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Sleepy driving in truck drivers: Insights from a self-report survey

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Background: There is increasingly more evidence to indicate that many Australian truck drivers may be working while sleepy. However, relatively little is known about their sleepiness-related experiences or why sleepy drivers continue to drive. Aims: This study examined the subjective experience of sleepiness and the motivation of truck drivers at work, with particular focus on the behaviour of persevering with driving despite being sleepy. Method: Two hundred and fifty-five Australian professional truck drivers (245 males, 10 females, mean age of 43.60 years, average of 19.11 years experience) completed a self-report survey that was distributed at Australian truck stops and transport organisations. Drivers were asked to report on a variety of sleepiness-related experiences during the previous three months of their work. Results: The results revealed that 49% of drivers felt too sleepy to drive on at least half of their trips, while 40% reported falling asleep while driving at least once in the previous three months of work. A regression analysis indicated that several psychosocial factors were related to sleepy driving behaviour (i.e. continuing to drive when sleepy). These included impaired judgement, perceived work and social pressures, driver attitudes, and, most notably, perceived lack of control over work schedule. The frequency of sleepy driving was also associated with reported occurrences of impaired driving performance, dozing off whilst driving, near misses, and perceived accident risk. Conclusions: These findings provide new direction for further investigations of truck driver attitudes and behaviour, as well as the management of driver sleepiness. ©Misa et al: Licensee HFESA Inc.
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R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 1 ]
Sleepy driving in truck drivers: Insights from a
self-report survey
Raymond Misa
1
, Russell Conduit
2
and Grahame Coleman
2
1
Transport Safety Victoria, Australia
2
Monash University, Australia
Abstract
Background: There is increasingly more evidence to indicate that many Australian truck drivers may be working while
sleepy. However, relatively little is known about their sleepiness-related experiences or why sleepy drivers continue to
drive. Aims: This study examined the subjective experience of sleepiness and the motivation of truck drivers at work, with
particular focus on the behaviour of persevering with driving despite being sleepy. Method: Two hundred and fifty-five
Australian professional truck drivers (245 males, 10 females, mean age of 43.60 years, average of 19.11 years experience)
completed a self-report survey that was distributed at Australian truck stops and transport organisations. Drivers were
asked to report on a variety of sleepiness-related experiences during the previous three months of their work. Results: The
results revealed that 49% of drivers felt too sleepy to drive on at least half of their trips, while 40% reported falling asleep
while driving at least once in the previous three months of work. A regression analysis indicated that several psychosocial
factors were related to sleepy driving behaviour (i.e. continuing to drive when sleepy). These included impaired judgement,
perceived work and social pressures, driver attitudes, and, most notably, perceived lack of control over work schedule. The
frequency of sleepy driving was also associated with reported occurrences of impaired driving performance, dozing off
whilst driving, near misses, and perceived accident risk. Conclusions: These findings provide new direction for further
investigations of truck driver attitudes and behaviour, as well as the management of driver sleepiness.
©Misa et al: Licensee HFESA Inc.
Background
While the concepts of fatigue and sleepiness are often
deemed to be closely related in both research and practice,
fatigue has been more extensively researched in safety critical
industries and is widely recognised as a major contributor to
road transport accidents [1-3]. Indeed, it would be difficult
to determine which fatigue-related incidents can be attributed
to sleepiness, and much of the available research on fatigue
is relevant to this study. With this in mind, the inquiry of
the present study focused on drivers being sleepy or in a
(subjective) state of needing sleep [4], which is likely to pose
a significant threat to truck driver error or incapacitation on
a public road.
Investigations of sleepiness have typically involved laboratory
studies using objective neurophysiological measures (e.g.
polysomnography) and simulated driving tasks [5-7]. These
studies have found falling asleep to be a gradual process that
is preceded by a series of reliable physiological events [5,8-
10]. As an individual becomes increasingly sleepy, intrusions
of drowsiness give way to fluctuations between wake and the
initial stages of sleep. During this period, arousal markedly
decreases and compensatory effort to remain awake must
be increased to fight sleep onset. As such efforts fail, the
individual eventually succumbs to longer and more intense
periods of sleep.
