Zaleplon

IN HUMAN ADULTS, many biological functions (i.e., internal temperature, secretion of certain hormones [e.g., growth hormone, adrenocorticotropi hormone, melatonin]) display a physiological circadian rhythm that oscillates in synchronism with the sleep-wake rhythm. (1) All transient or persisting situations that lead to a phase shift between these bioperiodic functions disturb the psychophysical balance of humans. (2)

Given that there is a need for an increase in productivity and a need for a decrease in cost, the distribution of working hours in industrialized countries has been altered progressively; there has been a growing tendency toward a 24-hr work cycle. There has been a concomitant increase in numbers of nightshift workers and, according to some European Union estimates, these additional nightshift workers constitute more than one-fifth of Europe's human resources. (3) Shiftwork frequently causes a desynchronization between the sleep-wake cycle, light-darkness rhythm, and other endogenous biological rhythms. Such work conditions determine the phase shift of an individual's connections with the social-environmental synchronizing stimuli (4-6) which, on the whole, are the signals that affect all the psychological and behavioral attitudes that induce sleep or strengthen the state of wakefulness.

It seems well established that shiftwork--particularly the nightshift--has a negative impact on workers' health and lowers their quality of life significantly. (7,8) Several studies have revealed that sleep disorders and daytime drowsiness are the main disorders reported by nightshift workers. (6,8-10) Shiftwork interferes with both the quality and quantity of sleep. Regarding sleep duration, there is a decrease in the number of hours spent sleeping, both during morning shifts (i.e., as a result of early awakening) and during night shifts (i.e., resulting from inversion of the normal sleep-wake cycle). (11) Increases in body temperature, starting from the early morning hours, and unfavorable environmental conditions (i.e., noise, family and social activities) make sleeping during the day difficult. (12,13) Several studies have reported that sleepiness and fatigue can increase the risk of human errors and accidents. (14,15) Night work and sleep deprivation may account for some serious accidents (e.g., Three Mile Island [1979], Chernobyl [1986], Exxon Valdez [1989]). (16)

The consequences of altered sleep quality, with possible reductions in vigilance levels, are particularly relevant when shiftwork is associated with tasks that (1) require high performance levels and (2) involve stressful work conditions. Police force personnel are consistently subjected to these work levels, as well as stress. (17,18) In this study, we used self-administered questionnaires to determine the (1) presence of sleep disorders, (2) occurrence of daytime drowsiness, and (3) use of hypnotics (i.e., Benzodiazepines, Zaleplon, Zolpidem, or Zoplicone) among the overall population of state police officers in the city of Genoa. We also identified the differences between shiftworkers and nonshiftworkers.

Subjects and Method

Subjects. We conducted this study during the period between September 1996 and February 1997, inclusive. The October 1996 census revealed that Genoa has a population of 653,000 inhabitants and is ranked third with respect to crime in Italy. (19) The subjects enrolled in this study were advised of the objectives and methods that would be used during the investigation. The study was documented with a self-administered questionnaire, and complete confidentiality was guaranteed during data collection and processing. The subjects were also supplied with telephone numbers that they could call if they desired further information (e.g., the physician in charge of the investigation was available); they could also call if they did not understand the wording of any of the questions or if the meaning was ambiguous. Of the original 2,165 individuals, 1,840 (85%) agreed to participate in the study, and they completed and returned the questionnaires. To minimize age-related differences, we attempted to make the subject population homogeneous. For this study, therefore, we evaluated only the questionnaires of subjects who were 20-39 yr of age. The population included 1,115 subjects who were subdivided as follows: 413 male non-shiftworkers, 483 male shiftworkers, 127 female non-shiftworkers, and 92 female shiftworkers. The average age of the total population was 31 yr (median = 31 yr and 10 mo); there were no age-related differences among the aforementioned 4 groups. Two hundred twenty-one subjects were excluded from evaluation because their ages were outside the chosen age interval, and 504 questionnaires could not be evaluated because (1) evident errors existed or (2) the questionnaires were incomplete. The sample of shiftworkers included personnel who were employed at tasks that were stressful from a psychophysical point of view. The evaluation was based on the description of tasks that involved risks (e.g., escort service, patrol service, flying squad). All personnel worked in counter-clockwise quick-rotating shifts (Table 1). In addition, special operational requirements could involve an extension of the work period without immediate leave. Non-shiftworkers performed mainly office work during regular working hours (8:00 A.M.-2:00 P.M.) and during 2 afternoons each week (3:00 P.M.-7:00 P.M.).

