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Truck Drivers Sleep-Wake TimeArrangementsC.R.C. Moreno , L. Matuzaki , F. Carvalho , R. Alves , I.Pasqua & G. Lorenzi-FilhoPublished online: 09 Aug 2010.

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Truck Drivers Sleep–Wake Time Arrangements

C.R.C. Moreno1, L. Matuzaki1, F. Carvalho1, R. Alves2, I. Pasqua1 and G. Lorenzi-Filho2

1Department Environmental Health, School of Public Health, University of São Paulo, Brazil; 2Sleep Laboratory, Pulmonary Division, Heart Institute, University of São Paulo

Abstract

Irregular working hours, including night work, change sleep–wake time arrangementswhich in turn might affect the ability to drive safely. This study aims to compare theeffects of an irregular and a fixed day shift system on the sleep–wake cycle of truckdrivers.

The investigation of sleep–wake cycle was carried-out with 37 truck driversworking on two transportation plants: 24 working on irregular working hours and 13on fixed day shift. The truck drivers filled out sleep logs and wore actigraphs for 10consecutive days to identify activity and rest episodes.

The group working in irregular hours showed more sleep episodes per 24h andthey were shorter compared to the fixed shift group (p < 0.05). No differences werefound between the two transportation plants. These results suggest an the influenceof working hours on specific sleep–wake patterns. The polyphasic sleep pattern shownby irregular shift group could be a strategy to cope with sleep deprivation, which mayaccount for their difficulty to resist falling asleep behind the wheel.

Keywords: Shiftwork, drivers, sleep-wake cycle, work schedule.

Introduction

To cope with the demands of their working schedules, shiftworkers must invert theirsleep-wake cycle, sleeping during the day and remaining awaken at night. Those whoare unable to get enough sleep suffer from partial sleep deprivation. Sleep depriva-tion can be worse in workers that are on an irregular working time which can includework at any time of the day or night depending on work demand. In a recent studyof train drivers, Kecklund and Åkerstedt (2000) concluded that a lack of schedule

Address correspondence to: Claudia R. de C. Moreno, Departamento de Saúde Ambiental, Faculdade deSaúde Pública, Universidade de São Paulo, Av Dr. Arnaldo, 715 São Paulo SP, Brazil, CEP 01246-904. E-mail: crmoreno@usp.br

Biological Rhythm Research 0165-0424/03/3402-137$16.002003, Vol. 34 No. 2. pp. 137–143 © Swets & Zeitlinger

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predictability and regularity, and the short break between shifts, causes poor andinsufficient sleep and extended wake periods.

Developing strategies that enable the drivers to adapt to this kind of work sched-ule are essential to allow them not only to perform their tasks in a satisfactory mannerbut also to minimize their sufferings. One of these strategies is to compensate forsleep loss by taking naps whenever possible (Dinges et al., 1987; Naitoh et al., 1982).Broughton (1989) reported experimental evidences supporting the hypothesis thatthere is a semi-circadian predisposition to fall asleep, constituting therefore a bi-phasic pattern of sleep. It is possible that in the human species there is an endoge-nous predisposition to sleep not only two but several times along the day. Thispossibility would be expressed in certain circumstances, particularly during sleepdeprivation (Stampi, 1992).

It is possible, however, that irregular naps could contribute to further desyn-cronization of the body’s timing system. Though, the individual flexibility in termsof entraining bedtime and wakeup time may reveal an endogenous capacity to adaptto sudden changes in environmental synchronizers, as for example work schedules.

The main purpose of Moreno’s doctorate thesis (1998) was to investigate whetherthis capacity to fragment the sleep episodes would allow individuals to cope with shiftwork. In this study, based on interviews, some night workers that considered them-selves well adapted took several naps in a 24-h period. On the other hand, it wasobserved that some workers who did not exhibit a polyphasic sleep patterns alsodeclared that they were well adapted. This suggests that taking naps may be a usefulstrategy for some workers, but not for all of them.

It seems logical that a polyphasic sleep pattern would thus occur in cases wheresleep deficits are present, associated with the role of social factors that would prevent(or make it more difficult) workers from having a single sleep episode. In short,human endogenous ability to sleep more than once on a 24-h period seems to expressitself in situations where a partial sleep deprivation occurs, as is the case of workerssubmitted to irregular work schedules. The experiments described here were aimedat evaluating the effects of an irregular shift schedule on sleep patterns in truckdrivers. In this study, we analyzed the number of sleep episodes during 24-h inter-vals, mean duration of sleep episodes and total minutes scored as sleep during 24-hintervals. This professional category has been chosen for this field study because itmakes up a group of workers within which taking naps is a well known practice tocompensate for irregular work shifts.