Studies that have used self-reports as subjective estimates of
sleepiness suggest that drivers are able to gauge sleepiness levels
reasonably accurately. Drivers’ ratings on measures, such as
the Karolinska Sleepiness Scale and the Epworth Sleepiness
Scale, have been found to be closely related to objective
indicators of sleepiness such as electroencephalography
[11,12]. Moreover, drivers have been found to recognise a
precursory state of feeling sleepy, but may underestimate the
likelihood of falling asleep in such a state [13].
There is overwhelming empirical evidence that sleepiness is
deleterious to cognitive performance [14], although these
performance effects depend on the nature of the task and
factors such as the level of novelty, complexity, uncertainty,
knowledge, and experience [4]. Sleepiness has been found
to have adverse effects on attention, vigilance, information
processing, strategy, and decision making [15,16]. Sleepiness-
induced impairment of driving has been demonstrated
convincingly in laboratory studies, and increasingly worse
performance on driving simulator tasks (in terms of slower
reaction, speed deviation, lapses, and lane drifting) has been
found to occur with extended periods of being awake [11,17].
While the effects of sleepiness on on-road driving have been
more difficult to quantify, self-reports from drivers have
confirmed such sleepiness-related impairment [18-20].
There are numerous factors that may contribute to sleepiness.
Lack of sleep (also referred to as sleep loss, deprivation or
curtailment) is an obvious contributor, and has been
extensively researched. Shiftwork features have been identified
as common causes of sleep loss [21,22], while time of day (i.e.
Corresponding author: Raymond Misa. Email – Raymond.misa@transportsafety.vic.gov.au
Research article
R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 2 ]
circadian effects) has been found to have a profound effect
on sleepiness. Numerous studies [19,20,23] have found the
experience of fatigue, sleepiness, and incidents related to
these to be most frequently reported for times of circadian
lows, that is, between midnight and 6am as well as (to a lesser
extent) between 2pm to 4pm.
Based on the above research, the potential causes of truck
driver sleepiness are summarised in Figure 1. Also summarised
are the effects of sleepiness on a driver’s physiological
wakefulness, cognitive and motor performance, behaviour,
and subsequently, error.
Note. The dashed boxes indicate those that were not examined
in the present study.
For professional truck drivers, sleep loss and chronic
sleepiness is likely to result from a combination of work-
related disruption, poor sleep hygiene, lifestyle and health.
Indeed, the work and lifestyle of these individuals make
them susceptible to sleepiness. Most drivers work alone
with very little or no supervision. They drive considerable
distances with trips lasting many hours to numerous days, a
considerable proportion of which may be under monotonous
conditions. They may also regularly work at times of the day
when the biological need for sleep is at its greatest, and may
be required to vary the timing of their sleep to accommodate
schedules and work demands.
Despite relatively low admission of sleepiness as a problem
by heavy vehicle drivers [24], there is some evidence that
a considerable proportion of Australian truck drivers may
regularly drive in a sleepy state. In a national study of heavy
vehicle drivers (n=613), 36% of drivers reported ‘nodding
off for a moment’ on some occasion in the previous 12
months of work [24]. In another study of 1,249 truck
drivers, approximately 14% admitted to falling asleep at
least occasionally whilst driving for work [25], while another
survey of 1,007 truck drivers found that nearly half reported
that they had dozed off in the previous year [20].
While sleepy driving may be related to driver attitudes toward
break-taking [18], pressures from managing organisations
and other parties in the supply chain may have substantial
influence on a driver’s perceived need to continue driving
[26]. Many drivers have reported that the schedules imposed
by management made it difficult to take breaks when sleepy
[18]. Another study found that a considerable proportion
(57%) of drivers regularly exceeded regulated maximum
hours, with “tight schedules” as the most common reason
cited, followed by “in order to return home” and “to do
enough trips to earn a living” [20]. Moreover, drivers reported
engaging in risky driving behaviours to the point of breaking
road rules in order to meet delivery deadlines [20]. Hence,
many professional truck drivers may lack sufficient control in
determining when they work.
The present study examined the self-reported experience
of sleepiness in Australian truck drivers as well as potential
contributors to sleepiness. The behaviour of persevering
with driving despite being sleepy (referred to in this study
as sleepy driving behaviour) was of particular interest, as were
the psychosocial and work-related factors that influenced
this behaviour. Potential consequences for safety were also
examined.
Figure 1. An overview of truck driver sleepiness in regards to its potential causes and effects on sleep onset and
performance impairment as precursors to error based on previous research.
