Could Externally Desynchronized Circadian Rhythm Be Resynchronized in Shift Workers?

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Could Externally DesynchronizedCircadian Rhythm BeResynchronized in Shift Workers?Arti Chandrawanshi & Atanu Kumar PatiPublished online: 09 Aug 2010.

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Biological Rhythm Research, 2000, Vol. 31, No. 2, pp. 160–176 0929-1016/00/3102-0160$15.00© Swets & Zeitlinger

Could Externally Desynchronized Circadian RhythmBe Resynchronized in Shift Workers?

Arti Chandrawanshi and Atanu Kumar Pati

School of Life Sciences, Pt. Ravishankar Shukla University, Raipur, India

ABSTRACT

The present study aimed at investigating the effect of shift work on circadian time structure of severalvariables, such as skin temperature (ST), heart rate (HR), peak expiratory flow rate (PEFR), subjectivedrowsiness (SDr), subjective fatigue (SF) and subjective attention (SA) in shift workers of a sub-urbancement factory. Six shift workers volunteered for this study. In each subject, above mentioned varia-bles were monitored at least 4–6 times per day for over a period of one week. The study was conductedin two different spells. In the first spell (1994), circadian time structure of six shift workers wasstudied about 14 months after slowing down of overall functioning of the cement factory. In thesecond spell (1996), the circadian time structure of the same subjects was reexamined following about30 months of slough in the cement factory. The results indicate that the rhythm desynchronization ofST, HR and PEFR was witnessed among shift workers in 1994. However, when all six shift workerswere monitored again in 1996, the desynchronized rhythm became synchronized in most of the shiftworkers. Further, in the present study it was noticed that subjective variables, such as SF and SA areless prone to desynchronization as compared to other objective variables. The relative stability ofrhythms in fatigue and attention could also be ascribed to the period of sleep-wake rhythm thatremained either 24 h or very close to 24 h irrespective of the year of study. In conclusion, the findingsof this study document rigorously that externally desynchronized circadian rhythms in shift workerscould become normal following their transfer from shift work to diurnal work.

KEYWORDS: Shift work, rhythm desynchronization, resynchronization, cement factory.

INTRODUCTION

Humans possess a timing device to keep their internal rhythms synchronized withthe light dark cycle and other oscillatory components of the environment. Humanbeings have been evolved to work or remain active during the day and sleep atnight. However, schedules of rotating shift work force the workers to phase theirwork timing (activity) away from the customary clock hours (Folkard et al., 1983;

Address correspondence to: Dr. Atanu Kumar Pati, Reader, School of Life Sciences, Pt. Ravishankar

Shukla University, Raipur-492 010, India. Fax: 91-771-234283. E-mail: [email protected]

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RHYTHM RESYNCHRONIZATION IN SHIFT WORKERS 161

Folkard, 1989, 1990). Eventually these conditions do upset the human circadianclock (Reinberg et al., 1984).

Today shift work has become a routine feature in industries, hospitals andmany other essential sectors (Gupta et al., 1997). Several studies have document-ed the phenomenon of desynchronization among various rhythms, namely axil-lary temperature, heart rate, subjective fatigue, attention and drowsiness, peakexpiratory flow and grip strength of both hands in shift workers from oil refinery,steel manufacturing, chemical engineering and photographic film manufacturingindustries (Motohashi et al., 1987; Reinberg et al., 1984, 1988, 1989). Further, thephenomenon of desynchronization of oral temperature, pulse and performancecircadian rhythm has also been reported among shift working Indian nurses (Patiand Saini, 1991; Gupta and Pati, 1994; Gupta et al., 1997). The desynchronizationis often marked by alteration in the rhythm parameters, such as in mesor, ampli-tude and acrophase of the variable under investigation. The desynchronized circa-dian rhythms may lead to certain clinical complications. Shift workers have beenshown to develop impaired metabolism and impaired tolerance or response tomedications (Phillips et al., 1991). It has also been reported that shift workers dohave increased risk for myocardial infarction and number of systemic illness,notably exacerbation of insulin-dependent diabetes, epilepsy, and neuropsychiat-ric disorders (Brief and Scala, 1986; Phillips et al., 1991). All these complicationsassociated with shift work are attributed to effects of shift work on sleep depriva-tion and disturbances of circadian rhythms.