Material and Methods

Data collection

Population

Thirty-seven male truck drivers working at two transportation companies participatedin this study: one of the companies was located in São Paulo municipality (n = 19; 9working on an irregular shift and 10 in a fixed day) and the other one in Campinas

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municipality (n = 18; 15 working on an irregular shift and 3 in a fixed day). In bothcompanies the irregular shift was defined as working anytime, according to the tasksdemands. The scheduled start time for the fixed shift was 07:30h and the end timewas 16:30h from Monday to Saturday. Although they also have variations due todemand, which can reach 50h a month, the actual working hours are always duringthe daytime.

The truck drivers that work on an irregular schedule drive on average for 9h andspend about 5h loading/unloading the truck. In contrast, the drivers on a fixed dayschedule drive an average of 6h and spend 4h loading/unloading the truck.

Tools for data collection

Activity Protocols The study on sleep-wake cycle was carried out through the com-pletion of Daily Activity Protocols comprising sleep logs where the drivers registeredtheir bedtime and wakeup time, as well as the activities performed while they wereawaken. Such protocols were filled out during 10 consecutive days.

Actigraphs During 10 consecutive days, the truck drivers wore wrist actigraphs, inorder to register both their activities and resting periods.

Data analysis

To perform the analysis of data relative to sleep, actograms have been drawn by usingthe program AW2, specific for actigraphic data. The comparison between suchactograms and the data obtained through the protocols allowed the accomplishmentof the referred analysis.

The sleep patterns were identified by analyzing: 1) the number of sleep episodesduring 24-h intervals; 2) mean duration of sleep episodes with sleep interruptions(time in bed); 3) excluding sleep interruptions (actual sleep duration); and 4) totalminutes scored as sleep during 24-h intervals. In cases 2 and 3 the duration of eachsleep episode was analyzed. The total sleep duration on 24h was analyzed on case 4.

The analysis of the number of sleep episodes considered that the 24-h periodstarted at 12:00 noon and ended at 12:00 noon the following day. The analysis of thesleep duration including sleep interruptions was performed considering each sleepepisode from its onset to its end; this was called the length of ‘time in bed’ (case 2).

Differences between the work schedules (irregular or fixed), and between theworkers from the two companies were analyzed by ANOVA.

Results

The results revealed distinct sleep patterns for workers on an irregular schedule andon a fixed schedule. The truck drivers who have an irregular work schedule showeda polyphasic sleep pattern on several days of the temporal series studied, whereas thetruck drivers who have a fixed work schedule showed a monophasic pattern. Suchpatterns relative to the number of sleep episodes could be summarized as can be seenon Table 1.

Truck Drivers Sleep–Wake Time Arrangements 139

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The calculation of the percentage of days making up the temporal series, accord-ing to the number of sleep episodes on each 24-h period, was obtained through actigraphic data and Daily Activity Protocols. It is interesting to point out that noneof the truck drivers working at São Paulo transportation company under fixed workschedules had more than one sleep episode in more than 50% of the days studied. AtCampinas transportation company five drivers under irregular schedules had morethan one sleep episode in all of the days studied. As for the truck drivers working atCampinas transportation company under fixed work schedules all of them had onlyone sleep episode in 100% of the days studied.

Table 2 shows that on both companies the mean duration of sleep episodes, includ-ing sleep interruptions (time in bed) among the group of truck drivers who had a fixedwork schedule is higher than that among the group of truck drivers who had irregu-lar work schedules. The same result can be seen with the mean duration of sleepepisodes excluding sleep interruptions (actual sleep duration).

The total sleep duration on a 24-h period, excluding sleep interruptions, is alsoshown in Table 2.

The results obtained through ANOVA showed no difference between the means of‘time in bed’ concerning truck drivers working at one company or the other; however,a significant difference between truck drivers submitted to irregular schedules andfixed schedules was found (Table 3).

Table 3 also shows a significant difference relative to actual sleep duration couldbe observed between the work schedules (p = 0.000038). No difference was found

140 C.R.C. Moreno et al.

Table 1. Sleep patterns concerning to the number of sleep episodes, according to companiesand work schedules.