3
Note. The dashed boxes indicate those that were not examined in the present study.
Figure 1. An overview of truck driver sleepiness in regards to its potential causes
and effects on sleep onset and performance impairment as precursors to error
based on previous research.
For professional truck drivers, sleep loss and chronic sleepiness is likely to result
from a combination of work-related disruption, poor sleep hygiene, lifestyle and
health. Indeed, the work and lifestyle of these individuals make them susceptible
to sleepiness. Most drivers work alone with very little or no supervision. They
drive considerable distances with trips lasting many hours to numerous days, a
considerable proportion of which may be under monotonous conditions. They may
also regularly work at times of the day when the biological need for sleep is at its
greatest, and may be required to vary the timing of their sleep to accommodate
schedules and work demands.
Despite relatively low admission of sleepiness as a problem by heavy vehicle
drivers [24], there is some evidence that a considerable proportion of Australian
truck drivers may regularly drive in a sleepy state. In a national study of heavy
vehicle drivers (n=613), 36% of drivers reported ‘nodding off for a moment’ on
some occasion in the previous 12 months of work [24]. In another study of 1,249
truck drivers, approximately 14% admitted to falling asleep at least occasionally
whilst driving for work [25], while another survey of 1,007 truck drivers found
that nearly half reported that they had dozed off in the previous year [20].
While sleepy driving may be related to driver attitudes toward break-taking [18],
pressures from managing organisations and other parties in the supply chain may
have substantial influence on a driver’s perceived need to continue driving [26].
Many drivers have reported that the schedules imposed by management made it
difficult to take breaks when sleepy [18]. Another study found that a considerable
proportion (57%) of drivers regularly exceeded regulated maximum hours, with
“tight schedules” as the most common reason cited, followed by “in order to
return home” and “to do enough trips to earn a living” [20]. Moreover, drivers
reported engaging in risky driving behaviours to the point of breaking road rules
STATE OF
SLEEPINESS
IMPAIRMENT OF
COGNITIVE/MOTOR
PROCESSES. E.g.
- Attention
- Vigilance
- Info processing
- Memory (STM)
- Reaction time
- Coordination
SHIFTS FROM
WAKE TO SLEEP
IMPAIRMENT OF
PERFORMANCE
MICROSLEEPS
AND SLEEP
ONSET
ERROR
CIRCADIAN
EFFECTS
COMPENSATORY
EFFORT TO
STAY AWAKE
BEHAVIOUR
CHANGES. E.g.
- Motivation / effort
- Mood changes
- Decision makin
g
REDUCED NEURO-
PHYSIOLOGICAL
WAKEFULLNESS
COPING
BEHAVIOURS
SLEEP LOSS
& SLEEP HYGEINE
TASK EFFECTS
-
Task demands
- Work conditions
HEALTH EFFECTS
- incl. general health
and sleep pathology
R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 3 ]
Method
Participants
The sample consisted of 255 (245 males, 10 females)
Australian professional truck drivers, with a mean age of
43.60 (SD=9.80) years and a mean of 19.11 (SD=10.75) years
experience in driving trucks. A summary of their driving
operations is provided in Table 1.
Table 1. Summary of Operation Type, Vehicle Type and
Method of Payment of the Driver Sample: Percentages
of Drivers in Each Category
Type of driver %
Employee drivers 80.0
Owner-drivers 15.3
Owner-operators 3.9
Unreported 0.8
Predominant driving operation %
Single driver for entire shift 91.4
Two-up driving 3.9
Staged /changeover driving 3.5
Unreported 1.2
Vehicle type %
Rigid – 2 axle 3.1
Rigid – 3 axle 1.6
Rigid – 4 axle 0
Articulated – 3axle 3.5
Articulated – 4 axle 2.0
Articulated – 5 axle 6.3
Articulated – 6 axle 31.4
B-Double – 5 axle 34.5
Road train double/triple 15.3
Other 2.3
Method of payment %
Hourly rate 16.6
Flat day rate 1.6
Day rate with overtime 1.6
Weekly rate w/ overtime 1.2
Flat rate per load 10.2
Based on kms and/or tonnage carried 61.2
Other 7.6
N = 255
Measures
This study was part of a larger self-report survey study,
for which initial interviews with seven experienced long-
distance drivers were undertaken in developing the survey.