The aim of present study was to investigate the effect of shift work on circadi-an time structure of several variables, such as skin temperature (ST), heart rate(HR), peak expiratory flow rate (PEFR), subjective drowsiness (SDr), subjectivefatigue (SF) and subjective attention (SA) in shift workers of a sub-urban cementfactory of Chhattisgarh region, India.

METHOD

Pattern of shift workFigure 1 demonstrates the pattern of rotation of work schedule of shift workers atthe cement factory. The shift workers were working in three shifts of 8 hourseach. The rotation of shifts was from night shift (00:00–08:00) to evening shift(16:00–00:00) and to morning shift (08:00–16:00), for 8 hours a day, 6 days aweek and one week for a shift.

Figure 2 depicts activity of the cement factory. It shows idealized temporal(monthly) pattern of cement production in the factory. In addition, Figures 3 and 4illustrate patterns of monthwise running hours in two important departments, suchas cement mills and packing plants of the factory. Both running hours and the rate

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A. CHANDRAWANSHI AND A.K. PATI162

of cement production exhibited apparently positive relationship. During the peri-od from November 1994 to April 1995 there was practically no cement produc-tion (Fig. 2) that accompanied near closer of cement mills (Fig. 3) and low profileactivity in packing plants (Fig. 4).

Fig. 1. Rotation schedule of shift workers in cement factory. NS = Night shift; ES = Evening shift;MS = Morning shift.

Fig. 2. Idealised temporal (monthly) profile of cement production in a suburban cement factory ofChhattisgarh region, during the period from April, 1993 to March, 1996. Left �– Spell 1;Right �– Spell 2. � – Indicates closure of the factory that accompanied almost zero produc-tion of both cement and clinker.

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A closer look at the activities of the factory revealed that after June 1993 therate of cement production of the factory started to sink and attained its nadir inNovember 1994. This happened as a result of acute power shortage and otherunknown technical problems. Thereafter, from November 1994 till April 1995 thecement factory experienced a near complete shut down that accompanied zeroproduction of cement and clinker. The factory resumed cement production par-tially in May 1995 and again the rate of cement production attained anothertrough in October 1995. The cement factory experienced yet another closure inFebruary 1996 before attaining partial recovery in January 1996. However, duringthis period, shift workers were assigned shift duties, irrespective of the work loadand activity of the factory. Despite on rotational shift duties, whenever there wasno work load, they slept at their work places. In other words they behaved as dayworkers with nocturnal sleep.

In the first spell, circadian time structure of 6 shift workers was studied in July-August, 1994, about 14 months after slowing down of overall functioning of thecement factory. In the second spell, the circadian time structure in the samesubjects was reexamined in January 1996, following about 30 months of sloughin the cement industry. There was a lag of about 16 months between spell 1 andspell 2. During this period the factory remained almost closed for nearly 8 monthswith moderate to low profile activities in the remaining months.

Subjects followed their normal routine, during the study period. No restrictionwas imposed upon the subjects concerning their food habits. Meal timing was

Fig. 3. Idealised temporal (monthly) profile of running hours in cement mills in a suburban cementfactory of Chhattisgarh region, during the period from April, 1993 to March, 1996. Left � –Spell 1; Right � – Spell 2. � – Indicates closure of the factory that accompanied almostzero production of both cement and clinker.

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considered; because it is well known that it affects the variable, such as heart rate.Data were collected at least after one hour of the meal. No restriction was alsoimposed upon their sleep schedules. They had ad libitum sleep during the studyspan. Various biometric characteristics of the subjects are shown in Table 1.

Data collection In each subject several variables, such as skin temperature (ST), heart rate (HR),peak expiratory flow rate (PEFR) and subjective drowsiness (SDr), fatigue (SF)and attention (SA) were monitored at least 4-6 times per day for over a period ofone week.

Skin temperatureSkin temperature (°C) was measured using a digital temperature monitor withskin probe for 3 minutes (Model No. DTM-100, VISCON Electromed Pvt. Ltd.,Thane, India).

Heart rateHeart rates (beats/minute) were monitored with the help of fingercheck electronicdigital blood pressure and pulse monitor (Model FC-1X, Anand Health Equip-ment Pvt. Ltd. Bombay).

Peak expiratory flow ratePeak expiratory flow rate (liter/minute) was monitored with the help of a miniWright peak flow meter (Airmed, Clement Clarke International Ltd., London).