Transportation company

São Paulo (n = 19) Campinas (n = 18)

Pattern concerning to theWork schedule Work schedule

number of sleep episodes Irregular Fixed Irregular Fixed

More than 1 sleep episodeper day

0–49% of studied days 5 – 4 –50–99% of studies days 4 – 5 –100% of studied days – – 5 –

1 sleep episode per day

0–49% of studied days – – – –50–99% of studies days – 6 – –100% of studied days – 4 1 3

Total sample 9 10 15 3

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between the two companies (p = 0.259633). The same result was obtained for the totalsleep duration on a 24-h period. With this measure, the work schedule factor was atthe limit for demonstrating significance (p = 0.051808).

Discussion

The sleep analyses have revealed differences between truck drivers working underirregular schedules and those working only during day time. Such differences are statistically significant and demonstrate the influence of the work schedule upon thesleep of the individual. It is also interesting to observe that workers from two distinctcompanies performing similar functions and having similar work schedules showedquite similar results.

Truck Drivers Sleep–Wake Time Arrangements 141

Table 2. Mean duration of truck drivers ‘time in bed’, mean actual sleep duration and mean of total sleep duration on each 24-h period, according to the work schedule and the transportation company.

Campinas São Paulo

Sleep categories Irregular Fixed Irregular Fixed

Time in bed (min) 235.77 468.88 251.32 390.9Actual sleep duration (min) 199.58 397.61 213.74 312Total duration scored as sleep on 303.04 398.2 327.6 350.7

each 24-h period (min)

Table 3. ANOVA of means relative to ‘time in bed’, actual sleep duration and total sleep duration concerning truck drivers at both companies, according to the company and the workschedule (1-Company, 2-Work schedule).

Sleep categories Effect F p

Time in bed 1. Company 0.84427 0.3648452. Work Schedule 30.18560 0.0000041.2. 1.89647 0.177745

Actual sleep duration 1. Company 1.31559 0.2596332. Work Schedule 22.60080 0.0000381.2. 2.56395 0.118856

Total sleep duration on 24h 1. Company 0.152642 0.6985352. Work Schedule 4.071335 0.0518081.2. 1.505251 0.228547

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Workers on fixed work schedules showed a monophasic sleep pattern, whereasworkers submitted to irregular schedules showed a polyphasic sleep pattern on alldays or on most days studied. In addition, workers submitted to irregular schedulessleep less on each 24-h period, and their sleep episodes are shorter. Such resultsdemonstrate that in the face of a strong social pressure – in this case the work schedule – the occurrence of a polyphasic pattern might be considered as a strategyto adapt to the work schedule. On the other hand, it is important to point out that theindividual tendency towards this sleep pattern may facilitate the onset of naps.

An earlier study carried out with women working at night under a fixed schedulesuggested that a relationship existed between the need of sleep and an individual’sability to fragment sleep (Moreno et al., 2000). In the study, the sleep polyphasicpattern was expressed by some workers, even when submitted to a fixed work sched-ule. The individual character associated with a double burden (work plus domesticwork) might explain the occurrence of such pattern.

In the present study, the truck drivers on a fixed work schedule exhibited amonophasic pattern occurred on almost all days studied. One explanation for the dif-ference might be that in the absence of other social factors (e.g., domestic work), theindividual tendency towards sleep fragmentation might not be expressed. In the caseof truck drivers submitted to irregular schedules, sleep deprivation generated by theirwork schedules would constitute a pressure strong enough to derive a polyphasicpattern.

Conclusion

The results obtained through this study suggest that in the face of a strong social pressure – in this case the work schedule – the manifestation of a polyphasic patterncan be considered as a strategy to adapt to the work schedule. Other studies suggestthat truck drivers make up a population who tend to stick to habits that lead to obesity:in general, they don’t practice exercises and carry an inadequate diet (Korelitz et al.,1993; Häkkänen et al., 2000; Häkkänen, 2000), which constitute risk factors withregard to some sleep pathologies. Particular sleep patterns associated with workschedule, therefore, need to be considered insofar as their potential contribution tothe individual’s health.

Acknowledgements

We would like to thank the workers who participated of this study and FAPESP forthe support (98/13053-3; 99/00033-7; 99/07418-1; 00/10391-7; 00/10560-3).

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