The survey inquired about a variety of sleepiness-related
experiences during the previous three months of a driver’s
work, including: demographics, job characteristics and work
demands (some of which were adapted from Williamson and
colleagues’ survey [20]); the frequency with which sleepiness
was experienced; various contributors and symptoms of
sleepiness; sleepiness-induced impairment of performance;
management of sleepiness; and continuing to drive when
sleepy, as well as psychosocial and work-related precursors of
this behaviour.
While the larger survey consisted of 195 items, a number of
scales were constructed from survey items using Principal
Components Analysis (PCA) (see [27]). These are listed in
Table 2 along with their respective reliability coefficients,
component loadings, and item-total correlations. Further key
measures are described in text following Table 2.
Table 2. Scales and Items Following Principal
Components Analysis and Respective Cronbach’s Alpha
Reliability Coefcients, Component Loadings, and Item-
Total Correlation Coefcients
Scales and items
(with Cronbach’s alpha coefficients)
Component
loading
Item-total
correlation
Driver Sleepiness (alpha = .75)
How often did you become sleepy while
driving for work?
.85 .62
How often did you feel that you were too
sleepy to drive?
.82 .60
How often was sleepiness a problem for
you personally in your work?
.78 .52
Sleepy Driving Behaviour (alpha = .85)
How often did you continue driving
when you were experiencing sleepiness
instead of taking a break?
0.92 0.80
How often did you continue driving even
when you felt that you may have been
too sleepy to drive safely?
0.90 0.75
I tended keep driving even when I was
fighting sleep.
0.81 0.62
Recognition of sleepy state (alpha = .68)
How often were you able to tell or
recognise when you were sleepy?
.87 .52
How often were you able to tell or
recognise when you were too sleepy to
drive safely for work?
.87 .52
Attitudes towards sleepy driving
(alpha = .84)
A driver should try not to drive when he
or she is sleepy.
0.81 0.63
A driver should rest if he or she is
fighting sleep.
0.82 0.65
Driving when sleepy is dangerous for
drivers.
0.87 0.78
Driving when sleepy is dangerous for
others on the road.
0.83 0.79
Perceived behavioural control
(alpha = .88)
I believe that I was able to stop driving
when I was sleepy.
0.92 0.80
I decided when I needed to stop driving
when sleepy.
0.90 0.75
I was confident that I could take rest
breaks when I needed.
0.81 0.62
Continuing to drive or stopping for rest
was beyond my control. (Reverse-
scored)
0.92 0.80
How I drove was totally determined
by me.
0.90 0.75
R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 4 ]
For the above (and the majority of) survey items, drivers
were asked to report the frequency in relation to each item or
statement based on their experiences and behaviours during
the previous three months of work. These were rated on a
5-point Likert scale where 0=Never (I have not experienced
this while driving for work before), 1=Rarely (I have
experienced this only a few times while driving for work),
2=Sometimes (I have experienced this on about half of my
driving trips for work), 3=Often (I have experienced this on
more than half of my driving trips for work), and 4=Very
Often (I have experienced this on almost all of my driving trips
for work). Attitudes and Perceived Behavioural Control items
were rated on another 5-point Likert scale, from 1=Strongly
Disagree to 5=Strongly Agree. An overall score for each scale
was obtained by computing the mean frequency rating for
the respective items. Principal components analyses indicated
the scales as single components (i.e. eigenvalue > 1.0), while
Pearson’s correlations between the items did not indicate
multicolinearity (see [27]).
Contributors to driver sleepiness and sleepy driving
behaviour. Drivers were asked to report on the contribution
of a number of factors on a 4-point Likert scale (0=No
contribution, 1=Slight, 2=Moderate, 3=Strong).
Contributors to sleepiness levels that drivers identified
included: accumulated sleep loss (sleep debt), work activities,
time spent waiting (e.g. for loading, unloading, checks,
queuing, etc.), time of day, weather, quality/ergonomics of
vehicle, roster/shiftwork, traffic conditions, route variability,
and fatigue management education/training. Contributors
to continuing to drive when sleepy included: delivering
time-sensitive loads, schedules/deadlines, roster/shiftwork,
employer/management pressures, work regulations, social/
family commitments, road safety regulations, confidence
in one’s own driving ability, financial incentives, and peers/
other drivers.
