Symptoms of sleepiness while driving and their relationship to prior sleep, work and individual characteristics

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1 Transportation Research Part F 9 (2006) Symptoms of sleepiness while driving and their relationship to prior sleep, work and individual characteristics Johannes van den Berg a,b, *, Ulf Landström a a Department of Work and the Physical Environment, National Institute for Working Life, Box 7654, Umeå, Sweden b Department of Public Health and Clinical Medicine, Umeå University, Umeå, Sweden Received 29 March 2005; received in revised form 4 January 2006; accepted 5 January 2006 Abstract In total, 154 lorry and bus drivers participated in a questionnaire study dealing with sleepiness in traffic. The questionnaire included questions about sleep before work, appearance of sleepiness, the type of sleepiness they experienced while driving, and ways to counter sleepiness while driving. Driver sleepiness was analysed with respect to age, type of driving, traffic intensity, experience of work, and sleep before work. About 14% of the drivers reported regular sleepiness while driving, 33% had occasionally fought sleepiness while driving, and 8% had experienced nodding of the head while driving. The majority of the drivers had once been so tired that they had to stop driving. Sleepiness normally occurred between and 06.00, at the end of longer trips, and was most frequently appeared in autumn. Poor sleep and poor working hours were considered as the most important causes to sleepiness. Eye tiredness, yawning, difficulties concentrating on the road, and difficulties keeping one s thoughts together: these were the most frequently reported symptoms of sleepiness. More sleeping hours before work, better working hours, naps during work, listening to the radio, conversations, and lowering the cabin temperature were the most frequently mentioned countermeasures. The appearances of sleepiness while driving was strongly correlated to lower sleep hours and lower sleep quality before work. Age, type of work, or work experience did not interfere with sleepiness in the investigated group in any systematic way. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sleepiness; Sleep hours; Drivers; Survey 1. Introduction Estimates of traffic accidents related to sleepy drivers varies: Summala and Mikkola (1994) 7 10%; Knipling and Wang (1995) %; Horne and Reyner (1995b) 16 20%; Lyznicki, Doege, Davis, Williams, and Fabrizio (1998) 1 3%; and Sagberg (1999) 3.9%. Prevalence of driver sleepiness has also been studied by surveys using questionnaires or interviews. McCartt, Ribner, Pack, and Hammer (1996) concluded that * Corresponding author. Address: Department of Work and the Physical Environment, National Institute for Working Life, Box 7654, Umeå, Sweden. Tel.: ; fax: address: johannes.berg@niwl.se (J. van den Berg) /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.trf

2 208 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) recently about 55% of the drivers had been driving while sleepy and 23% had even fallen a sleep at the wheel. Similarly, Dawn (2003) reports that 29% of the driving population had nodded off at least in the previous year while driving. Research of sleepiness among truck drivers has shown similar results. According to a study by Hakkanen and Summala (2001) regarding accidents related to sleepiness, a truck driver was estimated to have fallen asleep in 2% of the cases. About 4% were sleepy before the accident. In a questionnaire study by Hakkanen and Summala (2000), about 40% of the long-haul truck drivers reported problems staying alert every fifth drive and over 20% had dozed of more than once. Interviewing long-distance truck driver revealed that 25% had fallen asleep in the past year and about 47% had fallen asleep at the wheel of their truck (McCartt, Rohrbaugh, Hammer, & Fuller, 2000). The gap between drivers recollection of being sleepy while driving and the statistics of sleep related accident might be interpreted that just a small number of sleepy drivers had been involved in accidents. However, it has been pointed out that estimates of sleep related accident reports are underestimated and that sleepiness is the largest identifiable and preventable cause of accidents in transport operations (Akerstedt, 2000; Nguyen, Jauregui, & Dinges, 1998; Stutts, Wilkins, & Vaughn, 1999). Symptoms of sleepiness (eye problems, yawning, difficulties staying alert, and task focused) are well known and have been investigated in previous driving studies. Kecklund and Akerstedt (1993) used symptoms of sleepiness such as tired eyes, heavy eyelids, difficulties focusing one s eyes, and feeling irresistible sleepiness. Milosevic (1997) studied long-distance drivers and dump truck drivers by ranking their symptoms of fatigue. Their most common symptoms were back/leg pain, drowsiness/fatigue, bad mood, slowed-down activity, and pain/other eye problems. However, the order of appearance or in which way the symptoms developed was not evaluated in the study. In another study by Nilsson, Nelson, and Carlson (1997), the subjects rated the presence of 18 pre-named symptoms of fatigue over time. The most salient changes of symptoms over time were sore feet, tired eyes, and feeling drowsy. They were also asked to rate their level of fatigue over time and the authors concluded that the subjects stopped driving at the same level of subjective fatigue. The use of subjective ratings of sleepiness during driving has been tested in different studies. Williamson, Feyer, and Friswell (1996) used Standford Sleepiness Scale (SSS) showed an increase in fatigue from the beginning to the end of a trip; however, since no driver stopped due to sleepiness, the ratings do not reveal a certain stop-driving-sleepiness level. Epworth Sleepiness Scale (ESS) was used in a survey of truck drivers. One study showed that a higher level of daytime sleepiness was related to more frequent drowsy driving (McCartt, Hammer, & Fuller, 1997), but the study did not provide a suggested cut-off level. In an other study of truck drivers, the same ESS scale was used to estimate sleep problems rather than to find a certain level of sleepiness that resulted in the driver stopping (Hakkanen & Summala, 2000). Kecklund and Akerstedt (1993) used the Karolinska Sleepiness Scale (KSS). According to their study, two drivers took a nap (<30-min nap) when they preceded a KSS-rating P7, which corresponds to the verbal expression Sleepy, no effort to stay awake. Reyner and Horne (1998b) also used KSS. A majority of their subjects reported that they were likely to fall asleep when rating a sleepiness of KSS was 8 9 ( Sleepy but some effort to stay awake and Very sleepy, great effort to stay awake ). The drawback is that the KSS is a 9-point bipolar rating scale, a scale that measures sleepiness and alertness. This might have a negative impact on subjects ability make fine adjust levels of self-rated sleepiness. Recently, we have used the Category-Ratio Scale (CR-10) by Borg (1998), (van den Berg, Neely, Nilsson, Knutsson, & Landstrom, 2005). Such a scale may be more suited to measure sleepy subjects because there are more rating levels available for expressions of sleepiness and there is no set endpoint that could create a ceiling effect. To prevent the driver from falling asleep, several countermeasure strategies have been studied (Akerstedt, Landstrom, Bystrom, Nordstrom, & Wibom, 2003; De Valck & Cluydts, 2001; Feyer, Williamson, & Friswell, 1997; Hanks, Driggs, Lindsay, & Merrill, 1999; Horne & Reyner, 1996, 2001a; Reyner & Horne, 1998a, 2002). Still the safest countermeasure is to stop driving (Horne & Reyner, 1995a; Lyznicki et al., 1998). Therefore, drivers need to know the risk of sleepy driving. Education and information to the drivers have been proposed to improve the awareness of the risk of driving sleepy(akerstedt, 2000; Hakkanen & Summala, 2000; Lyznicki et al., 1998). This, however, is not enough and more needs to be added. For instance, the driver need to recognize the symptoms of sleepiness and cope with them in order to stop driving (Hakkanen & Summala, 2000; Horne & Reyner, 2001b; Stutts et al., 1999). This study investigates several aspects of driving while sleepy: drivers symptoms of sleepiness and how these symptoms develop while driving; drivers countermeasures and how often they are used; drivers

3 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) opinions and the extent of contributing factors to sleepiness; and whether there is a correlation between the drivers who stopped driving due to sleepiness and their ratings of sleepiness on the CR-10 scale. 2. Methods 2.1. Subjects and procedure To fulfil our purpose, we used long-distance drivers as our subjects. Using different registries and addresses for haulages and drivers in northern Sweden, we compiled a list of 227 truck and bus drivers. The questionnaire was coded so the authors could send a reminder to those who had not completed and returned the questionnaire within approximately two weeks. The first dispatch of the self-completion questionnaires was done in October 2004 and a reminder was sent in November Those who completed and returned the questionnaire were given a lottery ticket with a value of about 25 SEK. The answers were entered in a computer without identifying the respondents. Seven of the questionnaires were excluded because they were returned due to an unknown address. Of the remaining 220 questionnaires, 120 (54.5%) were returned after the first dispatch and 34 (15.5%) were returned after the reminder. The overall participation rate was 154 (70.0%) Questionnaire The questionnaire was based on the experience from questionnaires used in previous field studies of professional drivers (Landstrom, Akerstedt, Bystrom, Nordstrom, & Wibom, 2004; Landstrom, Englund, Nordstrom, & Astrom, 1999; Landstrom, Knutsson, & Lennernas, 2000). The authors developed a questionnaire based on their previous studies on sleepiness (van den Berg et al., 2005; van den Berg, Neely, Wiklund, & Landstrom, 2005). The finished seven pages self-completion questionnaire contained 28 questions with 84 items. The majority of the questions were multiple-choice questions with more than three alternatives. Two questions were open-ended. The questionnaire contained the following sections about individual and work characteristics: age, sex, year of driving experience, type of vehicle, type of driving (long distance, short distance or mixed driving), driving time before rest, type of work shift, self-rated health, tobacco use, exercise habits, usual amount of sleep hours prior to driving, and sleep pattern/quality. The questionnaire contained the following questions about sleepiness while driving: self-rated sleep quality prior to driving, experience of sleepiness- fighting sleepiness and head nodding while driving, experience of sleepiness that force the driver to stop driving, season and time of day when sleepiness is most pronounced. The CR-10 rating scale (Borg, 1998) was used and the drivers were asked to rate their sleepiness and when they had felt so sleepy that they