Fig. 4. Idealised temporal (monthly) profile of running hours of packing plants in a suburban cementfactory of Chhattisgarh region, during the period from April, 1993 to March, 1996. Left � –Spell 1; Right � – Spell 2. � – Indicates closure of the factory that accompanied almost zeroproduction of both cement and clinker.

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Subjective Drowsiness/Fatigue/AttentionSubjective drowsiness, fatigue and attention were measured with the help ofhorizontal rectangle (25 × 10 mm). Subjects were instructed to draw a vertical lineon this rectangle. A line drawn towards right and coinciding with the small arm ofthe rectangle in the extreme right indicates the maximum level of sleepiness,while a line drawn towards left coinciding with the small arm of the rectangle inthe extreme left indicates the lower level of sleepiness (alertness). Thus, theposition of the line drawn by the subject on the rectangle reflects the level ofsleepiness at given study point on a scale from 0 to 100% represented by 0 to 25mm, the distance of the drawn line from the small arm of the rectangle in theextreme left, respectively. A similar scale was used for measuring fatigue. In caseof attention, marking towards the right indicates more attentiveness, while mark-ing towards the left indicates less attentiveness. Scores were obtained by measur-ing the distance in mm from the left to another point in its right where the verticalline was drawn (Reinberg et al., 1989). The actual time of sleep onset and theawakening time was measured day-to-day for each subject.

Statistical analysesData were analyzed for documenting a circadian rhythm (τ = 24 h) with the helpof cosinor rhythmometry (Nelson et al., 1979; Gupta and Pati, 1992). A rhythmwas characterized by estimating three parameters, such as mesor (M, rhythm-adjusted mean), amplitude (A, half of the difference between the highest and thelowest value), and acrophase (Ø, the timing of the highest value). A power spec-trum method was also employed for detecting prominent periods in all the varia-bles under study (De Prins et al., 1986). This method is suitable for time serieswith missing data as well as for data collected with unequal time intervals (Moto-hashi et al., 1987). Other conventional statistical analyses were also performedwhenever required.

TABLE 1. Biometric characteristics of shift workers of the cement factory.

Subject Age Duration of Tolerance to Height Weight BSAcode shift work shift work in cm in Kg in m2

in years

SW # 01 48 34 Poor 175 80 1.956SW # 02 42 26 Good 187.5 70 1.943SW # 03 41 21 Good 163 62 1.667SW # 04 55 32 Poor 165 63 1.693SW # 05 44 26 Poor 150 58 1.526SW # 06 49 26 Poor 165 65 1.716

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RESULTS

PeriodTable 2 summarizes the results from power spectrum analysis of the time series inboth groups of shift workers of 1994 and 1996, respectively. In 1994, ST andPEFR in subject SW # 01 and 06, HR in SW # 03 and 06, SDr in SW # 05 and 06,SF in SW # 05 and SA in SW # 01, 04, 05 and 06 had a circadian period equal to24 h, while the remaining subjects exhibited either ultradian or non-circadiancomponents. In 1996, when the same subjects were monitored again, most ofthem exhibited prominent 24 h period. The phenomenon of frequency multiplica-tion was also prominent in 1996. Most of the variables showed except SA rhythmswith periods equal to 12 h or 8 h in at least one or two subjects (Table 2).

Figures 5 & 6 show results of spectral analysis on the data obtained from thesubject SW # 04 in 1994 and 1996, respectively. Results indicate that in 1994, 5out of 6 investigated variables had non-24 h τs (Fig. 5). However, circadianperiodicity (τ = 24 h) was observed in all 6 variables (Fig. 6) in 1996.

Table 3 shows the mean period of the sleep-wake rhythm. Interestingly, allsubjects exhibited sleep-wake rhythm with a τ very close to 24 h, irrespective ofthe year of study.

Rhythm detectionAt the group level statistically significant circadian rhythm was noticed in 5 out of6 variables in 1994. The null amplitude hypothesis was, however, accepted for

TABLE 2. Prominent circadian periodsa for six variables in shift workers of cement factory.

Subject Variablescode ST HR PEFR SDr SF SA

1994SW # 01 24 28 24 – 28 24SW # 02 12.92 8 8 8 8 8SW # 03 28 24 14 – 8 8SW # 04 36 12 9 – 20.6 24SW # 05 21 11.2 14 24 24 24SW # 06 24 24 24 24 28 24

1996SW # 01 24 24 24 24 24 24SW # 02 24 12 8 24 8 24SW # 03 12 12 9.6 24 9.6 24SW # 04 24 24 24 24 24 24SW # 05 24 28 12 24 24 24SW # 06 24 8 24 8 9.33 24

a by power spectrum analysis (De Prins et al., 1986).

