Internal desynchronization of circadian rhythms and tolerance of shift work

Fifteen male subjects including 12 shift workers (oil refinery operators) volunteered to document circadian rhythms in sleep-wake, grip strength of both hands, peak expiratory flow, heart rate, self-rated drowsiness, fatigue and attention. Each of these variables was measured 4 to 6 times/day for 2 to 3 weeks. In addition, both axillary temperature (with a shielded probe) and wrist activity were almost continuously recorded at 15 min intervals during the same time span. Individual time series were analyzed according to several statistical methods (power spectrum, cosinor, chi 2, etc.), in order to estimate the prominent circadian period tau and to evaluate both individual and subgroup differences with regard to tolerance of shift work, age, duration of shift work. The present study confirms for continuously recorded temperature and wrist activity, grip strength of both hands, heart rate and peak expiratory flow that intolerance of shift work is frequently associated with an internal desynchronization. However, this conclusion cannot be extended to circadian rhythms in self-rated drowsiness, fatigue and attention. The internal desynchronization among several circadian rhythms supports the hypothesis that these latter are driven by several oscillators, with presumable differences between right and left hemispheres as suggested by unequal values of tau in rhythms of both hand grip strength. Since an internal desynchronization can be observed in tolerant shift workers having no complaint, it is likely that symptoms of intolerance are related to the subject's sensitivity to internal desynchronization rather than to the desynchronization itself.     

Alteration of period and amplitude of circadian rhythms in shift workers. With special reference to temperature, right and left hand grip strength

48 male shift workers in various industries volunteered to document circadian rhythms in sleeping and working, oral temperature, grip strength of both hands, peak expiratory flow and heart rate. All physiological variables were self-measured 4 to 5 times a day for 2 to 4 weeks. Individual time series were analyzed according to several statistical methods (power spectrum, cosinor, chi squares, ANOVA, correlation, etc.) in order to estimate rhythm parameters such as circadian period (tau) and amplitude (A), and to evaluate subgroup differences with regard to tolerance to shift work, age, duration of shift work, speed of rotation and type of industry. The present study confirms for oral temperature and extends to other variables (grip strength of both hands, heart rate) that intolerance to shift work is frequently associated with both internal desynchronization and small circadian amplitude. The internal desynchronization among several circadian rhythms supports the hypothesis that these latter are driven by several oscillators. Many differences were observed between circadian rhythms in right and left hand grip strength: circadian tau in oral temperature was correlated with that in the grip strength of the dominant hand but not with that of the other hand; changes in tau s of the non-dominant hand were age-related but did not correlate with temperature tau; only the circadian A of the non-dominant hand was associated with a desynchronization. Thus, circadian rhythms in oral temperature and dominant hand grip strength may be driven by the same oscillator while that of the non-dominant hand may be governed by a different one. Internal desynchronization between both hand grip rhythms as well as desynchronization of performance rhythms reported by others provide indirect evidence that circadian oscillator(s) may be located in the human cerebral cortex.     

Could Externally Desynchronized Circadian Rhythm Be Resynchronized in Shift Workers?

The present study aimed at investigating the effect of shift work on circadian time structure of several variables, such as skin temperature (ST), heart rate (HR), peak expiratory flow rate (PEFR), subjective drowsiness (SDr), subjective fatigue (SF) and subjective attention (SA) in shift workers of a sub-urban cement factory. Six shift workers volunteered for this study. In each subject, above mentioned variables were monitored at least 4-6 times per day for over a period of one week. The study was conducted in two different spells. In the first spell (1994), circadian time structure of six shift workers was studied about 14 months after slowing down of overall functioning of the cement factory. In the second spell (1996), the circadian time structure of the same subjects was reexamined following about 30 months of slough in the cement factory. The results indicate that the rhythm desynchronization of ST, HR and PEFR was witnessed among shift workers in 1994. However, when all six shift workers were monitored again in 1996, the desynchronized rhythm became synchronized in most of the shift workers. Further, in the present study it was noticed that subjective variables, such as SF and SA are less prone to desynchronization as compared to other objective variables. The relative stability of rhythms in fatigue and attention could also be ascribed to the period of sleep-wake rhythm that remained either 24 h or very close to 24 h irrespective of the year of study. In conclusion, the findings of this study document rigorously that externally desynchronized circadian rhythms in shift workers could become normal following their transfer from shift work to diurnal work.     

Could Externally Desynchronized Circadian Rhythm Be Resynchronized in Shift Workers?