Questionnaire. The questionnaire was divided into 2 types of questions. The first group included personal/ administrative questions about sex, age, civil status, number of children, type of service, role, weight, and height. The second group of questions comprised 36 sleep-related questions. The latter group was organized into 3 main sections, and occasionally they were arranged into subsections. Some questions required a numerical answer (e.g., hours slept each night, hours of sleep during a 24-hr period, severity [i.e., hours] of insomnia, duration of time [i.e., mo] that sleeping pills were used, age when snoring began). Other questions (e.g., "Do you have any difficulty initiating sleep?" "Do you snore?") were organized into 4 ordinal levels (i.e., "never," "rarely," "sometimes," and "often") or divided into items that required a "yes" or "no" response. Within the ordinal levels, the subjects evaluated themselves with respect to time required to fall asleep, dose of sleeping pills taken, amount of alcohol consumed per day, and number of cigarettes smoked per day. In summary, the 3 main sections explored the following areas of sleep: Section 1--difficulty in falling and/or staying asleep; Section 2--sleepiness and accidents; and Section 3--specific sleep disorders (e.g., obstructive sleep apnea syndrome, narcolepsy). Sleepiness was evaluated with an Italian version of a subjective evaluation scale (Epworth Sleepiness Scale [ESS]). (20,22) The scale measures the tendency of an individual to doze or to fall asleep during various daily situations. In its original form, the ESS has been validated in normal subjects and in patients who suffer from different diseases that lead to daytime sleepiness. The scale has good discrimination sensitivity, and it is correlated excellently with results obtained with the objective neurophysiological procedures. (23)

Statistics. To assess the statistical significance of basic differences between male and female subjects that were independent of any disturbances ascribable to shiftwork, we conducted an initial analysis. The cumulative distributions between shift and non-shiftworkers were then compared within the same-sex category. With respect to questions that were organized according to ordinal levels and numerical values, we evaluated the significance of differences between groups with the Kolmogorov-Smirnoff test on the relative cumulative distributions. "Yes" and "no" answers were organized by groups, and the differences between the 2 groups were evaluated with the chi-square test.

Results

In response to the question "Do you usually complain of insomnia?" there were no statistically significant differences detected between male and female subjects or by shiftwork factor. Only 8.7% of the subjects responded affirmatively to that question (Fig. 1). Of the individuals for whom no statistically significant differences relative to sex or the shiftwork factor existed, 18.7% were not satisfied with their quality and quantity of sleep. There was no difference in use of hypnotics in men or women or in shiftworkers and non-shiftworkers. Use of hypnotic and/or hypno-inducing drugs was reportedly frequently in only 0.4% of the subjects; 92.7% of the subjects reported that they had never used such drugs.

[FIGURE 1 OMITTED]

Comparison of males and females. The comparison between female and male subjects revealed that 29.1% of females and 11.9% of males suffered from insomnia "sometimes-often" in the past (p < .001). Forty-five percent of the females and 30% of the males took more than 10 min to fall asleep (p < .05), and 33% of the females and 50% of the males thought they "never" had difficulty waking up in the morning (p < .001). Thirty-three percent of females and 20% of males reported having distressing dreams "sometimes-often" (p < .001). Sixty-three percent of females and 47.6% of males stated that they had "sometimes-often" experienced a headache when they got up in the morning (p < .001). Sixty percent of females and 31% of males reported that they had "never" snored (p < .001). Snoring was correlated significantly with body mass index ([r.sup.2] = .5, p < .01). Thirty-one percent of males and 17.3% of females indicated that they "never" had moments of fatigue or had "never" required sleep for several consecutive days (p < .01). Fifty percent of males and 25% of females drank alcohol (p < .001).

Differences between shift and non-shiftworkers: males. Difficulty in going to sleep was reported by 41% of male shiftworkers vs. 23.2% of male nonshiftworkers (p < .001), and the response of "often" was used by 8.7% of the male shiftworkers vs. 2.9% of the male non-shiftworkers. A sleep latency that exceeded 20 min was reported by 14% of the male shiftworkers vs. 7% of the male non-shiftworkers (p < .0001). Early waking (i.e., "sometimes-often") was reported by 54% of the male shiftworkers vs. 43.6% of the male nonshirtworkers (p < .01). Difficulty in waking up and getting up (i.e., "sometimes-often") was reported in 50.7% of male shiftworkers vs. 41.6% of the male nonshiftworkers (p < .05). The aforementioned results are summarized schematically in Table 2.