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HR only. When these individuals were monitored again in 1996 all variablesexhibited statistically validated circadian rhythms. Data from 6 shift workerswere pooled separately in both occasions (Tables 4–9).

MesorAn inter individual variation in mesors was marked for all variables (Tables 4–9).The average circadian mesors of ST for the groups were 35.22°C and 34.64°C in1994 and 1996, respectively (Table 4). Similarly, mesors at the group level were72.91 beats/min and 69.06 beats/min for HR, 496.05 l/min and 603.51 l/min forPEFR, 2.56 and 2.40 for SDr, 2.94 and 2.68 for SF, and 19.55 and 18.45 for SA in1994 and 1996, respectively (Tables 5–9). The difference between mesors of both

ST

HR

SF

SA

PE

FR

24 9

16

10.2

8

24

13.0

9

20.5

7

24

12 9

2420.57

36*

Fig. 5. Illustrative examples showing power spectra of five variables ( ST = skin temperature; HR =heart rate; PEFR = peak expiratory flow rate; SF = subjective fatigue; SA = subjectiveattention) in a shift worker (SW # 04) in 1994. *Period (τ).

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HR

ST

PE

FR

SD

r

SF

SA

12

8.3

9

12

8.3

9

24 24

21

12 8

16.7

9 8

9.8

8

14 8

24 24 24

24*

Fig. 6. Illustrative examples showing power spectra of six variables ( ST = skin temperature; HR =heart rate; PEFR = peak expiratory flow rate; SDr = subjective drowsiness; SF = subjectivefatigue; SA = subjective attention) in a shift worker (SW # 04) in 1996. *Period (τ).

groups of shift workers was statistically significant for ST (p < 0.001), HR (p <0.05) and PEFR (p < 0.001), while in case of subjective variables no statisticallysignificant difference was observed between both groups.

AmplitudeAn interindividual variation in circadian amplitudes was also observed for allvariables (Tables 4-9). The circadian amplitudes at group levels were 0.31 and0.59 for ST, 0.46 and 5.46 for HR, 14.88 and 19.06 for PEFR, 3.97 and 2.26 forSDr, 2.01 and 1.29 for SF, and 7.53 and 6.95 for SA in 1994 and 1996, respective-ly (Tables 4-9). The amplitude obtained for the group in 1994 did not differstatistically significantly from that obtained for the same group in 1996.

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TABLE 3. The mean period (τ) of the sleep-wake rhythm of shift workers. The timing of mid-sleepwas taken as the phase reference to compute period.

Lengthof time

Subject Year Sleep Days seriescode 1 2 3 4 5 6 Mean ± SE (in days)

SW # 01 1994a – – – – – – –1996 τmid 23 23.9 24 23.6 24.1 24.2 23.82 ± 0.18 7

SW # 02 1994a – – – – – – –1996 τmid 23.4 24.1 24.2 23.8 23.9 24.1 23.92 ± 0.11 7

SW # 03 1994a – – – – – – –1996 τmid 23.9 23.5 24.3 23.9 23.8 –a 23.92 ± 0.12 6

SW # 04 1994 τmid 23.7 24.6 23.9 23.8 24.1 –a 24.05 ± 0.16 61996 τmid 23.3 24.4 24.1 23.9 23.9 24.1 23.98 ± 0.14 7

SW # 05 1994 τmid 24.5 24.2 24.1 23.6 24.2 23.6 24.06 ± 0.14 71996 τmid 24.2 23.6 24.4 24 24 23.8 24.02 ± 0.11 7

SW # 06 1994 τmid 26.11 22.14 24 24.25 23.75 23.75 24.00 ± 0.52 71996 τmid 24.2 23.7 24.1 24.1 25.3 22.7 24.04 ± 0.35 7

TABLE 4. Cosinor summary: Circadian rhythm in skin temperature (°C) of shift workers of cementfactory.