Time of day effects on sleepiness. Adapted from
Williamson and colleagues’ fatigue survey [20], drivers were
asked to report whether they experienced sleepiness (i.e. ‘felt
quite sleepy’) during the various times of the day, using a
typical trip as a guide.
Epworth Sleepiness Scale (ESS). The ESS, developed
by Dr Murray Johns, is a self-report questionnaire used to
identifying excessive daytime sleepiness. Respondents were
asked to specify the likelihood of falling asleep across eight
situations using on a 4-point Likert scale (i.e. 0=would
never dose, 1=slight chance of dozing, 2=moderate chance
of dozing, 3=high chance of dozing). A total ESS score
(calculated by summing the ratings) of 11 or more suggests
that the individual may need further evaluation to determine
the risk of excessive daytime sleepiness and any underlying
sleep disorder. The ESS has been demonstrated to have
internal consistency as well as construct validity with other
indices of sleep propensity [6].
Body Mass Index (BMI). BMI (calculated by dividing
weight by height squared, or kg/m
2
) is a widely used and
validated index to classify underweight, overweight, and
obesity in adults. While BMI should be interpreted according
to population-specific norms, the following grading has
been issued as the general standard by the World Health
Organization [28]: underweight<18.50, normal range=18.50
to 24.99, overweight25.00, and obese30.00. Health risks
are generally believed to increase when BMI is outside the
normal range.
Impairment of driving performance. Drivers were asked
to report how often they believed driving was impaired by
sleepiness on the abovementioned 5-point Likert scale from
Never to Very Often. Level of impairment of driving tasks was
also indicated on a 4-point Likert scale (0=Not affected, or
same; 1=Slightly worse, or barely noticeable; 2=Moderately
worse, or noticeable; 3=A lot worse, or very obvious).
Impaired judgement. Drivers were asked to report how often
they believed their judgement was impaired by sleepiness on
the abovementioned 5-point Likert scale from Never to Very
Often. Level of impairment was also indicated on a 4-point
Likert scale (0=Not affected, or same; 1=Slightly worse, or
barely noticeable; 2=Moderately worse, or noticeable; 3=A
lot worse, or very obvious).
Perceived pressure to continue driving. The item ‘to what
extent did you feel that you had to continue driving despite
being sleepy?’ was rated on a 5-point Likert scale (0=No
extent, 1=Small extent, 2=Some extent, 3=Great extent,
4=Unavoidable).
Near-misses. Given that actual accidents, such as collisions,
are relatively infrequent, drivers were asked to report any near-
misses while driving for work in the previous three months as
‘yes’, ‘no’, or ‘prefer not to say’.
Worried about having an accident due to sleepiness.
Drivers were asked ‘how often have you been worried that you
might have an accident because you were sleepy while driving for
work?’, responding on the 5-point Likert scale (Never to Very
Often).
Procedure
Between April 2007 and August 2007, a total of 1,100 surveys
were distributed to 55 truck stops throughout Australia.
Operators of these sites were instructed to place the surveys
in locations frequented by drivers (e.g. drivers’ lounge, dining
area). To alert drivers to the study, advertisements were placed
on community radio programs and posters displayed on
truck stop noticeboards. The survey was expected to take
approximately 45 minutes to complete and participants were
instructed to return the completed survey to the researcher
anonymously using the enclosed reply-paid envelop. Of
the surveys distributed at truck stops, a stocktake indicated
that approximately 600 surveys were collected, while only
220 surveys (37%) of these were completed and returned by
mail. Twelve drivers opted to participate in the survey via
the telephone. Due to the relatively low response rate at
truck stops, transport organisations within the Melbourne
metropolitan area were approached, with 11 organisations
agreeing to forward the survey to employee drivers. Out of 80
surveys distributed to drivers at companies, 41 surveys (51%)
were completed and returned by mail. Overall, out of a total
of 273 surveys returned, 18 were largely incomplete and were
excluded from the sample, resulting in a sample of 255.
R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 5 ]
Results
Correlations among scores for Driver Sleepiness, Sleepy Driving
Behaviour, and other key variables were examined. These
are presented in Table 3, which also indicates sample means
and standard deviations for a number of shift features and
demographic variables.
Table 3. Pearson’s r correlations among Driver
Sleepiness (DS), Sleepy Driving Behaviour (SDB), and
other key variables, with sample means and standard
deviations indicated where applicable.