The present study aimed at investigating the effect of shift work on circadian time structure of several variables, such as skin temperature (ST), heart rate (HR), peak expiratory flow rate (PEFR), subjective drowsiness (SDr), subjective fatigue (SF) and subjective attention (SA) in shift workers of a sub-urban cement factory. Six shift workers volunteered for this study. In each subject, above mentioned variables were monitored at least 4-6 times per day for over a period of one week. The study was conducted in two different spells. In the first spell (1994), circadian time structure of six shift workers was studied about 14 months after slowing down of overall functioning of the cement factory. In the second spell (1996), the circadian time structure of the same subjects was reexamined following about 30 months of slough in the cement factory. The results indicate that the rhythm desynchronization of ST, HR and PEFR was witnessed among shift workers in 1994. However, when all six shift workers were monitored again in 1996, the desynchronized rhythm became synchronized in most of the shift workers. Further, in the present study it was noticed that subjective variables, such as SF and SA are less prone to desynchronization as compared to other objective variables. The relative stability of rhythms in fatigue and attention could also be ascribed to the period of sleep-wake rhythm that remained either 24 h or very close to 24 h irrespective of the year of study. In conclusion, the findings of this study document rigorously that externally desynchronized circadian rhythms in shift workers could become normal following their transfer from shift work to diurnal work.     

Pattern of shift rota modulates oral temperature circadian rhythm and sleep-wakefulness profiles in shift workers

Twenty four shift workers (8 from a steel industry and 16 from a Government hospital) participated in the study. The subjects were instructed to self-measure oral temperature, 4 6 times a day for about three weeks. Sleep quantity and quality for each subject were analysed with the help of an appropriate inventory. The data were analysed by cosinor and power spectrum methods. The frequency of circadian rhythm detection was in the order of 48% in senior nurses, 29% in steel plant workers and 14% in junior nurses. These were also complemented by the results of power spectrum analysis. Present results suggest that rhythms of subjective fatigue and subjective drowsiness are governed neither by oral temperature oscillator nor by the sleep/wake cycle oscillator. The results show that shift rotation pattern chiefly modulates the circadian time structure of shift workers. It is also suggested that the phenomenon of circadian rhythm desynchronization in oral temperature appears to be independent of per day total sleep length.     

Interindividual Differences in a Set of Biological Rhythms Documented During the High Arctic Summer (79°N) In Three Healthy Subjects

The High Arctic summer with its permanent sunlight provides a situation in which one of the natural synchronizers, the light-dark alternation, is minimal. During the summers of 1981 and 1982 three healthy right-handed geographers who were performing field studies in Svalbard as part of their own research volunteered to document, 4–6 times per 24 hr for respectively 63,141 and 147 days, a set of circadian rhythms: self-rated fatigue, oral temperature, grip strength of both hands, heart rate and times of awakening and retiring. Tests were performed before departure from France, in Svalbard (79°IN latitude) where their daily activities were often strenuous, and after returning to France. Time series were treated individually according to three methods: display of data as a function of time, cosinor analyses to quantify rhythm parameters, and spectral analyses to estimate component periods of rhythms. Circadian parameters such as period and acrophase of activity-rest, oral temperature and fatigue rhythms were not altered. On the other hand, the circadian rhythm in grip strength was altered: the period differed from 24 hr in one subject, while grip strength acrophase of the left, but not the right, hand of the other two subjects was phase shifted during the sojourn in Svalbard. A prominent circahemidian (about 12 hr) rhythm was observed in two subjects for their heart rate in Svalbard, while a prominent circadian rhythm (differing from exactly 24 hr) was observed in France associated with a small circahemidian component.     

Interindividual differences in a set of biological rhythms documented during the high arctic summer (79 degrees N) in three healthy subjects

The High Arctic summer with its permanent sunlight provides a situation in which one of the natural synchronizers, the light-dark alternation, is minimal. During the summers of 1981 and 1982 three healthy right-handed geographers who were performing field studies in Svalbard as part of their own research volunteered to document, 4-6 times per 24 hr for respectively 63,141 and 147 days, a set of circadian rhythms: self-rated fatigue, oral temperature, grip strength of both hands, heart rate and times of awakening and retiring. Tests were performed before departure from France, in Svalbard (79°IN latitude) where their daily activities were often strenuous, and after returning to France. Time series were treated individually according to three methods: display of data as a function of time, cosinor analyses to quantify rhythm parameters, and spectral analyses to estimate component periods of rhythms. Circadian parameters such as period and acrophase of activity-rest, oral temperature and fatigue rhythms were not altered. On the other hand, the circadian rhythm in grip strength was altered: the period differed from 24 hr in one subject, while grip strength acrophase of the left, but not the right, hand of the other two subjects was phase shifted during the sojourn in Svalbard. A prominent circahemidian (about 12 hr) rhythm was observed in two subjects for their heart rate in Svalbard, while a prominent circadian rhythm (differing from exactly 24 hr) was observed in France associated with a small circahemidian component.     