Subject Data Rhythm Rhythm-adjusted Amplitude, A Acrophase, ∅ in h code point detection mean, M ± 1 SE (95% CL) (95% CL)

1994SW # 01 41 <0.001 34.97 ± 0.09 0.40 (0.10, 0.69) 19.7 (15.6, 23.7)SW # 02 42 0.074 35.31 ± 0.08 0.35 15.8SW # 03 37 0.002 35.68 ± 0.09 0.36 (0.03, 0.69) 18.7 (13.8, 23.7)SW # 04 32 0.192 35.35 ± 0.12 0.21 10.4SW # 05 41 0.004 35.51 ± 0.10 0.58 (0.14, 1.02) 16.2 (13.3, 19.2)SW # 06 39 <0.001 34.54 ± 0.11 0.53 (0.24, 0.82) 21.4 (18.4, 00.4)ALL 232 <0.001 35.22 ± 0.06 0.31 (0.10, 0.52) 18.2 (14.7, 21.6)

1996SW # 01 40 0.003 34.39 ± 0.13 0.63 (0.18, 1.08) 17.4 (14.0, 20.8)SW # 02 42 <0.001 34.60 ± 0.08 0.45 (0.14, 0.76) 17.3 (13.8, 20.8)SW # 03 30 0.003 34.23 ± 0.15 0.76 (0.19, 1.33) 15.9 (12.5, 19.4)SW # 04 41 <0.001 35.12 ± 0.09 0.49 (0.18, 0.80) 16.9 (13.9, 19.9)SW # 05 37 <0.001 34.78 ± 0.08 0.67 (0.35, 0.99) 15.7 (13.7, 17.6)SW # 06 40 <0.001 34.59 ± 0.05 0.84 (0.65, 1.03) 14.9 (14.1, 15.7)ALL 230 <0.001 34.64 ± 0.04 0.59 (0.43, 0.75) 16.3 (15.2, 17.3)

anot recorded.

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TABLE 5. Cosinor summary: Circadian rhythm in heart rate (beats/min) of shift workers of cementfactory.

Subject Data Rhythm Rhythm-adjusted Amplitude, A Acrophase, ∅ in hcode point detection mean, M ± 1 SE (95% CL) (95% CL)

1994SW # 01 41 0.121 61.51 ± 1.53 3.25 19.3SW # 02 42 0.005 81.32 ± 1.35 4.92 (–0.08, 9.92) 11.5 (08.7, 14.4)SW # 03 37 0.007 77.25 ± 1.41 5.08 (–0.30, 10.46) 06.6 (01.1, 12.2)SW # 04 32 0.975 61.65 ± 1.38 0.51 16.2SW # 05 41 0.261 91.27 ± 1.11 3.14 02.5SW # 06 38 0.009 62.03 ± 0.87 3.24 (–0.16, 6.64) 23.8 (21.2, 02.5)ALL 231 0.972 72.91 ± 1.14 0.46 03.8

1996SW # 01 40 0.009 57.16 ± 0.86 3.77 (0.69, 6.85) 17.2 (13.2, 21.3)SW # 02 42 0.002 83.14 ± 1.29 7.82 (2.46, 13.17) 14.9 (12.5, 17.4)SW # 03 30 0.001 72.05 ± 1.85 12.00 (4.36, 19.64) 14.2 (12.2, 16.1)SW # 04 41 <0.001 57.21 ± 0.65 4.70 (2.20, 7.20) 15.1 (13.1, 17.1)SW # 05 37 0.203 87.56 ± 1.10 2.37 16.9SW # 06 40 0.07 59.20 ± 0.61 2.14 00.2ALL 230 0.003 69.06 ± 1.09 5.46 (1.39, 9.53) 15.6 (12.3, 18.9)

TABLE 6. Cosinor summary: Circadian rhythm in peak expiratory flow rate (l/min) of shift workersof cement factory.


1994SW # 01 41 0.01 542.35 ± 6.91 24.39 (–2.48, 51.27) 11.8 (08.9, 14.6)SW # 02 42 0.265 527.44 ± 4.56 7.09 10.1SW # 03 37 0.379 525.35 ± 4.35 10.39 15.8SW # 04 32 0.057 443.70 ± 6.28 16.15 07.2SW # 05 41 0.416 417.47 ± 10.98 14.53 07.6SW # 06 39 0.015 511.79 ± 7.43 27.37 (–2.65, 57.39) 12.3 (09.7, 14.9)ALL 232 0.02 496.05 ± 5.29 14.88 ( 8.36, 21.40) 11.1 (07.4, 14.7)