According to the International Classification of Sleep Disordersw1990 version (ICSD-90) and the Diagnostic and Statistical Manual of Mental Disorders (3rd [DSM III-R] and 4th [DSM IV] editions), (24-26) insomnia results from a combination of several criteria. Although at the time the questionnaire was completed (i.e., "interview") no significant differences were found between male non-shiftworkers and male shiftworkers who suffered from insomnia, we calculated the number of subjects in whom at least 3 of the following criteria were found: persistence of difficulty in falling asleep and in staying asleep, early waking, reduced number of hours during night sleep (i.e., < 6 hr), and dissatisfaction about sleep quality and quantity. We determined that the probable prevalence of clinically significant insomnia was 1.25% in male non-shiftworkers and 5% in male shiftworkers (p < .01 [Fig. 1]).

When we analyzed the replies to the questions about numbers of hours spent sleeping at night, 33% of male shiftworkers vs. 17.7% of male non-shiftworkers reported a sleep duration that exceeded 8 hr/night (p < .0001 [Fig. 2]). In addition, 65% of male shiftworkers vs. 47% of male non-shiftworkers reported that they remained in bed for a while ("sometimes--often") during the day (p < .01). The total sleep time during the 24-hr period was reported by this group to be greater than 9 hr in 26% of male shiftworkers vs. 11.9% of male non-shiftworkers (p < .0001 [Fig. 2]).

Differences between shift and non-shiftworkers: females. The results obtained for females appeared similar to results recorded in the males, although there was a lower significance for females (.01 < p < .05). This was likely the result of the smaller number of female subjects studied. The prevalence of insomnia, evaluated by the combination of several replies (according to DSM IV criteria), highlighted a prevalence of insomnia in 2.5% of female non-shiftworkers vs. 4.5% of female shiftworkers (Fig. 1c).

Comparison between the degrees of seniority in shiftworkers. The statistically significant effect of seniority was found by subdividing the shiftworkers' sample into 2 large subgroups: the first with a seniority factor of 0-59 mo (junior shiftworkers), and the second with a seniority of 60-119 mo (senior shiftworkers).

Males. When we analyzed single insomnia criteria, 59% of senior shiftworkers vs. 39% of junior shiftworkers (p < .01) reported that they awoke early ("sometimes--often"). Difficulty in staying asleep ("sometimes--often") and difficulty in awakening ("sometimes--often") were reported, respectively, in 37% of senior shiftworkers vs. 17% of junior shiftworkers (p < .05) and 43% of senior shiftworkers vs. 27% of junior shiftworkers (p < .05).

Females. We observed no statistically significant differences among females with respect to the aforementioned criteria.

Comparison between age groups in shiftworkers. Statistically significant differences were found in 2 age groups: (1) younger shiftworkers (20-29 yr), and (2) older shiftworkers (30-39 yr).

Male shiftworkers. Forty-seven percent of male older shiftworkers vs. 26% of male younger shiftworkers (p < .001) reported early morning waking ("sometimes--often"). Difficulty staying asleep ("sometimes--often") was reported, respectively, in 55% of older shiftworkers vs. 43% of younger shiftworkers (p < .01) and in 38% of older shiftworkers vs. 29% of younger shiftworkers (p < .05). Diurnal naps were reported by 61% of older shiftworkers vs. 52% of younger shiftworkers (p < .05).

Female shiftworkers. In this group, we found statistically significant differences only for frequent diurnal naps (47% of older shiftworkers vs. 27% of younger shiftworkers [p < .05]) and for staying in bed for a long time during the day (53% of older shiftworkers vs. 46% of younger shiftworkers [p < .05]).

Sleepiness and accidents. On the basis of scores reported on the ESS, no statistically significant differences were observed between males and females or between shiftworkers and non-shiftworkers. ESS scores that exceeded 10 were observed in 10% of the sample (Fig. 3); this value indicated the first level of pathological sleepiness. At least 1 accident at home or when the subject was driving a car was reported by 51.5% of the sample. No statistically significant differences were observed relative to the shift factor. Accidents were attributed to sleepiness or to a sudden fit of drowsiness by 10% of shiftworkers vs. 6.7% of non-shiftworkers.