Pearson’s r with Mean (SD)
(if applicable)
No. Variable
1. DS 2. SDB
Sleepy driving
1 Driver Sleepiness (DS) - 0.52***
2 Sleepy Driving
Behaviour (SDB)
.52*** -
Shift features
3 Total hours worked per
week
.16** .15* 70.60
(18.18)
4 Total hours worked at
night per week
.16** .11 37.75
(19.25)
5 Average shift length (hrs) .11 .11 13.23 (3.97)
6 Average distance per
shift (kms)
.20** .06 884.59
(290.69)
7 Ave. time spent awake
before shift (hrs)
.15** .08 4.09
(3.04)
Demographics
8 Daytime sleepiness
(measured by ESS)
.32*** .31*** 9.20
(4.50)
9 Body Mass Index (BMI) -.01 -.09 29.93 (5.28)
10 Experience in driving
trucks (years)
-.12 -.20** 19.11
(10.75)
Pearson’s r with Mean (SD)
(if applicable)
No. Variable
1. DS 2. SDB
Performance and safety
11 Impaired driving
performance
.66*** .64***
12 Drifting out of lanes .46*** .41***
13 Dozing off (falling asleep
while driving)
.47*** .41***
14 Worried about having
accident
.57*** .38***
15 Reports of near-misses .18** .22***
Psychosocial factors
16 Recognition of sleepiness .06
17 Impaired judgement .38***
18 Perceived pressure to
continue driving
.65***
19 Attitudes towards sleepy
driving
-.29***
20 Perceived behavioural
control
-.58***
*p < .05, **p < .01, ***p < .001.
A multiple regression analysis (F(5,253)=66.46, p<0.001)
indicated a significant link between Sleepy Driving Behaviour
and the psychosocial variables listed in Table 3. These variables
accounted for 57% of the variance (i.e. adjusted R
2
=.57) in
Sleepy Driving Behaviour. Perceived Pressure to Continue Driving
(β=.44, p<0.001, part correlation=.34) was the strongest
predictor. This was followed by Perceived Behavioural Control
(β=-.23, p<0.001, part correlation=-.17), then Attitudes (β=-
.22, p<0.01, part correlation=-.21), then Impaired Judgement
(β=.20, p<0.01, part correlation=.18). Recognition of sleepiness
was not predictive of Sleepy Driving Behaviour (p>0.05).
Figure 2. The percentages of drivers who reported experiencing sleepiness at various times throughout the 24-hour period.
9
Figure 2. The percentages of drivers who reported experiencing sleepiness at
various times throughout the 24-hour period.
A non-parametric Friedman test was used to test for variability in reported
sleepiness at various periods of the day, that is: (1) 12am-6am, (2) 6am-12pm,
(3) 12pm-6pm, and (4) 6pm-12am. This test revealed an overall time of day
effect in reported sleepiness (Friedman Χ
23
=167.91, p<0.001), while Wilcoxon
Signed Ranks post hoc tests indicated that sleepiness was more frequently
reported (p<0.001) in time block 1 (12am-6am) than in time blocks 2, 3, and 4
(Wilcoxon Ζ= 9.03, 6.81, and 9.05, respectively).
Discussion and conclusion
Drivers reported experiencing sleepiness at work relatively frequently. On
average, sleepiness was reported on approximately half of trips. Approximately
23% of drivers reported experiencing sleepiness often (i.e. on more than half of
their trips). On average, drivers reported beginning to feel sleepy approximately
7 hours (SD=3.55) into the driving trip. The results also revealed that 49% of
drivers felt too sleepy to drive on at least half of their trips, while 40% reported
falling asleep while driving at least once in the previous three months of work.
The frequency with which drivers reported sleepiness was also positively
correlated with a number of shift features including average total hours worked
per week, total hours worked at night, average distance driven per shift, and
average time spent awake before commencing shift. In identifying specific
contributors to sleepiness levels, drivers rated accumulated sleep loss (or sleep
debt) as having the greatest contribution (46.5% of the drivers rated this to
strongly contribute to sleepiness). This was closely followed by waiting (e.g. for
loading and unloading of goods, queuing, etc.) and work activities (e.g. driving
and other work tasks).