1996SW # 01 40 0.005 635.04 ± 4.76 23.33 ( 5.59, 41.07) 12.0 (09.4, 14.7)SW # 02 42 0.105 629.62 ± 4.50 14.42 12.4SW # 03 30 0.03 625.44 ± 3.83 16.05 ( 0.47, 31.63) 12.8 (09.6, 16.0)SW # 04 41 0.017 564.94 ± 4.55 20.12 ( 2.56, 37.68) 12.2 (09.2, 15.2)SW # 05 37 0.015 565.60 ± 6.67 34.16 ( 5.68, 62.64) 13.1 (10.4, 15.9)SW # 06 40 0.003 600.16 ± 5.56 24.41 ( 7.38, 41.44) 08.1 (04.8, 11.3)ALL 230 <0.001 603.51 ± 2.99 19.06 ( 7.85, 30.27) 12.0 (10.3, 13.8)

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TABLE 7. Cosinor summary: Circadian rhythm in subjective drowsiness of shift workers of cementfactory.


1994SW # 01 41 0.003 3.15 ± 0.90 4.92 ( 1.04, 8.80) 01.5 (23.8, 03.2)SW # 02 42 0.002 2.33 ± 0.63 3.50 ( 0.77, 6.23) 01.6 (24.0, 03.3)SW # 03 37 <0.001 3.90 ± 0.87 6.51 ( 2.70, 10.32) 01.7 (00.5, 02.9)SW # 04 32 0.070 1.32 ± 0.62 2.18 01.4SW # 05 41 0.027 2.68 ± 0.97 4.07 (–0.15, 8.29) 01.6 (23.2, 04.1)SW # 06 39 0.004 1.89 ± 0.52 2.61 ( 0.41, 4.81) 01.1 (23.3, 03.0)ALL 232 <0.001 2.56 ± 0.32 3.97 ( 2.66, 5.28) 01.5 (00.8, 02.3)

1996SW # 01 40 0.001 2.82 ± 0.70 3.86 (1.32, 6.40) 23.4 (21.1, 01.7)SW # 02 42 0.196 0.51 ± 0.23 0.47 21.1SW # 03 30 0.011 1.62 ± 0.50 2.20 (0.26, 4.14) 23.8 (21.3, 02.2)SW # 04 41 0.022 2.15 ± 0.67 2.48 (0.13, 4.83) 22.2 (18.6, 01.8)SW # 05 37 0.005 3.58 ± 1.03 4.25 (0.54, 7.96) 21.7 (18.9, 00.5)SW # 06 40 0.315 3.70 ± 0.67 1.23 20.2ALL 230 <0.001 2.40 ± 0.29 2.26 (1.28, 3.24) 22.5 (21.0, 24.0)

TABLE 8. Cosinor summary: Circadian rhythm in subjective fatigue of shift workers of cementfactory.


1994SW # 01 41 0.279 2.65 ± 0.96 1.87 00.3SW # 02 42 0.001 2.59 ± 0.59 3.14 ( 0.64, 5.64) 01.2 (23.5, 03.0)SW # 03 37 <0.001 1.39 ± 0.29 1.94 ( 0.71, 3.17) 01.0 (23.6, 02.4)SW # 04 32 0.022 4.34 ± 0.77 2.16 ( 0.29, 4.03) 21.9 (17.5, 02.3)SW # 05 41 0.036 3.26 ± 0.91 4.00 (-0.04, 8.04) 02.2 (23.4, 05.0)SW # 06 39 0.131 3.50 ± 0.65 1.62 17.7ALL 232 <0.001 2.94 ± 0.32 2.01 ( 0.77, 3.25) 00.5 (22.9, 02.1)

1996SW # 01 40 0.005 2.25 ± 0.34 1.47 (0.37, 2.57) 21.2 (17.9, 00.5)SW # 02 42 0.284 1.42 ± 0.47 0.83 19.1SW # 03 30 0.08 0.82 ± 0.32 0.89 16.9SW # 04 41 0.028 1.96 ± 0.60 2.03 (0.09, 3.97) 21.0 (16.8, 01.2)SW # 05 37 0.123 1.88 ± 0.96 2.13 19.9SW # 06 40 0.02 7.18 ± 0.43 1.53 (0.16, 2.90) 18.4 (13.7, 23.2)ALL 230 <0.001 2.68 ± 0.28 1.29 (0.51, 2.07) 20.2 (17.2, 23.2)

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AcrophaseThe acrophase spread for ST was very wide, i.e., of about 11.0 h in 1994. Howev-er, in 1996, the acrophase timings appeared to be much more stable with a spreadof only 2.5 h (Table 4). Similarly, the acrophase timings appeared with a spreadequal to 21.3 h and 17.0 h for HR, 8.6 h and 5.0 h for PEFR, 0.6 h and 3.6 h forSDr, 21.6 h and 4.3 h for SF, and 2.0 h and 2.2 h for SA in 1994 and 1996,respectively (Tables 5–9).