needed to stop driving and take a nap. The questionnaire contained the following questions about the symptoms of sleepiness while driving: when these symptoms occurred, and countermeasures taken by the driver. The following symptoms of sleepiness were listed: (1) yawning; (2) difficulties with keeping track of time; (3) difficulties with keeping one thoughts together; (4) difficulties concentrating on the road; (5) eye tiredness; (6) feeling of heavy eyelids; (7) difficulties to sitting still; (8) difficulties keeping head upright; (9) feeling of sickness; (10) feeling of dizziness. Countermeasures in the list for stop driving were (a) take a coffee break and (b) take a nap/sleep. Countermeasures in the list for driving were (a) drink coffee; (b) using tobacco; (c) eat snacks, food, or something else; (d) turn up the volume of the stereo; (e) call and talk to someone; (f) lower the temperature in the cabin; (g) increase the temperature in the cabin; (h) open the side window; (i) increase the light in the cabin; (j) change driving speed; and (k) others (open-ended). Questions about factors contributing to sleepiness while driving included the following: (1) not enough sleep; (2) poor-sleep quality prior driving; (3) poor work schedules; (4) too long driving shift; (5) annoying sound in the cabin; (6) annoying sound from the road; (7) vibrations in the cabin; (8) too warm in the cabin; (9) poor light conditions; (10) poor eating habits; (11) too little exercise; (12) too much exercise, (13) monotonous driving; (14) other factors (open-ended).

4 210 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) The constructions of general variables for sleepiness symptoms, countermeasures and contributing factors Sleepiness symptoms and time of occurrence of the symptoms contained the same 10 symptoms, but there were 12 countermeasures and 13 sleepiness contributing factors to rate. To get an overall picture of these four categories, each item in the category was dichotomized (for instance, 1 = ratings of a symptom > never, 0 = never. See Table 4 for the first five options). When summarizing the dichotomized items in the category, each driver received a score, which for sleepiness symptoms could range from 0 to 10. This score was then divided by the amount of items that the drivers had completed; that is, if the drivers had rated all 10 sleepiness symptoms and the sum of the dichotomized items was 7, the average score became 0.7 for that driver as general score for sleepiness symptoms. Drivers who had completed zero or one item were excluded The constructions of independent variables in each of the three areas; prior sleep, driving conditions and individual characteristics When analysing the way in which sleepiness, symptoms of sleepiness and sleepiness related aspect are affected and/or interferes with prior sleep aspects, driving conditions and individual characteristics, independent variables were created within these three areas. Each of these independent variables were analysed against general sleepiness symptoms, specific sleepiness symptoms, countermeasures, and contributing factors to sleepiness Area; prior sleep aspects Four independent variables were defined: (1) prior sleep time before a driving shift, (2) general feeling of sleep quality, (3) prior sleep quality to a shift and (4) rated sleep problems Prior sleep time; short-sleep/long-sleep drivers. The drivers were grouped depending on how much sleep they usually had prior to a driving shift. Keenan (1999) states that most people report a need of between 7 and 8.5 h of sleep. In the present study the lower value is used to group the drivers into two groups. Short-sleep, consisted of drivers that usually had less than 7 h of sleep prior to driving (mean: 5 h 43 min, SD 38 min), and Long-sleep was comprised of drivers who normally slept equal to or more than seven hours (mean: 7 h 25 min, SD 34 min). The difference between the two groups sleep hours was significant (p <.001) General sleep quality; good-sleep quality/poor-sleep quality. The drivers were grouped depending on how they rated their sleep quality in general. Good-sleep quality were those who had rated good or better while Poor-sleep quality were those who had rated their general sleep quality as fairly or poor Prior sleep quality; good-sleep prior driving/poor-sleep prior driving. The drivers were grouped depending on how often they usually had a good sleep prior to a driving shift. Good-sleep prior driving were those who had rated that they often or more get a good sleep prior to driving while Poor-sleep prior were those how had rated that they occasionally or less often have a good sleep prior to driving Sleep problems; low-frequency sleep problems/high-frequency sleep problems. The questionnaire contained three questions about sleepiness problems. The drivers were asked to rate how often each of these sleep problems had occurred during the last year. The ratings alternative were; Very rarely/never (1), occasionally once or twice each month (2), sometimes each week (3) or daily/almost daily (4). When summarising the ratings from the three questions, the driver received an overall sleep problem score that could range from 3 to 12. The drivers were grouped by the median score of all the drivers. Low-frequency sleep problem group were those below the median score and High-frequency group were those above the median score for all the drivers Area; driving conditions aspects Three independent variables in this area were defined: (1) driving time before a break, (2) type of driving and (3) driving shift.