Circadian Rhythms in Competitive Sabre Fencers: Internal Desynchronization and Performance

During a 7-10 day span, circadian rhythms of sleep-wake, self-rated fatigue and mood, oral temperature, eye-hand skill and right and left hand grip strength were investigated in eight subjects: five males (21-28 years of age), members of the French sabre fencing team selected for the 1984 Olympic Games in Los Angeles, and three females (19-26 years of age) practicing fleuret (foil) fencing as a sports activity. On the average six measurements/day/variable/subject were performed. The single cosinor method showed that a circadian rhythm was detectable for only 26 out of the 56 time series (46.4%). Power spectrum analysis gave almost the same figure (19 out of 48: 39.5%) with regard to rhythms with τ=24hr indicating that with one exception (subject JFL) rhythms were internally desynchronized including differences τ between right and left hand grip strength rhythms for three subjects. Results suggest: (a) a physiologic synchronization of circadian rhythms may be a predictor of good performance; (b) however, internal desynchronization as shown previously may be a trivial phenomenon and thus does not imply in itself alterations of either health or performance; (c) chronobiologic methods should be recommended for a better understanding of changes in performance by those participating in competitive and other sports.     

Circadian Rhythms in Competitive Sabre Fencers: Internal Desynchronization and Performance

During a 7-10 day span, circadian rhythms of sleep-wake, self-rated fatigue and mood, oral temperature, eye-hand skill and right and left hand grip strength were investigated in eight subjects: five males (21-28 years of age), members of the French sabre fencing team selected for the 1984 Olympic Games in Los Angeles, and three females (19-26 years of age) practicing fleuret (foil) fencing as a sports activity. On the average six measurements/day/variable/subject were performed. The single cosinor method showed that a circadian rhythm was detectable for only 26 out of the 56 time series (46.4%). Power spectrum analysis gave almost the same figure (19 out of 48: 39.5%) with regard to rhythms with τ=24hr indicating that with one exception (subject JFL) rhythms were internally desynchronized including differences τ between right and left hand grip strength rhythms for three subjects. Results suggest: (a) a physiologic synchronization of circadian rhythms may be a predictor of good performance; (b) however, internal desynchronization as shown previously may be a trivial phenomenon and thus does not imply in itself alterations of either health or performance; (c) chronobiologic methods should be recommended for a better understanding of changes in performance by those participating in competitive and other sports.     

Temporal structuring of delivery in the absence of a photoperiod: preparturient myometrial activity of the rhesus monkey is related to maternal body temperature and depends on the maternal circadian system.

No convincing evidence exists that the shift from myometrial contractures to contractions, which determines the synchronized 24-h rhythm in the dynamics of the primate uterus, may be attributed to an endogenous circadian rhythm. We therefore wished to ascertain whether a 24-h periodic shift would also occur in the myometrial activity of animals kept under constant conditions. We studied five pregnant rhesus monkeys, kept in continuous darkness from 56-77 days gestational age until delivery at 117-167 days gestational age. During the last week before delivery we determined the individual phase, level, and amplitude of circadian changes in maternal body temperature and 24-h myometrial activity patterns in the form of contractions. In all five monkeys, a rhythm with a period of 24-h characterized the temporal incidence of preparturient contraction activity. A consistent phase lag of 6-7 h from the temperature crest was observed in four out of the five animals. The circadian phase of all individual rhythms was idiosyncratic among animals. We conclude that endogenous rhythms in body temperature and preparturient myometrial activity are truly circadian. In addition, these rhythms are either interdependent or subject to the same maternal timekeeping mechanism, supporting the hypothesis that the exact time of the day at which birth occurs in the rhesus monkey depends on the maternal circadian system.     