DISCUSSION

Results of the present study document the phenomenon of external desynchroni-zation of several rhythms, such as skin temperature (ST), heart rate (HR) and peakexpiratory flow rate (PEFR) from 24 h. Thus the results of this study could befavourably compared with other reports those document circadian rhythmdesynchronization among shift workers (Reinberg et al., 1984, 1988, 1989; Patiand Saini, 1991; Gupta and Pati, 1993, 1994; Gupta et al., 1997). Furthermore, thepresent paper documents rhythms in human subjects longitudinally thus provid-ing individual and distinct time series spanning over 7 consecutive days for eachsubject. Such a procedure of data gathering adopted in the present study allows toquantify the period of the rhythm in each shift worker. Thus the present contribu-

TABLE 9. Cosinor summary: Circadian rhythm in subjective attention of shift workers of cementfactory.


1994SW # 01 41 <0.001 19.26 ± 1.05 8.62 (4.17, 13.07) 13.3 (12.1, 14.4)SW # 02 42 <0.001 18.67 ± 1.09 10.14 (5.50, 14.78) 13.5 (12.5, 14.6)SW # 03 37 <0.001 17.13 ± 1.20 12.63 (7.51, 17.75) 13.5 (12.6, 14.4)SW # 04 32 <0.001 17.98 ± 1.32 6.71 (1.82, 11.60) 11.5 (09.2, 13.7)SW # 05 41 0.053 21.45 ± 1.30 4.70 13.4SW # 06 39 <0.001 22.52 ± 0.47 2.83 (0.95, 4.71) 12.5 (10.9, 14.1)ALL 232 <0.001 19.55 ± 0.48 7.53 (5.63, 9.43) 13.1 (12.5, 13.8)

1996SW # 01 40 <0.001 20.09 ± 0.85 6.11 (3.14, 9.08) 10.8 (09.0, 12.6)SW # 02 42 <0.001 19.45 ± 1.01 7.53 (3.68, 11.38) 11.6 (10.2, 13.1)SW # 03 30 <0.001 21.36 ± 0.79 4.44 (1.48, 7.40) 11.2 (09.3, 13.2)SW # 04 41 <0.001 16.87 ± 0.93 8.58 (5.28, 11.88) 10.7 (09.3, 12.0)SW # 05 37 <0.001 18.60 ± 1.30 7.51 (2.76, 12.26) 10.1 (08.1, 12.1)SW # 06 40 <0.001 14.69 ± 0.56 8.23 (6.40, 10.06) 09.4 (08.4, 10.3)ALL 230 <0.001 18.45 ± 0.41 6.95 (5.56, 8.34) 10.6 (09.9, 11.4)

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tion becomes one among very few studies (Motohashi, 1990; Motohashi et al.,1995) that documents τs of rhythms in shift workers. Motohashi et al. (1995) havecategorically mentioned in their paper that short data-gathering spans of only 2 to4 days are insufficient to quantify τ precisely.

A highly statistically significant linear relationship (P < 0.001) was validatedbetween skin temperature and oral temperature. Thus, results clearly reveal that tounderstand human circadian time structure, skin temperature could be taken as aconvenient end point. However, when work is of clinical significance, oral tem-perature variable should be more relevant. In the present investigation, skin tem-perature rhythm desynchronized in many shift workers. The results of the presentstudy corroborate those reported earlier by numerous authors where externaldesynchronization has been documented in the circadian rhythm of oral tempera-ture self-measured by the group of tolerant as well as non-tolerant shift workers(Reinberg et al., 1979, 1984, 1988). Gupta and Pati (1994) have also documentedexternal as well as internal desynchronization of circadian rhythms among shiftworking nurses. Similar pattern of desynchronization was observed in HR andPEFR rhythm also. In 1994, in some shift workers HR and PEFR rhythms didnot exhibit 24 h periods. This corroborates those published earlier where desyn-chronization in the circadian rhythm of HR and PEFR has been documented inshift workers (Reinberg et al., 1988, 1989; Pati and Saini, 1991; Gupta and Pati,1994).