[FIGURE 3 OMITTED]

We looked at the differences between senior shiftworkers and junior shiftworkers, and we found a statistically significant difference in the male group, who had at least 1 accident at home or at work or when driving a car (68% of senior shiftworkers vs. 24% of junior shiftworkers [p < .001]). However, 83% of the senior shiftworkers believed that the accidents were not related to sleepiness. In the female group, no statistically significant tendencies were found; this likely resulted from the fact that there were only a few female subjects in our study sample.

Discussion

The analysis of data shows that approximately 9% of subjects' complaints of habitual insomnia were not related to differences in sex or type of work. This percentage is quite similar to the prevalence of chronic insomnia observed .in the overall Italian population for this age group (i.e., approximately 10%). (27) The sex differences confirm the data reported in the international literature, which show a higher prevalence of difficulty in falling and staying asleep in female subjects. (28,29) In fact, sleep in the female group was more disturbed by difficulty in falling asleep, by difficulty in awakening, and by a higher number of distressing dreams. Moreover, female subjects experienced longer periods that were characterized by severe fatigue and by the need for consecutive days of diurnal sleep.

Our data confirmed the higher prevalence of snoring in male subjects and the relationship between snoring and higher body mass index. (30) The high prevalence in cephalalgia upon awakening that was detected in the female group could simply be the result of the higher prevalence of this disorder in females. (31)

The results obtained when we compared the 2 types of workers provided interesting and partly unexpected results. In corroboration with previous studies, (6,8,9) shiftworkers--especially those who were older and had more seniority--showed a higher prevalence of disorders relative to both falling and staying asleep--regardless of their gender. Despite the presence of significant differences in such complaints, there seemed to be no differences relative to the subjective perception of the quantity and quality of sleep. Similarly, the use of hypnotic or hypno-inducing drugs was the same in the 2 groups, and it seems comparable to what was reported in recent epidemiological studies that were carried out in the overall population. (32,34)

Although no differences were observed in the replies to the questions that investigated the presence of habitual insomnia, the results obtained from the combination of several criteria showed a higher prevalence of insomnia in shiftworkers of both sexes. The percentage of insomnia evaluated in this manner, which probably reflects more objectively the presence of a possibly clinically remarkable disorder, was present in approximately 5% of both groups of shiftworkers. This percentage, relative to this age group, was comparable to the percentage obtained in a recent epidemiological study of an older population. (34)

The higher percentage of insomnia did not appear to have had any repercussions on the levels of daytime vigilance. This result clearly diverged from results of studies that have been conducted in populations of shiftworkers. (35) In fact, a comparison of the results obtained from the ESS between the 2 groups of workers was not significant statistically. In both groups, the percentage of subjects with pathological sleepiness was approximately 10%--a value that even in this case mirrors the results obtained in other worker populations. (36) No differences were detected in the prevalence of accidents in the 2 groups.

An additional, unexpected result, which was not in agreement with studies that were conducted in shiftworkers, (8,9,11,37,38) was the finding that shiftworkers reported a higher number of hours of sleep--both at night and during 24-hr periods. In fact, 33.0% of shiftworkers vs. 17.7% of non-shiftworkers required more than 8 hr of sleep, and 26.0% vs. 11.5% reported that they slept more than 9 hr during a 24-hr period.

In conclusion, although shiftworkers presented a higher prevalence of complaints relative to difficulty in falling and staying asleep, as well as a probable higher percentage of insomnia assessed on the basis of the combination of several factors, they did not report a lower quality of sleep than non-shiftworkers. We believe that such a perception resulted from several variables, of which the most influential might be motivation and awareness. In fact, it is well known that motivation can influence the threshold perception of fatigue and sleepiness, and this has enabled shiftworkers to maintain sufficient vigilance levels--even during unfavorable situations. (39) On the other hand, individual differences in tolerance to shiftwork likely existed in police shiftworkers. (40) Perhaps these differences in tolerance existed because there were selection and autoselection of police officers, by higher adaptability to shiftwork with the ability to maintain high performance levels during shifts. In addition, there was most likely an awareness of the difficulties associated with (1) tasks, (2) working rhythms (i.e., schedules), and (3) risks associated with police shiftwork. The fact that our study subjects needed more hours of sleep during a 24-hr period than non-shiftworkers may reflect an attempt to compensate for and prevent disturbances in the sleep-wake cycle. It could also explain the absence of increased levels of sleepiness among our subjects.