5 Driving time before a break; driving time > 2 h/driving time <= 2 h. The drivers were grouped depending on how long they usually drove before taking a break Type of driving; long-distance driving/densely-build-up drivers. The drivers were grouped depending on what kind of driving they usually had been involved in Driving shift; night-shift drivers/day-shift drivers. The drivers were grouped depending on time of day they usually drove. Those drivers usually driving between 6 p.m. and 6 a.m. were defined as night-shift drivers and those who usually drove between 6 a.m. and 6 p.m. were defined as day-shift drivers Area; individual characteristics aspects Five independent variables in this area were defined: (1) age, (2) driving experience, (3) exercise habits, (4) tobacco use and (5) rated health Age; young-drivers/old drivers. The drivers were grouped in two groups; young drivers (less that 35 years of age) and old drivers (35 years or older) Experience; short-experience/long-experience. The drivers were grouped in two groups; short-experience drivers (5 years or less as a professional driver) and long experience drivers (more than 5 years as a professional driver) Exercise habits; low-frequency/high-frequency exercise habits. The drivers were grouped into two groups; low-frequency exercise (once a week or less) and high-frequency exercise drivers (twice a week or more) Tobacco habits; tobacco users/non-tobacco users. The drivers were grouped in two groups; Tobacco users (smokers or use of moist snuff) and Non-tobacco users Rated health; good health/poor health. The drivers were grouped in two groups: those who rated their health in general as good or very good, Good health, and those drivers who rated their health as fair or poor, Poor health Statistical methods and data analysis J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) The statistical analyses were made using SPSS software version 11.0 (SPSS, Chicago, IL). Besides descriptive statistics, Fischer s exact test was used for R C contingency tables (nominal and ordinal data). Independent t-test was used for the interval data; age, year of experience and sleep hours. To compare scores of general variables for sleepiness symptoms, countermeasures and contributing factors and CR-10 ratings between groups, Mann Whitney test for 2 independent samples were used. 3. Results Included in the results is a description of the ratings of individual parameters, work, experience, health, sleep, and sleepiness. The descriptive part is followed by the results from the analyses of the way in which the different variables are related to fatigue and fatigue related to conditions Descriptive results Age and work experience On average, the drivers (149 men and 5 women) were 44.5 years old (SD 13.00) and had been driving for 18.9 years (SD 12.9).

6 212 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) Type of vehicles, driving and traffic The majority of the drivers (84%; n = 127) were normally engaged in long-distance driving. 62% (n = 96) were long-distance truck drivers and 20% (n = 31) long-distance bus drivers. 10% (n = 16) were engaged in different types of mixed driving using different types of vehicles while 7% (10 truck drivers and 1 bus driver) normally were driving in densely built-up areas. The drivers identified the following as their typical traffic conditions: 69% (n = 107) mostly long-distance driving ; 7% (n = 11) mostly densely built-up areas ; 23% (n = 36) mostly a mixed of these two conditions. Transporting mixed cargo (including timber and ore) engaged 65% (n = 102) of the drivers, 23% (n = 36) were engaged in passenger traffic, 8% (n = 13) mostly preformed food transportation, and 2% (n = 3) of the drivers were engaged in other types of transports. Most of the drivers 58% (n = 90) had work shifts that involved mixed driving hours between and and between and 06.00, 24% (n = 38) of the drivers normally carried out their driving between and 18.00, and 17% (n = 26) of the drivers drove between and Over half the drivers 57% (n = 87) drove between 2 and 4 h without a break, 20% (n = 31) drove between 1 and 2 h, and 16% (n = 25) normally drove for more than 4 h without a break. Only 7% (n = 11) of the driver drove less than 1 h without a break Health Subjective health status was rated as good or very good by 77% (n = 121) of the drivers, good by 18% (n = 28), and fairly or poor by 3% (n = 5) of the drivers. Over half 59% (n = 91) of the drivers were non-smokers/non-moist snuff users, 38% (n = 59) were either smokers or used moist snuff, and 3% (n = 4) were both smokers and moist