Postshift Day Research Articles

The Effect of 24 h Shift Work on the Nutritional Status of Healthcare Workers: An Observational Follow-Up Study from Türkiye.

This study aimed to determine changes in energy and nutrient intakes over three consecutive days, including the day of the shift, and the days before and after the shift, in healthcare workers working in a 24 h shift system. This study is an observational follow-up study conducted with a total of 500 volunteer healthcare professionals. Food consumption records were taken over 3 consecutive days: pre-shift (off day), shift day (24 h shift), and post-shift (off day). Mean daily intakes of energy, carbohydrate, protein, fat, saturated fat, caffeine, vitamins B1, B2, niacin, B6, folate, and B12, potassium, magnesium, phosphorus, iron, and zinc are listed from highest to lowest as shift day > pre-shift > post-shift (p < 0.05 for all pairwise comparisons). While fiber, vitamin C, and calcium intakes were similar on the shift day and pre-shift day, they were significantly lower on the post-shift day (p < 0.05). The lowest dietary reference intake percentages on the post-shift day were calcium, fiber, and folate, respectively. In the present study, significant differences were detected in the energy, micronutrient-intake, and macronutrient-intake levels between the pre-shift day, shift day, and post-shift day of healthcare workers. Awareness should be increased regarding the decreased nutrient intake seen especially on the first day after a 24 h shift, and appropriate precautions should be taken to increase calcium, fiber, and folate intake levels.

Assessment of triphenyl phosphate (TPhP) exposure to nail salon workers by air, hand wipe, and urine analysis.

Triphenyl phosphate (TPP or TPhP) is commonly used as an additive plasticizer or organophosphate flame retardant (OPFR) in consumer products including nail polish. We evaluated exposure to TPhP from 12 nail salon technicians working at four nail salons located in California over a period of two work days. Bulk samples of 15 nail polishes and other nail products were collected. Study participants also provided two personal air samples, two hand wipe samples (pre- and post-shift on day two), and two urine samples (pre-shift day one and post-shift day two). The geometric mean (GM) of TPhP air sampling concentrations was 7.39ng/m3. Post-shift TPhP hand wipe concentrations (GM 1.35 μg/sample) were significantly higher (p=0.024) than pre-shift hand wipe concentrations (GM 0.29 μg/sample). Diphenyl phosphate (DPP or DPhP), a urinary metabolite of TPhP used in this study as a biomarker of exposure, was detected in all post-shift urine samples and 75% of urine pre-shift samples. DPhP post-shift concentrations (GM 1.35μg/g creatinine) were significantly higher than pre-shift concentrations (GM 0.84μg/g creatinine; p=0.012). In addition, DPhP post-shift concentrations were correlated with TPhP post-shift hand wipe concentrations, suggesting dermal contact may be a relevant exposure pathway for nail salon workers.

Assessment of spray polyurethane foam worker exposure to organophosphate flame retardants through measures in air, hand wipes, and urine

Tris(1-chloro-2-propyl) phosphate (TCPP, also referenced as TCIPP), a flame retardant used in spray polyurethane foam insulation, increases cell toxicity and affects fetal development. Spray polyurethane foam workers have the potential to be exposed to TCPP during application. In this study, we determined exposure to TCPP and concentrations of the urinary biomarker bis(1-chloro-2-propyl) phosphate (BCPP) among 29 spray polyurethane foam workers over 2 work days. Work was conducted at residential or commercial facilities using both open-cell (low density) and closed-cell (high density) foam. Study participants provided two personal air samples (Day 1 and Day 2), two hand wipe samples (Pre-shift Day 2 and Post-shift Day 2), and two spot urine samples (Pre-shift Day 1 and Post-shift Day 2). Bulk samples of cured spray foam were also analyzed. Sprayers were found to have significantly higher TCPP geometric mean (GM) concentration in personal air samples (87.1 μg/m3), compared to helpers (30.2 μg/m3; p = 0.025). A statistically significant difference was observed between TCPP pre- and post-shift hand wipe GM concentrations (p = 0.004). Specifically, TCPP GM concentration in post-shift hand wipe samples of helpers (106,000 ng/sample) was significantly greater than pre-shift (27,300 ng/sample; p < 0.001). The GM concentration of the urinary biomarker BCPP (23.8 μg/g creatinine) was notably higher than the adult male general population (0.159 μg/g creatinine, p < 0.001). Urinary BCPP GM concentration increased significantly from Pre-shift Day 1 to Post-shift Day 2 for sprayers (p = 0.013) and helpers (p = 0.009). Among bulk samples, cured open-cell foam had a TCPP GM concentration of 9.23% by weight while closed-cell foam was 1.68%. Overall, post-shift BCPP urine concentrations were observed to be associated with TCPP air and hand wipe concentrations, as well as job position (sprayer vs. helper). Spray polyurethane foam workers should wear personal protective equipment including air-supplied respirators, coveralls, and gloves during application.