However, when we reexamined all six shift workers of 1994 in 1996, most ofthem exhibited statistically significant circadian rhythm in all variables. The lagamong 1994 and 1996 studies witnessed shut downs of the cement factory fre-quently. Further, in 1994 shift workers experienced only 14 months of industrialslough whereas in 1996 the same shift workers experienced about 30 months ofindustrial slumber that accompanied 8 months of near complete closure. Duringthis period of internal turbulence marked by instability shift workers had lesswork load and had opportunity to have ‘on job’ nocturnal sleep. In other words,they behaved reasonably like day workers during the period between two spells ofstudies. Perhaps this situation was an unexpected benefit and permitted us to testthe null hypothesis that transferring shift workers from shift duties to diurnal typeday duties would not resynchronize the desynchronized circadian rhythms. Thus,in 1996, circadian rhythms were restored in all subjects who were sampled in1994. Our results reconfirm the findings reported by Reinberg et al. (1984) inthat when a shift worker with poor tolerance was transferred from shift work todiurnal work, the desynchronized rhythm in oral temperature became resynchro-nized after about 1 year exhibiting prominent period equal to 24 h in OT rhythm.However, there are not several studies for favour of further consolidation of thishypothesis. Furthermore, the observation that internal desynchronization canbe observed in apparently healthy subjects who have not undergone zeitgeber

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manipulation still makes the situation complicated in favour of validation of theabove hypothesis (Bicakova-Rocher et al., 1981; Reinberg et al., 1984; Moto-hashi, 1990).

In addition, in the present study an increase in circadian amplitudes of ST, HRand PEFR was noticed in most of the shift workers in 1996 as compared with therespective values obtained in 1994. Similarly, the amplitude increased approxi-mately twice in ST and five folds in HR in 1996, at the group levels. This clearlyshows that in 1996 shift workers of cement factory were able to tolerate shift workbetter than in 1994. It was not possible to compare circadian amplitude on indi-vidual basis because statistically validated rhythms in some of the cases wereabsent. Several studies have reported that the large amplitude of body temperatureis the marker of tolerance to shift work, since it helps the subjects to maintaintheir internal synchronization (Reinberg et al., 1978). It has been documented thatworkers with good tolerance had a greater amplitude of their temperature rhythmthan those with poor tolerance (Andlauer et al., 1979; Reinberg et al., 1979,1981). In the present study, of the 6 shift workers only two were having goodtolerance. However, our results did not confirm the relationship between ampli-tude and tolerance reported by the above authors. Could it be that the size of thesample is not enough to test such a possibility?

Furthermore, in the present study, it is noteworthy that in most of the casescircadian rhythms in subjective variables, such as drowsiness, fatigue and atten-tion did not undergo external desynchronization. It seems, therefore, that thesevariables are less prone to desynchronization as compared to other objectivevariables. These results corroborate earlier findings reported by Gupta and Pati(1994). The relative stability of rhythms in fatigue and attention could also beascribed to the period of sleep-wake rhythm that remained either 24 h or veryclose to 24 h irrespective of the year of study.

In the present study, a decrease in amplitudes was noticed in all three subjec-tive variables in 1996. That is to say very strict and rigid time windows for eithermore fatigue or less fatigue was abolished with fatigue randomly and reasonablyuniformly distributed over the circadian time scale. Similar pattern was witnessedin case of subjective attention. It seems that night shift tolerance of the shiftworkers improved as a result of temporary shut down of the cement factory. Thisresults favourably compared with those reported by Vidacek et al. (1993) in thatworkers with a smaller amplitude of fatigue rhythm could tolerate night shiftbetter.

In conclusion, findings of the present study suggest strongly that the problemof circadian rhythm desynchronization could be overcome by simply withdrawingthe affected shift workers from rotational shift duties and assigning them withnormal day duty instead.

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ACKNOWLEDGMENTS

We are grateful to University Grant Commission (UGC), New Delhi, for awarding Research Associ-ateship to AC. We are grateful to Prof. M.L. Naik, Chair, School of Life Sciences, Pt. RavishankarShukla University, Raipur, who provided facilities.

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Documents

Could Externally Desynchronized Circadian Rhythm Be Resynchronized in Shift Workers?