snuff users. Most of the drivers 40% (n = 61) never or rarely exercised 36% (n = 55) exercised once a week, and 25% (n = 28) of the drivers exercised two of more times a week Sleep On average, the drivers reported that they generally slept for 6 h and 28 min (SD 1 h 2 min) prior to their driving shifts. General sleep quality was rated as good or very good by 62% (n = 95) of the drivers, fair by 27% (n = 44), and fairly poor or very poor by 10% (n = 15). The majority of the drivers (51%) reported that they never/almost never had difficulties falling a sleep (n = 77), and (58%) reported that they had never/almost never difficulties falling a sleep after waking (n = 88). The most common sleep problem was feeling of not having slept enough when waking up ; 31% (n = 48) of the drivers reported this happened about once or twice a week. Having had a good night sleep prior to driving occurred occasionally or more seldom (38%; n = 57) Incidences of sleepiness The experience of feeling sleepy while driving was reported to occur often or more than often by 14% (n = 21) of the drivers. Almost one-third (n = 46) of the drivers occasionally had to fight sleepiness while driving, and 8% (n = 12) of the drivers reported occasionally head nodding/drops off while driving. More than one-fifth (n = 35) of the drivers occasionally or more often had experiences of such sleepiness so often they had to stop driving Time when sleepiness appears Autumn was the time of year when 66% (n = 102) of the drivers considered sleepiness to be most troublesome and 21% (n = 33) reported winter as the most troublesome period of the year (Fig. 1). The period between 3 a.m. and 6 p.m. was the time of day when 46% (n = 71) of the drivers considered sleepiness to be most severe, 14% (n = 22) reported the period 12 p.m. to 3 a.m., and 10% (n = 16) reported the period 6 9 a.m. to be the period when sleepiness would be most severe (Fig. 1). When dividing the drivers trip into five phases (start early phase mid-phase later phase finish), 36% (n = 53) of the drivers reported being most sleepy in the later phase of the trip, 28% (n = 41) reported the finish phase, and 24% (n = 35) reported the middle phase (Fig. 1).

7 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) % 100% 100% 80% 80% 80% 60% 60% 60% 40% 40% 40% 20% 20% 20% 0% Spring Summer Autumn Winter 0% 12 am - 3 pm 9 am - 12 am 6 am - 9 am 3 pm - 6 pm 9 pm - 12 pm 6 pm - 9 pm 12 pm - 3 am 3 am - 6 am At the start Early stage Mid stage Later stage At the finish Fig. 1. The distribution of the drivers experiences when sleepiness is considered to be most troublesome. Left frame shows the distribution of troublesome sleepiness in relation to time of the year, middle frame in relation to time of day and right frame the relation to stage of the drive Characteristics of sleepiness One hundred and eighteen (118) of the drivers reported that they at least once had been so sleepy that they needed to stop their driving. Mean rated value of fatigue on the CR-10 rating scale when these drivers stopped their driving was 5.0 (SD 2.4). The rated value corresponded to an experience of fatigue of strong, heavy and difficult. The three sleepiness symptoms most frequently rated by the drivers to occur during their driving were yawning (94%), eye tiredness (83%), and feeling of heavy eyelids (75%). Yawning occurred often or very often by 35% of the drivers, eye tiredness 11%, and feeling of heavy eyelids 10%. These three symptoms appeared early or very early in a degree of 38%, 17%, and 10%, respectively Causes of sleepiness The three main contributing causes of sleepiness reported by the drivers were not enough sleep (95%, n = 145), poor-sleep quality prior to driving (91%, n = 139), and poor work schedules (87%, n = 131). Of these three factors, 30% (n = 46) reported that not enough sleep was the strongest contributing cause to sleepiness while driving, 20% (n = 30) poor-sleep quality prior driving, and 21% (n = 31) poor work schedules Countermeasures to sleepiness The three most frequently reported countermeasures to sleepiness used by the drivers were to eat snacks, food, or something else (87%), stop for a break (85%), and turn up the volume on the stereo (80%). Other countermeasures reported were drinking coffee while driving (37%), tobacco use (31%) Analyses of interference caused by sleepiness This part of the results is a description of the way in which sleepiness, symptoms of sleepiness and sleepiness related aspect are affected and/or interfere with prior sleep aspects, and work related aspects of driving and individual characteristics.