Time of day was also reported as a major contributor. Drivers reported propensity
for sleepiness to be greatest between 12am and 6am. A Friedman test confirmed
that significantly more drivers reported feeling sleepy during this period than
Percentage of drivers who reported feeling sleepy
0
5
10
15
20
25
30
35
40
45
12am to 1am
1am to 2am
2am to 3am
3am to 4am
4am to 5am
5am to 6am
6am to 7am
7am to 8am
8am to 9am
9am to 10am
10am to 11am
11am to 12pm
12pm to 1pm
1pm to 2pm
2pm to 3pm
3pm to 4pm
4pm to 5pm
5pm to 6pm
6pm to 7pm
7pm to 8pm
8pm to 9pm
9pm to 10pm
10pm to 11pm
11pm to 12am
Time of day
Time Block 1 (12am-6am) Time Block 2 (6am-12pm) Time Block 3 (12pm-6pm) Time Block 4 (6pm-12am)
R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 6 ]
To illustrate the propensity for sleepiness throughout the 24-
hour period, the graph in Figure 2 presents the percentages of
drivers who reported being sleepy at various times.
A non-parametric Friedman test was used to test for variability
in reported sleepiness at various periods of the day, that is: (1)
12am-6am, (2) 6am-12pm, (3) 12pm-6pm, and (4) 6pm-12am.
This test revealed an overall time of day effect in reported
sleepiness (Friedman Χ
23
=167.91, p<0.001), while Wilcoxon
Signed Ranks post hoc tests indicated that sleepiness was
more frequently reported (p<0.001) in time block 1 (12am-
6am) than in time blocks 2, 3, and 4 (Wilcoxon Ζ= 9.03, 6.81,
and 9.05, respectively).
Discussion and conclusion
Drivers reported experiencing sleepiness at work relatively
frequently. On average, sleepiness was reported on
approximately half of trips. Approximately 23% of drivers
reported experiencing sleepiness often (i.e. on more than half
of their trips). On average, drivers reported beginning to feel
sleepy approximately 7 hours (SD=3.55) into the driving trip.
The results also revealed that 49% of drivers felt too sleepy
to drive on at least half of their trips, while 40% reported
falling asleep while driving at least once in the previous three
months of work.
The frequency with which drivers reported sleepiness was also
positively correlated with a number of shift features including
average total hours worked per week, total hours worked at
night, average distance driven per shift, and average time
spent awake before commencing shift. In identifying specific
contributors to sleepiness levels, drivers rated accumulated
sleep loss (or sleep debt) as having the greatest contribution
(46.5% of the drivers rated this to strongly contribute to
sleepiness). This was closely followed by waiting (e.g. for
loading and unloading of goods, queuing, etc.) and work
activities (e.g. driving and other work tasks).
Time of day was also reported as a major contributor. Drivers
reported propensity for sleepiness to be greatest between
12am and 6am. A Friedman test confirmed that significantly
more drivers reported feeling sleepy during this period
than other times of the day. Reported sleepiness was also
elevated between 2pm and 4pm, although to a lesser extent.
Research has consistently found the biological need for sleep
to be greatest during these periods [19,22]. Moreover, these
findings are consistent with truck drivers’ reports of fatigue
[20], and lend further support to already substantial evidence
that circadian effects play a central role in sleepiness and
sleepiness-related accident risk.
As expected, driver sleepiness ratings were also found to
be moderately correlated (r = .52, p < 0.001) with reported
instances of sleepy driving behaviour. The results indicated that
a considerable proportion of drivers persevered with driving
despite being sleepy on a regular basis. On average, drivers
reported continuing to drive rather taking a break on about
half of their trips. Approximately 26% of drivers reported
continuing to drive despite being sleepy on more than half of
their trips. Interestingly, continuing to drive when sleepy was
not correlated with shift features with the exception of total
hours worked per week.
It was found that more experienced drivers tended not to
persevere with driving when sleepy (r = -.20, p < 0.01). Drivers
who tended to persevere with driving on a regular basis also
tended to be more prone to daytime sleepiness as measured
by the ESS (r = .31, p < 0.001). A sample mean of 9.20 was
found for ESS scores, for which a score of 11 or more suggests
that the individual may need further evaluation to determine
the risk of excessive daytime sleepiness and any underlying
sleep disorder.
