We found that Alternating day shift bus drivers suffered from more sleepiness while driving from lunch to last driving than Daily two shift bus drivers, which may be because Alternating day shift workers displayed characteristics of more irregular work schedules and longer working hours per day and week. After adjusting for job duration, alcohol consumption and sleeping time on workdays, Alternating day shift was still associated with severe sleepiness while driving and showed 3.97 to 18.26 of PRs from lunch to last driving relative to Daily two shift. But it should be careful not to give undue meaning to the PR figures themselves, since KSS was a subjective questionnaire and not the medical standard.
There have been many studies about the factors associated with sleepiness while driving. In one study of commercial bus and truck drivers , 61% of drivers who worked longer than 12 h daily and faced long hours of driving including more than 4 h at a time reported feeling drowsiness while driving. These results are similar to the findings of our study. The very long driving hours of Alternating day shift (daily mean: 17.1 h) may be a crucial factor for the high proportion of severe sleepiness reported from lunch to last driving in this study.
In another study that administered polysomnography (PSG) to shiftwork bus drivers , the daytime sleep of shiftwork bus drivers was of a poorer quality than their nighttime sleep. In our study, Alternating day shift had more irregular schedules with alternately arranged workdays and off-days. Therefore, daytime sleep that occurs on off-days may be compensation for the long work time of workdays in this group. It is also possible that daytime sleeping on an off-day made drivers more tired by interrupting their circadian rhythms.
Sleep deprivation and sleep disorders [21,22,23,24] are known risk factors for sleepiness while driving. In our study, Alternating day shift had short sleeping times on both workdays and off-days. This trend may be related to their long driving and working hours and irregular working schedules. In this study, we did not adjust for whether the drivers had a sleep disorder. However, there was not much difference in this factor between the two groups, and it may be of little influence because we adjusted the participants’ sleeping time that correlated with the effect of sleep disorders. Obstructive sleep apnea (OSA) [2,3,4,5,6,7,8,9,10,11,12,13, 33] is a particularly well known major risk factor for sleepiness while driving and traffic accidents. Although we did not ask respondents about having OSA, we used their BMI as a surrogate variable because it is closely related to OSA [33, 34]. The proportion of overweight (BMI ≥ 25) drivers on Alternating day shift was lower than that of Daily two shift. Therefore, it seems that the prevalence of alternating shift drivers that had OSA was rarely higher than that of Daily two shift drivers.
A young age and short job duration [19,20,21,22, 35] were also strong risk factors for sleepiness while driving and traffic accidents. However, the mean age of Daily two shift and Alternating day shift were similar. In addition, age had a strong correlation with job duration in this study (results not shown). After adjusting for job duration, the difference in sleepiness between the two groups changed little.
Because Alternating day shift had an irregular work schedule and long driving hours , they were likely to be more vulnerable to cumulative fatigue than workers on Daily two shift. According to our study, cumulative fatigue does not occur in the morning but instead takes place after lunch and may cause sleepiness while driving. The lower sleeping time (both on workdays and off-days) for those on Alternating day shift may also be a factor that produces cumulative fatigue. An irregular work schedule and long working hours can cause sleep problems [37, 38] and therefore directly cause cumulative fatigue or indirectly influence cumulative fatigue to produce sleep deprivation.
Most countries regulate the daily and weekly work times of occupational drivers according to the ILO recommendations for road transport . Our results suggest a lack of regulation may threaten public safety. Therefore, exceptions to the Labor Standards Act in Korea should be removed to reduce the risk of traffic accidents due to sleepiness while driving. Bus companies that still use Alternating day shift should also change this shiftwork pattern to reduce the driving time of individual employees.
To our knowledge, this study was the first to investigate the sleepiness of occupational drivers at different times of day. We discovered that severe sleepiness in the long working hours group (Alternating day shift) markedly increased after lunch. This study also verified the influence of working shift on severe sleepiness. In addition, we showed that drivers on Alternating day shift may suffer from severe sleepiness and are more likely to experience traffic accidents than workers on Daily two shift. These results were still valid after adjustment for job duration, an important confounding factor.
This study has some limitations. First, it was a cross-sectional study that used questionnaires. Therefore, the data used in this investigation may reflect recall bias because answers depended on participant memory. However, the information that this study required (e.g. degree of sleepiness while driving, driving time, working time, and number of workdays) is usually easy to recall.
Second, it is difficult to know what the definite cause is that Alternating day shift had a higher proportion of severe sleepiness. This shift had a longer daily driving (working) time due to the characteristics of shiftwork (repeating workdays and off-days by alternating days). However, the actual mean weekly driving (working) time of Alternating day shift was also higher than Daily two shift. In addition, sleeping time (both on workdays and off-days) of Alternating day shift was lower than that of Daily two shift. As a result, it is difficult to pinpoint what made drivers on Alternating day shift sleepier. It was unlikely that just one factor produced the markedly high PR for severe sleepiness in Alternating day shift drivers. Additional studies will be required to better isolate the reason behind this finding.
Third, it was difficult to evaluate the effect of a relatively lower working time on sleepiness because the working time of Alternating day shift was abnormally high. Further study is needed on the relationship between working time and daytime sleepiness in a relatively low working time condition and whether the relationship demonstrates a deterministic or stochastic trend.
Fourth, we did not sufficiently adjust for confounding factors like sleep disorders (including OSA) which could have influenced drivers’ sleepiness. Most sleep disorders are correlated with sleeping time. Therefore, we only adjusted for sleeping time instead of adjusting for sleep disorders as well. In addition, it is not problematic that other factors were not adjusted because the difference in the degree of severe sleepiness between Daily two shifts and Alternating day shift was so high.