8 214 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) The interference of prior sleep to general variables of sleepiness In this section, 36 comparisons were made. Significant differences were found in 17 comparisons (47%). Irrespective of how the drivers were grouped in the area of prior sleep aspects, there were significant differences between each of these groups when comparing the experience of feeling sleepy, fighting sleepiness and symptoms of sleepiness in general while driving. That is, drivers who reported short-sleep, poor-sleep quality in general, poor-sleep prior a driving shift or having high-frequency of sleep problems, also reported more occasions of sleepiness, fighting against sleepiness and in general having more symptoms of sleepiness while driving (Table 1). Short-sleep drivers also reported more occasions when they had to stop driving due to sleepiness compared to long-sleep drivers. This is also true for drivers with poor-sleep quality prior driving compared to those with good sleep prior driving The interference of driving conditions to general variables of sleepiness Similar analyses as presented in Table 1 were done for the variable in the area of driving conditions, i.e. the drivers were grouped differently in three ways and for each of the grouping, comparison were done against the nine dependent variables (same as in first column in Table 1). The analyses included in total 27 comparisons. Significant differences between groups were found in 5 comparisons (18%). When grouping the drivers by driving time, no significant group difference in sleepiness could be found. Drivers engaged in long-distance driving however reported significantly more often experience of sleepiness while driving (p <.05) than those driving in densely built-up areas. They also in general used more countermeasures than densely built-up drivers (p <.001) and in general reported more contributing factors to sleepiness while driving (p <.01). When grouping the drivers depending on time of day they usually drive, night-shift drivers in general had more often having symptoms of sleepiness (p <.05) and more often use countermeasures to sleepiness while driving compared to day-shift drivers (p <.05). No other significant differences could be found The interference of individual characteristics to general variables of sleepiness Similar analyses as presented in Table 1 were done for the variable in the area of individual characteristics, i.e. the drivers were grouped in five different ways and for each of the grouping, comparison were done against the nine dependent variables (same as in first column in Table 1). Totally, 45 comparisons were made. In 5 comparisons, significant group differences were found (11%). Drivers in the Poor-health group reported a higher occurrence of sleepiness symptoms in general (p <.001) and a higher usage of countermeasures (p <.05) while driving than the drivers in the good-health group. Tobacco users significantly used more countermeasures in general against sleepiness than Non-tobacco users (p <.01). Younger drivers had significantly higher sleepiness ratings, made on the CR-10 scale, when their sleepiness forced them to stop driving compared to older drivers (p <.05). This is also true when grouping the drivers by experience, i.e. less experienced drivers perceives their sleepiness more than more experienced drivers when sleepiness force them to stop driving (p <.05) The interference of prior sleep to specific symptoms of sleepiness The analyses in total included 80 comparisons. Significant group differences were found in 37 comparisons (46%). Most of the differences (7 of 10) were found when the drivers were grouped to low and high frequency of sleep problems (Table 2). Difficulties to keep track of time, to keep ones thoughts together, to concentrate on the road, to sit still, to keep the head upright, felling of eye tiredness and feeling of heavy eyelids occurred more often in the group that reported high frequency of sleep problems. Drivers who reported poor sleep prior a driving shift had significant more often sleepiness symptoms compared to drivers who in general reported good sleep prior to a driving shift (Table 2). This was most evident for the symptoms; eye tiredness and feeling of heavy eye lids. Drivers in the group that reported general poor-sleep quality differed from drivers who in general had good-sleep quality as far as the following symptoms were concerned; eye tiredness, feeling of heavy eyelids, difficulties of sitting still of keeping the head upright and feeling of dizziness. Drivers with short sleep prior driving differed from those with long sleep prior driving as far as the following symptoms were concerned: difficulties of keeping ones thoughts together, difficulties with keeping track of time and difficulties to concentrate on the road (Table 2). Regarding the appearance of the sleepiness symptoms the pattern is similar. Drivers who reported high-frequency sleep problems reported that the symptoms, difficulties to keep

9 Table 1 Analyses of prior sleep aspects interference with sleepiness and sleepiness related variables General variables of sleepiness, countermeasures and contributing factors to sleepiness Experience of feeling sleepy while driving Experience of fighting sleepiness while driving Experience of head nodding/drop off while driving Experience of feeling so sleepy while driving that stop driving was necessitated General occurrence of sleepiness symptoms General appearance of sleepiness symptoms General use of countermeasures General opinion of contributing factors to sleepiness Short-sleep 86 drivers Long-sleep 67 drivers Good-sleepQ 95 drivers Poor-sleepQ 59 drivers Good-sleep prior driving 95 drivers Poor-sleep prior driving 58 drivers Low-freq. sleep problems 95 drivers Always/almost always Often Occasionally Rarely Never p <.001 p <.001 p <.001 p <.001 Always/almost always Often Occasionally Rarely Never p <.001 p <.001 p <.001 p <.001 Always/almost always Often Occasionally Rarely Never n.s n.s p <.05 n.s Always/almost always Often Occasionally Rarely Never p <.001 n.s p <.05 n.s Mean ranks (n) p <.01 p <.01 p <.001 p <.001 Mean ranks (n) Mean ranks (n) p <.001 n.s n.s p <.05 Mean ranks (n) High-freq. sleep problems 59 drivers CR-10 rating scale Mean ranks (n) The drivers were grouped in four different ways (first row) and analyzed against sleepiness/sleepiness related variables (first column). Frequency distributions of the four first variables are shown and the significant level of Fischer s Exact test are below each distribution. For the five lower variables, mean ranks and the significant level of the Mann Whitney U-test are shown. J. van den Berg, U. Landström / Transportation Research Part F 9 (2006)