Circadian rhythm of the grip strength of the right and left hands: desynchronization in some shift workers

38 male shift workers and former shift workers volunteered to self-measure 4 to 8 times/24 hrs. their oral temperature (OT) as well as right and left hand grip strength (HGS) best performance during a 16 to 30 day span. Time series were analyzed individually according to two methods: day by day circadian acrophase drift and power spectrum. Mainly, but not exclusively, subjects with poor tolerance to shift work exhibited an internal desynchronization with a circadian period tau different from 24 hrs. which was the case for OT as well as right and left HGS; each could be different in tau between one another and from 24 hrs. These results suggest that oscillatory systems may be influenced by the neocortex apparently with difference between right and left side.     

The genetic background of individual variations of circadian-rhythm periods in healthy human adults.

As a group phenomenon, human variables exhibit a rhythm with a period (tau) equal to 24 h. However, healthy human adults may differ from one another with regard to the persistence of the 24-h periods of a set of variables' rhythms within a given individual. Such an internal desynchronization (or individual circadian dyschronism) was documented during isolation experiments without time cues, both in the present study involving 78 male shift workers and in 20 males and 19 females living in a natural setting. Circadian rhythms of sleep-wake cycles, oral temperature, grip strength of both hands, and heart rate were recorded, and power-spectra analyses of individual time series of about 15 days were used to quantify the rhythm period of each variable. The period of the sleep-wake cycle seldom differed from 24 h, while rhythm periods of the other variables exhibited a trimodal distribution (tau = 24 h, tau > 24 h, tau < 24 h). Among the temperature rhythm periods which were either < 24 h or > 24 h, none was detected between 23.2 and 24 h or between 24 and 24.8 h. Furthermore, the deviations from the 24-h period were predominantly grouped in multiples of +/- 0.8 h. Similar results were obtained when the rhythm periods of hand grip strength were analyzed (for each hand separately). In addition, the distribution of grip strength rhythm periods of the left hand exhibited a gender-related difference. These results suggested the presence of genetically controlled variability. Consequently, the distribution pattern of the periods was analyzed to elucidate its compatibility with a genetic control consisting of either a two-allele system, a multiple-allele system, or a polygenic system. The analysis resulted in structuring a model which integrates the function of a constitutive (essential) gene which produces the exact 24-h period (the Dian domain) with a set of (inducible) polygenes, the alleles of which, contribute identical time entities to the period. The time entities which affected the rhythm periods of the variables examined were in the magnitude of +/- 0.8 h. Such an assembly of genes may create periods ranging from 20 to 28 h (the Circadian domain). The model was termed by us "The Dian-Circadian Model." This model can also be used to explain the beat phenomena in biological rhythms, the presence of 7-d and 30-d periods, and interindividual differences in sensitivity of rhythm characteristics (phase shifts, synchronization, etc.) to external (and environmental) factors.     

Circadian rhythms and their association with body temperature and time awake when performing a simple task with the dominant and non-dominant hand

The two main aims of the study were to compare the dominant and non-dominant hand with regard to circadian rhythms of accuracy of performance at a task that required eye-hand coordination and sub-maximum muscle contraction, as well as to investigate if there were differences between the dominant and non-dominant hands in the associations between circadian rhythms of performance and core temperature and time awake. The task consisted of using a larger counter to flick a set of 20 smaller counters to land as near as possible to the center of a target. The nearer to the center of the target a counter landed, the higher the score awarded. Three measures of accuracy were calculated: the total score, the number of times the counter missed the target altogether, and the mean score for those counters that hit the target. Seventy-eight healthy participants performed the task at each of six test sessions distributed every 4 h throughout the day (at 08:00, 12:00... . 04:00 h), the participants then having been awake for about 1, 4... . 20 h, respectively. Before each test session, sub-lingual temperature (an estimate of core temperature) was measured, and estimates of the individual's alertness and fatigue were obtained. Temperature, alertness, and fatigue all showed circadian rhythms that were phased conventionally. Measures of accuracy of performance also showed significant circadian rhythms that were phased closer to the rhythms of alertness and fatigue than to that of oral temperature. In addition, and in support of our previous work, there were significant associations between performance and temperature (positive) and time awake (negative) for most measures of accuracy. Even though circadian rhythms of performance accuracy and effects of oral temperature and time awake were generally very similar between the dominant and non-dominant hand, there was a suggestion that time awake affected some aspects of performance of the non-dominant hand to a greater extent. There was little evidence to support the view that the 24-h rhythmicity was less marked in the non-dominant hand, which argues against internal desynchronization, at least for the task used in this study.     