Submitted for publication April 17, 2000; revised; accepted for publication December 29, 2000.

Requests for reprints should be sent to Dr. Sergio Garbarino, Cattedra di Neurofisiopatalogia, Centro di Medicina del Sonnon, Ospedale S. Martino, Largo R, Benzi, 10, I-16132 Genoa, Italy.

References

(1.) Minors DS, Waterhouse JM. Circadian rhythms in general. Occup Med State Art Rev. 1990; 5:162-82.

(2.) Folkard S, Minors DS, Waterhouse JM. Chronobiology and shiftwork: Current issues and trends. Chronobiologia 1985; 123-54.

(3.) Wedderburn A (Ed). Statistic and news. Bulletin of European Studies on Time #6. Dublin, Ireland: European Foundation for the Improvement of Living and Working Conditions, 1993.

(4.) Akerstedt T. Adjustments of physiological circadian rhythms and sleep-wake cycle to shiftwork. In: Folkard S, Monk TH (Eds). Hours of Work. Temporal Factors in Work Scheduling. New York: John Wiley and Sons, 1985; pp 199-210.

(5.) Aschoff J. Zeitgeber der tierischen tagesperiodik. Naturwissenshaften 1954; 41:4-56.

(6.) Akerstedt T. Work hours, sleepiness and the underlying mechanisms. J Sleep Res 1995; 4(suppl 2):15-22.

(7.) Cervinka R. Nightshift dose and stress at work. Ergonomics 1993; 36(1-3):155-60.

(8.) Costa G. The impact of shift and night work on health. Ergonomics 1996; 27(1):6-16.

(9.) Kogi K. Sleep problems in night and shift work. J Human Ergol 1982; 11 (suppl)217-31.

(10.) Rutenfranz J. Occupational health measures for night and shiftworkers. J Human Ergol 1982; 11 (suppl):67-68.

(11.) Akerstedt T. Psycliological and psychophysiological effects of shiftwork. Scand J Work Environ Health 1990; 16(suppl 1):67-73.

(12.) Lavie P. Ultrashort sleep-waking schedule III: "gates" and "forbidden zone" for sleep. Electroenceph Clin Neurophysiol 1986; 63:414-25.

(13.) Knauth P, Rutenfranz J. The effects of noise on sleep of shiftworkers. In: Colquhourn WP, Folkard S, Knauth P, Rutenfranz J (Eds). Experimental Studies of Shiftwork. Wiesbaden, Germany: Westdeutscher, 1975; pp 57-65.

(14.) Leger D. The cost of sleep-related accidents: A report for national commission on sleep disorders. Sleep 1994; 17(1):84-93.

(15.) Dinges DF. An overview of sleepiness and accidents. J Sleep Res 1995; 4(suppl 2):4-14.

(16.) Milter MM, Carskadon MA, Czeiler CZ, et al. Catastrophes, sleep and public policy: Consensus report. Sleep 1988; 11:100-09.

(17.) Phillips B, Magan L, Gerhardstein C, et al. Shiftwork, sleep quality, and worker health: a study of police officers. Southern Med J 1991; 84(10):1176-84.

(18.) Garbarino S, Nobili L, Balestra V, et al. Sleep habits in an Italian population of state policemen. Comparison between shift and non-shiftworkers. Shiftwork Intern Newsletter 1997; p 50.

(19.) Ministero dell'Interno, Dipartimento di Pubblica Sicurezza, Ufficio per il coordinamento e la pianificazione delle Forze di Polizia. "Andamento della criminalita in Italia 1996. [Minister of the Interior, Department of Security Forces, Office for Police Coordination. Crime in Italy 1996]. Roma Ist. Poligrafico Zecca della Stato.

(20.) Murray WJ. A new method for measuring daytime sleepiness. The Epworth Sleepiness Scale. Sleep 1991; 14(6):540-45.

(21.) Garbarino S, Nobili L, Rosadini G, et al. Incidenza del disturbi del sonno e della veglia in due popolazioni di operatori della Polizia di Stato agenti in differenti condizioni operative. [Incidence of Sleep Disorders and Excessive Daytime Sleepiness among the Two Populations of Police Officers in Two Different Work Conditions.] In: Di Perri R, Raffaele M, Silvestri R, Smirne S (Eds). II Sonno in Italia 19,94; Milan, Italy: Poletto Edizioni, 1995; pp 240-45.