10 Table 2 Analyses of prior sleep aspects interference with specific sleepiness symptoms Symptom of sleepiness Items Experience of symptoms occurrence Experience of symptoms appearance Shortsleep Longsleep GoodsleepQ PoorsleepQ Goodsleep prior driving Poorsleep prior driving Lowfrequency sleep problems Highfrequency sleep problems Shortsleep Longsleep GoodsleepQ PoorsleepQ Goodsleep prior driving Poorsleep prior driving Lowfreq. sleep problems Yawn Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s n.s p <.05 n.s p <.01 n.s n.s n.s Difficulties with keeping track of time Difficulties to keep ones thoughts together Difficulties to concentrate on the road Eye tiredness Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear p <.05 n.s n.s p <.05 n.s n.s n.s p <.05 Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear p <.01 n.s p <.05 p <.05 p <.05 n.s n.s p <.05 Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear p <.05 n.s n.s p <.05 p <.05 n.s n.s p <.05 Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s p <.01 p <.001 p <.05 n.s n.s p <.001 n.s Highfreq. sleep problems 216 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006)

11 Feeling of heavy eyelids Difficulties to sit still Difficulties to keep the head upright Feeling of sickness Feeling of dizziness Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s p <.01 p <.001 p <.01 p <.05 n.s p <.01 p <.05 Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s p <.01 p <.01 p <.01 n.s p <.05 p <.05 n.s Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s p <.05 p <.01 p <.05 Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s p <.05 n.s n.s Very often/very early Often/early Occasionally/rather late Rarely/very late Never/never appear n.s p <.001 n.s n.s n.s p <.01 p <.001 p <.05 The drivers were grouped in four different ways (first row) and analyzed against the specific sleepiness symptoms (first column). Frequency distributions of the occurrence of the symptoms are to the left and the appearance to the right. Significant level of Fischer s Exact test are below each distribution. J. van den Berg, U. Landström / Transportation Research Part F 9 (2006)

12 218 J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) track of time, to keep ones thoughts together, to concentrate on the road, felling of heavy eyelids and dizziness, to appear significantly more earlier than for the drivers with low-frequency sleep problems (Table 2). Drivers in the group Poor-sleep prior to driving differed from those with good sleep prior to driving in 4 of 10 sleepiness symptoms. This also holds for the comparisons between short- and long-sleep drivers (Table 2) The interference of driving conditions to specific symptoms of sleepiness Similar analyses as presented in Table 2 were done for the variable in the area of driving conditions. Totally, 60 comparisons were made for both the occurrence and the appearance of the sleepiness symptoms. Significant differences betweens groups were found in 4 of these comparisons (7%). Drivers who usually drove more than 2 h before taking a brake reported that yawning occurred more often than those driving less than 2 h(p <.05). Night shift drivers reported eye tiredness to occur more often than day shift drivers (p <.05). They also reported that the appearance of the symptom, difficulties with keeping track of time, to appear earlier than the day shift drivers (p <.05). Long distance drivers reported that the symptom, difficulties to keep ones thoughts together, appeared earlier than densely area drivers (p <.05) The interference of individual characteristics to specific symptoms of sleepiness Similar analyses as presented in Table 2 were done for the variable in the area of individual characteristics. In total, 100 comparisons were made. In 19 comparisons significant group differences (19%) were found for the occurrence or appearance of sleepiness symptoms. Most of these differences, 7 of 19, were found when comparing drivers with good health with those with poor health. Drivers with good health reported significantly less occurrence of the symptoms; eye tiredness (p <.01), difficulties of keeping the head upright (p <.05), feeling of sickness (p <.001) and feeling of dizziness (p <.001) than drives with poor health. Difficulties to concentrate on the road, feeling of sickness and feeling of dizziness appeared significantly earlier for the drivers with poor health (p <.01, p <.05, p <.001, respectively). Younger drivers differed significantly from older drivers in the occurrence of 3 out of the 10 sleepiness symptoms; yawning, difficulties with keeping track of time and feeling of dizziness The interference of prior sleep to the use of countermeasures The drivers rated the how often they used different countermeasures to sleepiness. The analysis included 48 comparisons in total. Significant group differences were found in 14 comparisons (29%). Drivers with short sleep, poor general sleep quality, poor sleep prior a driving shift or driver with high frequency of sleep problems, significantly more often used changing driving speed as a countermeasure (Table 3) compared to the opposite group. Lowering the temperature in the cabin or turning up the volume on the radio were two other countermeasures that were significantly more frequently used more by drivers in the groups previously mentioned The interference of driving conditions to the use of countermeasures Similar analyses as presented in Table 3 were done for the variable in the area of driving conditions. In total, 36 comparisons were made and significant differences were found in 8 out of these comparisons (22%). Long distance drivers stopped for a break or a nap, lowered cabin temperature and increased the light in the cabin more frequently than drivers in densely built-up areas (p <.001, p <.05, p <.05, p <.05, respectively). Night shift drivers drank coffee, called and talked to someone and increased the light in the cabin more often than day time drivers (p <.001, p <.05, p <.05, respectively) The interference of individual characteristics to the use of countermeasures Similar analyses as presented in Table 3 were done for the variable in the area of individual characteristics. The analysis in total included 60 comparisons. Significant group differences were found in 8 cases (13%). Four of these were found when comparing younger drivers with older drivers. Older drivers used tobacco more often as a countermeasure than younger drivers (p <.01), while younger drivers eat, talked on the radio/phone and turned up the radio volume more often than older drivers (p <.05, p <.001, p <.001, respectively). While tobacco users used tobacco as a countermeasure, non-tobacco users stopped driving for a nap significantly more often than tobacco users (p <.001).