0091 Inflammatory markers and exposure to air pollutants among workers in a Swedish pulp and paper mill

ObjectivesStudy the relationship between inhalation of airborne particles in a pulp and paper mill and markers of inflammation and coagulation in blood.MethodPersonal sampling of inhalable dust was performed for 72...

Absence of a serum melatonin rhythm under acutely extended darkness in the horse

BackgroundIn contrast to studies showing gradual adaptation of melatonin (MT) rhythms to an advanced photoperiod in humans and rodents, we previously demonstrated that equine MT rhythms complete a 6-h light/dark (LD) phase advance on the first post-shift day. This suggested the possibility that melatonin secretion in the horse may be more strongly light-driven as opposed to endogenously rhythmic and light entrained. The present study investigates whether equine melatonin is endogenously rhythmic in extended darkness (DD).MethodsSix healthy, young mares were maintained in a lightproof barn under an LD cycle that mimicked the ambient natural photoperiod outside. Blood samples were collected at 2-h intervals for 48 consecutive h: 24-h in LD, followed by 24-h in extended dark (DD). Serum was harvested and stored at -20°C until melatonin and cortisol were measured by commercial RIA kits.ResultsTwo-way repeated measures ANOVA (n = 6/time point) revealed a significant circadian time (CT) x lighting condition interaction (p < .0001) for melatonin with levels non-rhythmic and consistently high during DD (CT 0-24). In contrast, cortisol displayed significant clock-time variation throughout LD and DD (p = .0009) with no CT x light treatment interaction (p = .4018). Cosinor analysis confirmed a significant 24-h temporal variation for melatonin in LD (p = .0002) that was absent in DD (p = .51), while there was an apparent circadian component in cortisol, which approached significance in LD (p = .076), and was highly significant in DD (p = .0059).ConclusionsThe present finding of no 24 h oscillation in melatonin in DD is the first evidence indicating that melatonin is not gated by a self-sustained circadian process in the horse. Melatonin is therefore not a suitable marker of circadian phase in this species. In conjunction with recent similar findings in reindeer, it appears that biosynthesis of melatonin in the pineal glands of some ungulates is strongly driven by the environmental light cycle with little input from the circadian oscillator known to reside in the SCN of the mammalian hypothalamus.

An unexpected reduction in sucrose concentration activates the HPA axis on successive post shift days without attenuation by discriminative contextual stimuli

Previous studies have shown that the successive negative contrast procedure, in which food-restricted rats entrained to once daily, brief presentations of 32% sucrose are unexpectedly shifted to a 4% solution, results in an adrenocortical response on the second, but not the first postshift day. We attempted to generalize that finding in our own procedure. In Experiment 1, two groups of rats were given a 32% sucrose solution once daily in their home cages for 14 days before being shifted to a 4% solution. One group was killed 10 min after the first 4% solution and one was killed after the second 4% solution. In addition, two groups receiving either 32% or 4% sucrose throughout the experiment served as unshifted controls. In contrast to previous findings, both shifted groups exhibited prominent adrenocorticotropin hormone (ACTH) and adrenocortical (B) responses on both postshift days compared to unshifted controls, which did not differ from one another. Experiment 2a employed distinctive contexts to test if the lack of generality of the delayed HPA axis response was due to suppressive effects of S(+) on the first postshift day. Rats were given once daily 32% sucrose in S(+) and equal exposure time in S(−). Half of these rats were shifted to 4% sucrose in S(+) and half were shifted in S(−). These two groups were compared to home cage controls. Half of each group was killed after their first 4% sucrose, and half after the second 4% sucrose. All rats showed ACTH and B responses comparable to shifted rats in Experiment 1. S(+) failed to suppress the HPA axis, and the stress response was higher on the first compared to the second day of the shift. Experiment 2b established that distinctive contexts predicting sucrose, S(+), or not predicting sucrose, S(−), controlled behavioral choice and contextual discrimination. Thus, there was no evidence that issues of stimulus control could explain the lack of generality of previous findings. The data indicate that thwarting sucrose expectancies is stressful, and that this stress response habituates across days.