(22.) Casagrande M, Violani C, Testa P, et al. Validita e attendibilita di una versione Italiana della Epworth Sleepiness Scale. [Validity and Reliability of an Italian Version of the Epworth Sleepiness Scale.] In: Ferrillo F, Nobili L, Schiavi G, Smirne S (Eds). II Sonno in Italia 1996. Milan, Italy: Poletto Edizioni, 1997; pp 145-47.

(23.) Murray WJ. Sleepiness in different situations measured by the Epworth Sleepiness Scale. Sleep 1994; 17(8):703-10.

(24.) Diagnostic Classification Steering Committee. Thorpy MJ, Chairman. International Classification of Sleep Disorders: Diagnostic and Coding Manual. Rochester, MN: American Sleep Disorder Association, 1990.

(25.) American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 3rd ed., revised (DSM-III-R). Washington, DC: American Psychiatric Association, 1987.

(26.) American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 4th ed (DSM-IV). Washington, DC: American Psychiatric Association, 1994.

(27.) Granella F, Spaggiari C, Parrino L, et al. Insomnia e uso di ipnoinducenti. Studio epidemiologico sulla popolazione Italiana. [Insomnia and Hypnotic Drugs Intake. An Epidemiological Study of the Italian Population.] In: Di Perri R, Raffaele M, Silvestri R, Smirne S (Eds). II Sonno in Italia 1994. Milan, Italy: Poletto Edizioni, 1995; pp 2-11.

(28.) Lugaresi E, Cirignotta F, Mondini S, et al. Good and poor sleepers. An epidemiological survey of the San Marino population. In: Guilleminault C, Lugaresi E (Eds). Sleep-Wake Disorders: Natural History, Epidemiology, and Long-Term Evolution. New York: Raven Press, 1983; pp 29-36.

(29.) Ohayon M. Epidemiological study on insomnia in the general population. Sleep 1996; 19(3):S7-15.

(30.) Ohayon MM, Guilleminault C, Priest RG, et al. Snoring and breathing pauses: telephone interview survey of a UK population study. Br Med J 1997; 22:860-63.

(31.) Linet MS, Stewart WE Migraine headaches. Epidemiologic perspectives. Epidemiol Rev 1984; 6:107-39.

(32.) Andersen FE (Ed). Three-Shiftwork. Copenhagen, Denmark: Socialforskningsinstitutet, 1970.

(33.) Mellinger GD, Baiter MB, Uhlenhuth EH. Insomnia and its treatment. Prevalence and correlates. Arch Gen Psych 1985; 42:225-32.

(34.) Ohayon M, Caulet M, Guilleminault C. How the general population perceives its sleep and how it relates to the complaint of insomnia. Sleep 1997; 20(9):715-23.

(35.) Akerstedt T, Gillberg M. Subjective and objective sleepiness in the active individual. Int J Neurosci 1990; 52:29-37.

(36.) Murray J, Hocking B. Daytime sleepiness and sleep habits of Australian workers. Sleep 1997; 20(10):844-49.

(37.) Foret J, Latin G. The sleep of train drivers: an example of the effects of irregular work schedules on sleep. In: Colquhoun WP (Ed). Aspects of Human Efficiency. Diurnal Rhythm and Loss of Sleep. London, UK: The English University Press, Ltd., 1972; pp 273-81.

(38.) Tilley A, Wilkinson RT, Drud M. Night and day shifts compared in terms of the quality and quantity of sleep disorders in the home and performance measured at work: a pilot study. In: Reinberg N, Wieux P, Andlauer P (Eds). Night and Shift Work. Biological and Social Aspects. Oxford, UK: Pergamon Press, 1981; pp 187-96.

(39.) Dinges DT. The nature of sleepiness. Causes, contexts and consequences. In: Stunkard A, Baum A (Eds). Perspectives in Behavioral Medicine: Eating, Sleeping and Sex. Hillsdale, NJ: Lawrence Erlbaum, 1989; pp 147-79.

(40.) Harma M. Individual differences in tolerance to shiftwork. A review. Ergonomics 1993; 36:97-110.

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