A regression analysis identified a number of psychosocial
predictors of sleepy driving behaviour. Most notable of these
were perceived pressures and a perceived lack of control over
driving schedules. Indeed, these findings provide additional
evidence that pressures and schedules imposed by management
and other parties in the supply chain have a considerable
impact a driver’s decision to persevere with driving and can
make it difficult to take breaks when needed [18,20,26].
Drivers’ attitudes towards sleepy driving also predicted sleepy
driving behaviour. Drivers who agreed that driving while
sleepy was undesirable and dangerous were also less likely
to persevere with driving. Impaired judgement was also a
predictor, as drivers who reported more frequent instances
of impaired judgement also tended to continue driving.
Impaired judgement was also associated with frequency
ratings for sleepiness. This may suggest that sleepiness may
impair a driver’s decision making, for instance, when deciding
to continue or cease driving. However, this cannot be known
without further analysis.
Despite some early research suggesting that a sleepy driver
may continue driving because he or she cannot recognise
the state of being sleepy (see [13]), 64% of drivers reported
that they were often able to tell or recognise when they were
‘too sleepy to drive safely for work’. Such self-reported ability
to recognise a sleepy state, however, did not predict sleepy
driving behaviour in the present study. Previous studies have
found that sleepy drivers tend to underestimate the likelihood
of falling asleep (rather than just being sleepy) [13], and
the effect of this underestimation on a driver’s decision to
continue driving also requires further examination.
A number of factors were noted to have at least moderate
contribution to continuing to drive when sleepy by at least
half of the drivers. These included schedules/deadlines, work
regulations (e.g. logbooks, other work-related requirements),
confidence in one’s driving ability, and time-sensitive loads.
Sleepy driving may also have a considerable impact on driving
safety. The frequency of which drivers reported continuing
to drive when sleepy was positively correlated with measures
of perceived performance and safety. This included reported
occurrences of: impaired driving performance (r
=.66);
drifting out of lanes (r
=.41); difficulties staying alert (r =.50);
dozing off whilst driving (r
=.41); and near misses (r =.22).
Drivers who persevered with sleepy driving more often were
also more likely to report being worried about having an
accident (r
=.38). Similarly, drivers’ ratings of sleepiness at
work were also positively related with reported occurrences
of these events.
These findings may provide further insight into the
contributors and consequences of sleepy driving. However, a
R. Misa et al
HFESA 47th Annual Conference 2011. Ergonomics Australia - Special Edition
[ 7 ]
number of methodological issues need to be acknowledged.
Like most surveys of truck driver behaviour, the data in this
study were based on self reports which rely on participants’
willingness to report accurately as well as accurate
recollection of events. Therefore, the data was susceptible to
self-presentation and response biases. Moreover, sleepiness
was measured subjectively using rating scales which are
considered to be less precise than objective measures such as
polysomnography.
Nonetheless, the results may have implications for the
management of driver sleepiness and safety in the trucking
industry. Education strategies that increase drivers’ awareness
of the risks associated with driving in a sleepy state, as well
as promote attitudes favouring break taking, are expected
to be useful. The results also suggest that targeting drivers’
perceived (or better yet, actual) control is likely to have the
greatest impact on discouraging sleepy driving. While there
is no substantial evidence that greater control over driving
schedules would translate to safer trucking practices [29],
greater flexibility in break-taking is expected to, at the very
least, encourage drivers to rest when needed. Indeed, the
present study has found perceptions of work pressures and
decreased control to also correspond with self-reports of
increased accident risk. With this in mind, drivers reported
a number of specific constraints and incentives to have had
considerable contribution to their sleepy driving behaviour,
including work, regulatory, social, and financial factors.
In regards to future research, these findings suggest that sleepy
driving involves a conscious voluntary decision to engage in
risky behaviour. If this is indeed the case, arguably, sleepy
driving behaviour can be accounted for by an appropriate
decision making or behavioural model. The above findings
suggest that the Theory of Planned Behaviour [30] may be
such a model. The applicability of this model to sleepy driving
behaviour in professional truck drivers will be explored as an
extension of the present study.
Acknowledgments
We are grateful to all the drivers and organisations for their
time in participating in the study. The opinions expressed in
this article are those of the authors and do not necessarily
represent the official positions of any government agency.
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... RMSEA=0.59). Misa et al. (2011) developed the "Sleepy Driving Behavior" scale. The scale consists of 3 statements in total. ...
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