Evidence for zolpidem-induced hyperphagia, but not anxiolysis, in a successive negative contrast paradigm

Zolpidem is an imidazopyridine which binds to certain benzodiazepine receptor types with varying degrees of affinity. The effect of zolpidem on successive negative contrast was investigated in three experiments. In each experiment, a contrast group was given brief access to 32% sucrose for 10 days, then shifted to 4% sucrose for 2 days; a procedure that elicits anxiety primarily on the second postshift day. One control group was given only 4% sucrose. Experiments 2 and 3 included a 2% sucrose group as an intake rate-dependent control. In Experiment 1, zolpidem (4.0 and 0.5 mg/kg) dose-dependently reduced contrast on the two postshift days. Contrast occurred during the first postshift consummatory burst. Zolpidem prolonged the first postshift burst equally in both shifted and unshifted groups, suggesting a general facilitation of intake masked by a ceiling effect in controls. In Experiment 2, zolpidem's (4.0 mg/kg) anti-contrast action was equivalent to its hyperphagic effect in the 2% control group. Zolpidem prolonged the first postshift burst equally in all three groups, again consistent with general intake facilitation. In Experiment 3, 8.0 mg/kg zolpidem produced an anti-contrast effect not present in 2% controls on both postshift days. This does not appear attributable to anxiolysis, however, as the effect was equivalent during stressful and non-stressful phases of the postshift period, and zolpidem extended the duration of the first postshift burst equally in all three sucrose groups. Thus, unlike benzodiazepines, zolpidem is not anxiolytic in this paradigm.

Urinary 1-hydroxypyrene as a biomarker of internal dose of polycyclic aromatic hydrocarbons in carbon black workers.

In this study, a total of 30 workers were selected, including eight wet pelletizing workers and 22 packaging workers. For all selected workers, urine samples were collected on the first day pre-shift, first day post-shift and fifth day post-shift, and their urinary 1-hydroxylpyrene levels (1-OHP) were determined (denoted as BM1pre, BM1post and BM5post, respectively). Personal respiratory exposures, including both inhalable particle-bound PAHs (Cinh) and gaseous PAHs (Cgas), together with dermal exposure to particle-bound PAHs (Cskin) were measured. Personal background information, including age, sex and smoking habit, was carefully registered. Pyrene exposure was statistically significantly correlated with exposure to PAHs and carcinogenic PAHs. Multiple linear regression analysis results showed that the BM1post values could not be explained by workers' exposures. For BM5post in packaging workers, both the regression model (R2 = 0.73) and the regression coefficients for Cgas, Cinh and Cskin were statistically significant (P < 0.05). For pelletizing workers, the R2 value was higher but was not statistically significant because of the smaller number of workers. The resultant regression coefficients for 'sex', 'smoking habit' and 'age' were statistically insignificant (P >> 0.05), which could be because these variables made relatively small contributions to BM5post. In conclusion, this study suggests BM5post could be a suitable indicator for PAH exposures of carbon black workers, on the condition that both respiratory (including gaseous PAHs and particle-bound PAHs) and dermal exposures have been assessed.