Late-night presentation of an auditory stimulus phase delays human circadian rhythms

Although light is considered the primary entrainer of circadian rhythms in humans, nonphotic stimuli, including exercise and melatonin also phase shift the biological clock. Furthermore, in birds and nonhuman mammals, auditory stimuli are effective zeitgebers. This study investigated whether a nonphotic auditory stimulus phase shifts human circadian rhythms. Ten subjects (5 men and 5 women, ages 18-72, mean age +/- SD, 44.7 +/- 21.4 yr) completed two 4-day laboratory sessions in constant dim light (<20 lux). They received two consecutive presentations of either a 2-h auditory or control stimulus from 0100 to 0300 on the second and third nights (presentation order of the stimulus and control was counterbalanced). Core body temperature (CBT) was collected and stored in 2-min bins throughout the study and salivary melatonin was obtained every 30 min from 1900 to 2330 on the baseline and poststimulus/postcontrol nights. Circadian phase of dim light melatonin onset (DLMO) and of CBT minimum, before and after auditory or control presentation was assessed. The auditory stimulus produced significantly larger phase delays of the circadian melatonin (mean +/- SD, -0.89 +/- 0.40 h vs. -0.27 +/- 0.16 h) and CBT (-1.16 +/- 0.69 h vs. -0.44 +/- 0.27 h) rhythms than the control. Phase changes for the two circadian rhythms also positively correlated, indicating direct effects on the biological clock. In addition, the auditory stimulus significantly decreased fatigue compared with the control. This study is the first demonstration of an auditory stimulus phase-shifting circadian rhythms in humans, with shifts similar in size and direction to those of other nonphotic stimuli presented during the early subjective night. This novel stimulus may be a useful countermeasure to facilitate circadian adaptation after transmeridian travel or shift work.     

Twenty-four-hour rhythms of plasma glucose and insulin secretion rate in regular night workers

To determine whether the ultradian and circadian rhythms of glucose and insulin secretion rate (ISR) are adapted to their permanent nocturnal schedule, eight night workers were studied during their usual 24-h cycle with continuous enteral nutrition and a 10-min blood sampling procedure and were compared with 8 day-active subjects studied once with nocturnal sleep and once with an acute 8-h-shifted sleep. The mean 24-h glucose and ISR levels were similar in the three experiments. The duration and the number of the ultradian oscillations were influenced neither by the time of day nor by the sleep condition or its shift, but their mean amplitude increased during sleep whenever it occurred. In day-active subjects, glucose and ISR levels were high during nighttime sleep and then decreased to a minimum in the afternoon. After the acute sleep shift, the glucose and ISR rhythms were split in a biphasic pattern with a slight increase during the night of deprivation and another during daytime sleep. In night workers, the glucose and ISR peak levels exhibited an 8-h shift in accordance with the sleep shift, but the onset of the glucose rise underwent a shift of only 6 h and the sleep-related amplification of the glucose and ISR oscillations did not occur simultaneously. These results demonstrate that despite a predominant influence of sleep, the 24-h glucose and ISR rhythms are only partially adapted in permanent night workers.     

The Relation of Shift Work Tolerance to the Circadian Adjustment

The amplitude and phasing of circadian rhythms are under discussion as possible predictors of tolerance to night work. In a field study, subjective sleepiness and oral temperature of 147 female nurses were measured at 2-hour intervals during a period with one morning shift and two consecutive night shifts. The nurses also filled out a questionnaire. Two types of tolerance indices were constructed: The “health index” was based on questions referring to general fatigue, gastrointestinal symptoms, and sleep disturbances, and the “sleepiness index” on the actual subjective ratings of sleepiness. According to the health index, the group with good tolerance had a larger circadian amplitude of the oral temperature rhythm on the day of the morning shift than the group with poor tolerance. However, with regard to the sleepiness index, the corresponding difference between the groups with good or poor tolerance was not significant. The data did not confirm the hypothesis that predicts a quick adjustment of the circadian rhythm when the circadian amplitude is small before the change to night work. The contradictory results found in this and in other studies do not yet permit prediction of tolerance to night work.     

Nonentrained circadian rhythms of melatonin in submariners scheduled to an 18-hour day.

