermeasure to reduce fatigue. For example, the UK Highway Code states “the most effective ways to counter sleepiness are to drink, for example, two cups of caffeinated coffee and to take a short nap (at least 15 min)” (UK Government, 2018). The benefits of caffeine consumption for truck drivers has been reported in several studies. For example, in a case control study of truck crashes, drivers who used caffeine were 63% less likely to crash than those who did not (Sharwood et al., 2013). Similarly, in a naturalistic study of truck drivers, those who self-reported using caffeine were half as likely to be involved in safety critical events as drivers that did not use caffeine (Camden et al., 2014). Another naturalistic driving study reported a 6% reduction in safety critical events per 8 oz of caffeinated beverage consumed (Heaton & Griffin, 2015). Although there are clear benefits for caffeine use by truck drivers compared to no caffeine at a population level, little is known about the potential for disadvantages of high caffeine consumption.

A high level of caffeine consumption has the potential for negative health consequences, with one key problem being the impact on sleep. Caffeine has a half-life of 3–7 h (Kaplan et al., 1997), meaning that 7 h after consumption half the amount of caffeine could still be present in the body. Consequently, people who consume high amounts of caffeine during the day will likely still have the caffeine in their system at night time which may impair their sleep. This may affect both the quantity and quality of sleep (Carrier et al., 2009). Within controlled laboratory experiments caffeine has consistently been shown to increase sleep latency, shorten sleep duration and reduce the amount of deep sleep obtained (for review see Roehrs and Roth, 2008).

A further problem associated with caffeine is its addictive nature. Caffeine dependency develops at relatively low daily levels (Roehrs and Roth, 2008) and it is difficult to determine the impact that habitual high caffeine consumption might have on the use of additional caffeine as an acute countermeasure to driver sleepiness. The more caffeine a person consumes, the more accustomed to it a person becomes, and they may need to consume greater volumes of caffeine to maintain the same baseline alertness level. Despite the potential concern, no studies were identified that specifically considered the influence of habitual caffeine consumption on the use of acute caffeine as a countermeasure to driver sleepiness. However, it has been reported that in regularly administering caffeine to both habitual and non-habitual consumers there was no difference in the acute benefits of caffeine on attention, reaction time and working memory between the groups (Haskell et al., 2005). Also, the impact of acute caffeine on athletic performance is not influenced by habitual caffeine consumption (Gonçalves et al., 2017). This suggests that caffeine can continue to improve alertness even with habitual consumption. Beyond this, evidence is limited as protocols to investigate the impact of caffeine commonly require participants who are usually caffeine consumers to abstain from caffeine for a number of days before the study. As such, there is some discussion over whether the findings of such laboratory studies are confounded by the reversal of caffeine withdrawal, rather than benefit of the caffeine per se (Heatherley, 2011).

Although there is evidence that caffeine use reduces crash risk, it is naïve to recommend caffeine as the only answer to crash safety without considering the broader implications for driver’s health. High levels of caffeine consumption place pressure on the body, the long-term impacts of which are complex. For example, in a controlled laboratory study, high caffeine consumption was shown to increase the blood pressure of infrequent caffeine consumers (Corti et al., 2002). Although it is not known if similar effects would be found with high caffeine consumers, any potential influence of caffeine consumption on driver health is concerning because poor health and chronic conditions such as cardiovascular disease and obstructive sleep apnoea (OSA) are associated with increased crash risk (Apostolopoulos et al., 2013, Burks et al., 2016, Howard et al., 2004, Thiese et al., 2015), and the prevalence of these conditions is often high amongst truck drivers, likely due to their associations with obesity and the higher obesity rates among drivers. Furthermore, caffeine has been shown to influence behaviour. For example, there is some evidence that habitual caffeine consumers are more impulsive (Giles et al., 2017) and that high caffeine consumption in young adults is associated with sensation seeking behaviour (Jones and Lejuez, 2005). If these behaviours are translated to driving then high caffeine consumers may be more likely to exhibit risk taking behaviour which is associated with an increased crash risk (Jonah, 1997).

The current study focuses on high (≥5 drinks per day) caffeine consumers, that is, those who daily consume more caffeine than 90% of the US population (Mitchell et al., 2014). By focusing on this group, the current study seeks to identify whether high caffeine consumption is of concern for truck drivers in relation to driving safety indicators and health behaviours. High caffeine consumers will be compared to low (1 drink per day) caffeine consumers. Low caffeine consumers are defined as those that consume less than the mean (2 drinks per day) daily caffeine intake of the US population (Mitchell et al., 2014). Nil caffeine users are not considered in this study because they are unusual compared to the majority of the US population who do consume caffeine (92%) (Mitchell et al., 2014). By limiting the study to caffeine consumers the results will be focused on the influence of caffeine itself, rather than on a comparison with nil users, which has the potential to identify trait differences between those who consume caffeine and those who choose to abstain from caffeine (Roehrs and Roth, 2008).

Section snippets

Participants

Data were collected from a convenience sample of 11,414 truck drivers participating in the Commercial Driver Safety Risk Factors (CDSRF) study (Mabry, J.E., 2019). Drivers were recruited at eight locations across the USA: Illinois, Ohio, Kentucky, New Jersey, Virginia, Georgia, Texas and California. Seven of the recruitment sites were associated with the same carrier, and the eighth site was an independent occupational health clinic located in Virginia. All drivers completed an initial

Results

In total, 1,653 drivers met the inclusion criteria for the low caffeine consumers group and 1,354 drivers for the high caffeine consumers group. Medical Examiner Reports were available for 1,358 (82.2%) of the low caffeine consumers and 1,137 (84.0%) of the high caffeine consumers. Participant demographics are presented in Table 1. In total 1,432 (86.7%) of the low caffeine group and 1,183 (87.4%) of the high caffeine group provided their age. The majority (83.7%) of the high caffeine group

Discussion

This paper considers associations between truck drivers’ high caffeine consumption, driving safety and health. 1,354 high (≥5 drinks per day) caffeine consumers were compared to 1,653 low (1 drink per day) caffeine consumers. Participants completed a self-report questionnaire including several standardised survey tools, as well as undertaking a medical examiner report and three years of follow up of crash and violation data. Results demonstrate that high caffeine consumption is associated with

Conclusion

Shorter sleep duration, irregular sleep schedules and higher daytime sleepiness (ESS) are apparent in high (≥5 drinks) compared with low (1 drink) caffeine consumers in a truck driving population. High caffeine consumption was also associated with poor health behaviours, including tobacco use, frequency of alcohol consumption, less exercise and poorer diet. Safety indicators of DDDI score and self-reported crashes in the previous three years are greater in high caffeine consumers. Work place

Acknowledgements

The Federal Motor Carrier Safety Administration funded part of this research (Contract DTMC75-10-H-00007). The opinions expressed in this manuscript are those of the authors and do not reflect the opinions of any U.S. government agency Dr Filtness was funded by an internal Loughborough University grant from the Transport Technologies Beacon to attend VTTI and undertake this work.

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