Caffeine for the prevention of injuries and errors in shift workers: Cochrane systematic review

Abstract

Assessed as up to date: 2010/04/01

Background

Sleepiness leads to a deterioration in performance and attention, and is associated with an increased risk of injury. Jet lag and shift work disorder are circadian rhythm sleep disorders which result in sleepiness and can elevate injury risk. They create a need for individuals to operate at times which are different to those dictated by their circadian rhythms. Consequently there is also a need for interventions to help ensure that these persons can do so safely. Caffeine has a potential role in promoting alertness during times of desired wakefulness in persons with jet lag or shift work disorder, however its effects on injury and error are unclear.

Objectives

To assess the effects of caffeine for preventing injuries caused by impaired alertness in persons with jet lag or shift work disorder.

Search methods

We searched the Cochrane Injuries Group Specialised Register, CENTRAL (The Cochrane Library), MEDLINE, EMBASE, PsycINFO, CINAHL, TRANSPORT (to July 2008); and PubMed databases (to April 2010). We also searched the Internet and checked reference lists of relevant papers.

Selection criteria

Randomised controlled trials investigating the effects of caffeine on injury, error or cognitive performance in people with jet lag or shift work disorder.

Data collection and analysis

Two authors independently screened search results and assessed full texts for inclusion. Data were extracted and risk of bias was assessed. Estimates of treatment effect (odds ratio and standardised mean difference (SMD)) and 95% confidence intervals (CI) were calculated and pooled using the fixed-effect model.

Main results

Thirteen trials were included. None measured an injury outcome. Two trials measured error, and the remaining trials used neuropsychological tests to assess cognitive performance. The trials assessing the impact on errors found that caffeine significantly reduced the number of errors compared to placebo. The pooled effect estimates on performance by cognitive domain suggest that, when compared to placebo, caffeine improved concept formation and reasoning (SMD -0.41; 95% CI -1.04 to 0.23), memory (SMD -1.08; 95% CI -2.07 to -0.09), orientation and attention (SMD -0.55; 95% CI -0.83 to -0.27) and perception (SMD -0.77; 95% CI -1.73 to 0.20); although there was no beneficial effect on verbal functioning and language skills (SMD 0.18; 95% CI -0.50 to 0.87). One trial comparing the effects of caffeine with a nap found that there were significantly less errors made in the caffeine group. Other trials comparing caffeine with other active interventions (for example nap, bright light, modafinil) found no significant differences. There is a high risk of bias for the adequacy of allocation concealment and presence of selective outcome reporting amongst the trials.

Authors' conclusions

Caffeine may be an effective intervention for improving performance in shift workers however, there are no trials from which we can assess its effect on injuries. The results largely originate from studies involving young participants under simulated conditions, and the extent to which the findings are generalisable to older workers and real world shift work is unclear. Based on the current evidence, there is no reason for healthy individuals who already use caffeine within recommended levels to improve their alertness to stop doing so. The assessment of the relative effects of caffeine to other potential countermeasures should be a focus of future research.

Author(s)

Ker Katharine, Edwards Philip James, Felix Lambert M, Blackhall Karen, Roberts Ian

Summary

Caffeine for preventing injuries and errors in shift workers

Sleepiness leads to a deterioration in performance and is associated with an increased risk of error and injury. Shift work is an major cause of sleepiness as it requires workers to be awake at times which are different to those dictated by their 'body clock'. This in turn can compromise the safety of themselves and of others - sleepiness is a risk factor for events such as traffic crashes, occupational injuries and medical errors.

The identification of interventions which can reduce the risk of error and injury is necessary to help ensure that those who are required to work through the night, can do so safely. Caffeine has been proposed as one such intervention, although how effective it is in shift workers is unknown.

For this systematic review, the authors searched for randomised controlled trials which investigated the effects of caffeine on injury, error and cognitive performance in shift workers. They found 13 trials - none of the trials looked at the effect on injury, two trials measured error, while the remaining trials used neuropsychological tests to assess cognitive performance.

The results of the trials suggest that compared to no intervention, caffeine can reduce the number of errors and improve cognitive performance in shift workers. No difference in effect was found by the trials comparing caffeine with other interventions (such as nap, bright light and modafinil). However, due to some methodological weaknesses of the trials, some caution is required when interpreting the results.

The authors of the systematic review conclude that caffeine may be an effective intervention for improving performance in shift workers however, there are no trials from which they could assess its effect on injuries. Based on the current evidence, the review authors judge that there is no reason for healthy shift workers who already use caffeine within recommended levels to improve their alertness, to stop doing so. They go on to suggest that it would be useful for further trials to be undertaken to assess the effects of caffeine against other potential countermeasures.

Reviewer's Conclusions

Implications for practice

The results of this systematic review suggest that caffeine may be effective in improving performance in persons engaged in shift work or suffering from jet lag, although it may not be possible to confidently translate such an improvement in performance to a reduced injury risk. The current evidence arises largely from trials conducted under simulated conditions thus caution is required when extrapolating the findings to real world shift work environments. Furthermore, the lack of trials involving persons aged 40 years and over, who are particularly vulnerable to the adverse effects of shift work, limits the ability to draw confident conclusions regarding the effects of caffeine in this group.

We judge that shift workers who currently use caffeine as a countermeasure to sleepiness can continue to do so with the knowledge that their risk of error should not increase as a result, and they may experience improvement in cognitive performance.

Implications for research

Future research in this area should focus on:

  • identifying the most effective method for the administration of caffeine as an alertness management intervention, and
  • exploring the relative effects of caffeine against other potential countermeasures.

The review authors are currently working on a number of systematic reviews investigating the effects of other interventions for preventing injuries in shift workers. On the completion of the reviews we will be able to identify the interventions which, according to the existing evidence, are the most effective.

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