13 The interference of prior sleep to reporting contributing factors to sleepiness The drivers rated the appearance of thirteen contributing factors to sleepiness while driving. In total, 52 comparisons were made. Significant group differences were found in 12 of the comparisons (23%). Irrespective of how the drivers were grouped, drivers with short sleep, poor general sleep quality, poor sleep prior driving or high frequency of sleep problems more often reported poor work schedule as contributing factor to sleepiness (Table 4). Poor sleep prior driving and long driving shift were reported more often as a contributing factor to sleepiness while driving by drivers with poor-sleep quality The interference of driving conditions to reporting contributing factors to sleepiness Similar analyses as presented in Table 4 were done for the variable in the area of driving conditions. The analysis included 39 comparisons of which 4 comparisons (10%) showed significant differences betweens groups. Driver that drove less than 2 h before a break more often reported annoying sound from the road as a contributing factor to sleepiness compared to those driving longer than 2 h (p <.05). Densely built up area drivers reported the same as well as that too little exercise contributed to sleepiness while driving compared to long-distance drivers (p <.05, p <.05, respectively) The interference of individual characteristics to reporting contributing factors to sleepiness Similar analyses as presented in Table 4 were done. In total, 65 comparisons were made. Significant differences between groups were found in three cases (5%). Drivers with low exercise habits reported that too much exercise more often contributed to sleepiness compared to those with high-frequency exercise habits (p <.05). Contrarily, drivers with high-frequency exercise habits reported that poor-sleep quality prior driving more often contributed to sleepiness while driving (p <.05). 4. Discussion J. van den Berg, U. Landström / Transportation Research Part F 9 (2006) This study illuminates a serious problem with respect to sleepiness and traffic. The number of drivers with experiences of sleepiness is high. This study supports a number of previous studies (Dawn, 2003; Hakkanen & Summala, 2000, 2001; McCartt et al., 1996). Almost one-third of the drivers occasionally had to fight sleepiness while driving and about 8% of the drivers reported occasional head nodding/drops while driving. This must be considered as a serious traffic problem. The same conclusion could be drawn from the result that more than one-fifth of the drivers occasionally or more often had to stop their driving due to sleepiness. The results could be evaluated from the perspective of prior sleep features, working features and individual characteristics. Of these three areas, prior sleep aspects contributed the most to sleepiness while driving. Drivers reporting poor-sleep quality prior to driving, poor-sleep quality in general, high frequency of sleep problems and/or short sleep prior to driving were more likely to experience sleepiness and sleepiness symptoms while driving compared to drivers that reported long sleep, low frequency of sleep problems and/or good-sleep quality both in general and prior to driving. They also used countermeasures more often against sleepiness and pointed out poor work schedules and poor-sleep quality prior to driving as the most contributing factors to sleepiness while driving. When grouping the drivers by different driving aspects or individual characteristics, no clear differences in pattern could be found regarding sleepiness, symptoms of sleepiness, countermeasures or contributing factors to sleepiness. Drivers identified two of the countermeasures to counteract early morning sleepiness as most important: sleep before work and better working hours. Some of the measures suggested by the drivers are aimed to reduce sleepiness that has developed during their driving: speech communications, naps, radio listening, lowering the cabin temperature, and changing the driving speed. The drivers more commonly suggested these countermeasures with respect to less sleep prior to work. This relation shows that there is a well-established awareness of sleepiness as a problem among the drivers and also a strong motivation for measures against driving sleepiness. Additionally, more than one-fifth of the drivers stopped their driving due to sleepiness. Taking a break in the driving, however, normally was only used in the case when the drivers had reached a relatively high degree of sleepiness (strong, heavy, difficult). The indicators of sleepiness used by the drivers are well known: yawning reduced concentration, problems focusing on ideas, and eye tiredness. All these symptoms functionally are based on experiences that they signal