The human circadian timing system has previously been shown to free run with a period slightly longer than 24 h in subjects living in the laboratory under conditions of forced desynchrony. In forced desynchrony, subjects are shielded from bright light and periodic time cues and are required to live on a day length outside the range of circadian entrainment. The work schedule used for most personnel aboard American submarines is 6 h on duty alternating with 12 h off duty. This imposed 18-h cycle is too short for human circadian synchronization, especially given that there is no bright-light exposure aboard submarines. However, crew members are exposed to 24-h stimuli that could mediate synchronization, such as clocks and social contacts with personnel who are living on a 24-h schedule. The authors investigated circadian rhythms of salivary melatonin in 20 crew members during a prolonged voyage on a Trident nuclear submarine. The authors found that in crew members living on the 18-h duty cycle, the endogenous rhythm of melatonin showed an average period of 24.35 h (n = 12, SD = 0.18 h). These data indicate that social contacts and knowledge of clock time are insufficient for entrainment to a 24-h period in personnel living by an 18-h rest-activity cycle aboard a submarine.     

Circadian rhythms in the green sunfish retina

We investigated the occurrence of circadian rhythms in retinomotor movements and retinal sensitivity in the green sunfish, Lepomis cyanellus. When green sunfish were kept in constant darkness, cone photoreceptors exhibited circadian retinomotor movements; rod photoreceptors and retinal pigment epithelium (RPE) pigment granules did not. Cones elongated during subjective night and contracted during subjective day. These results corroborate those of Burnside and Ackland (1984. Investigative Ophthalmology and Visual Science. 25:539-545). Electroretinograms (ERGs) recorded in constant darkness in response to dim flashes (lambda = 640 nm) exhibited a greater amplitude during subjective night than during subjective day. The nighttime increase in the ERG amplitude corresponded to a 3-10-fold increase in retinal sensitivity. The rhythmic changes in the ERG amplitude continued in constant darkness with a period of approximately 24 h, which indicates that the rhythm is generated by a circadian oscillator. The spectral sensitivity of the ERG recorded in constant darkness suggests that cones contribute to retinal responses during both day and night. Thus, the elongation of cone myoids during the night does not abolish the response of the cones. To examine the role of retinal efferents in generating retinal circadian rhythms, we cut the optic nerve. This procedure did not abolish the rhythms of retinomotor movement or of the ERG amplitude, but it did reduce the magnitude of the nighttime phases of both rhythms. Our results suggest that more than one endogenous oscillator regulates the retinal circadian rhythms in green sunfish. Circadian signals controlling the rhythms may be either generated within the eye or transferred to the eye via a humoral pathway.     

Relationships between the circadian rhythms of finger temperature, core temperature, sleep latency, and subjective sleepiness

Skin temperature circadian rhythms have been explored relatively recently. It has been suggested that distal and proximal skin temperature changes play a role in the regulation of the core temperature circadian rhythm and sleepiness. The authors investigated the circadian finger and core temperature rhythms in conjunction with the circadian rhythms of subjective and objective sleepiness. Fourteen healthy, young, good sleepers participated in a modified constant-routine procedure in which palmar finger temperature, rectal temperature, subjective sleepiness, and objective sleep latency were measured half-hourly across a 48-h period of enforced wakeful bed rest. Individual curves were adjusted to the group mean temperature minimum time of 0500 h and averaged to create the 4 mean curves. The 5 possible cross-correlation curves between these 4 measures were calculated for half-hourly phase lags from 12 h before to 12 h after the group mean core temperature minimum time. Maximum cross-correlations for each curve suggested that finger temperature preceded core temperature by 3 h (r = -0.22), and subjective sleepiness followed core temperature by 0.5 h (r = -0.33) and objective sleepiness by 2 h (r = 0.29). Although these data are correlational, they are consistent with the notion that finger temperature changes drive core temperature changes, which determine changes of subjective and objective sleepiness.     

The Relation of Shift Work Tolerance to the Circadian Adjustment

The amplitude and phasing of circadian rhythms are under discussion as possible predictors of tolerance to night work. In a field study, subjective sleepiness and oral temperature of 147 female nurses were measured at 2-hour intervals during a period with one morning shift and two consecutive night shifts. The nurses also filled out a questionnaire. Two types of tolerance indices were constructed: The “health index” was based on questions referring to general fatigue, gastrointestinal symptoms, and sleep disturbances, and the “sleepiness index” on the actual subjective ratings of sleepiness. According to the health index, the group with good tolerance had a larger circadian amplitude of the oral temperature rhythm on the day of the morning shift than the group with poor tolerance. However, with regard to the sleepiness index, the corresponding difference between the groups with good or poor tolerance was not significant. The data did not confirm the hypothesis that predicts a quick adjustment of the circadian rhythm when the circadian amplitude is small before the change to night work. The contradictory results found in this and in other studies do not yet permit prediction of tolerance to night work.