Relationship of negative contrast to animal models of fear and anxiety

The relatedness of behavior elicited by reward reduction (successive negative contrast procedure) and behaviors produced by three animal models of anxiety (open-field emergence, elevated plus-maze, and context-shock fear conditioning) was examined by correlational and factor analytic procedures. Factor analysis (oblique rotation) indicated substantial independence among the tests: Trials 1 and 2 of the plus-maze loaded on two different factors unaccompanied by any other test; open-field emergence and context-shock fear loaded on the same factor; and negative contrast loaded on a fourth factor. However, negative contrast proved to be a dynamic process, with factor loadings changing across a 4-day postshift period—moving from an independent loading on the 1st postshift day to being clustered with context-shock fear and open-field emergence on the 2nd and 3rd postshift days to being clustered with just context-shock fear on the last postshift day. These latter data support a multistage theory of successive negative contrast.

Temporal dynamics of corticosterone elevation in successive negative contrast

MITCHELL, C. AND C. FLAHERTY. Temporal dynamics of corticosterone elevation in successive negative contrast. PHYSIOL BEHAV 64(3) 287–292, 1998.—A negative contrast effect in consummatory behavior that occurred when rats were shifted from 32% to 4% sucrose was accompanied by elevations in corticosterone that were measurable at 10 and 20 min but not at 40 min after the second postshift day. No corticosterone elevations were found to accompany contrast at 10 or 15 min after the first postshift day in these experiments nor immediately after the first postshift day in an earlier experiment. The present study also found an anticipatory elevation in plasma corticosterone in shifted animals just before their placement in the apparatus on the second postshift day. These results are discussed in terms of a multistage hypothesis of successive negative contrast. The anticipatory elevation is discussed in terms of uncontrollability and unpredictability of aversive events.

Ibotenic acid lesions of the amygdala basolateral complex or central nucleus differentially effect the response to reductions in reward

The present study examined the role of the amygdala in the acquisition and expression of the Crespi effect (Crespi, L.P., Quantitative variation in incentive and performance in the white rat, Am. J. Psychol., 55 (1942) 467–517), also known as successive negative behavioral contrast. In Experiment One rats with bilateral amygdala cannulae were trained to run a straight alley for either a large (ten pellet) or small (one pellet) food reward. After 8 days of training, half of the rats in each reward condition received vehicle or ibotenic acid administered bilaterally into the amygdala. After 4 days of recovery from the induction of the lesions, training resumed. On Day 12 of training, the reward for rats in the large reward condition was shifted to one pellet and this reward level was maintained for the next 4 days of training. Both the lesioned and unlesioned shifted rats exhibited increased latencies to the reduction. However, shifted lesioned rats displayed a more persistent increase in latencies than shifted unlesioned rats, exhibiting significantly longer latencies than those of unlesioned rats by Day 15. This finding suggests that large amygdala lesions may impair learning of the appetitive value of the small reward. Experiment Two examined the effects of discrete ibotenic acid lesions of either the central nucleus or basolateral/lateral complex of the amygdala. Lesions of the central nucleus produced results similar to those of Experiment One. However, in Experiment Two the performance of shifted unlesioned and lesioned groups diverged significantly 1 day earlier, on Day 14. In contrast, lesions of the basolateral/lateral complex reduced the duration of the contrast effect. Shifted lesioned rats exhibited significantly lower latencies than shifted unlesioned rats by the first postshift day, Day 13. This finding suggests that the basolateral/lateral complex may be involved in learning about, or expressing the response to, the aversiveness of reward reduction.

Effect of Chlordiazepoxide on the Response to Repeated Reductions in Sucrose Concentration in Free-Fed Rats

Free-fed rats were shifted from brief access to 32% sucrose (5 min per day for 3 days) to 4% sucrose (5 min per day for 2 days) 8 separate times. During preshift, rats given access to 32% sucrose licked less than rats given access to 4% sucrose (the opposite of what is typically obtained with deprived rats). The shift from 32% sucrose to 4% sucrose resulted in a further decline in lick frequency, suggesting a successive negative contrast effect. Degree of negative contrast diminished with repeated downshifts. Administration of chlordiazepoxide (CDP), 8 mg/kg, on either the first or second postshift day completely offset the effect of reward reduction. The diminution of contrast and the pattern of effectiveness of CDP differed from results obtained with food-deprived rats. The reduction of contrast by CDP was interpreted as being separate from an appetite-stimulation effect of the drug. Finally, all groups showed tolerance to the sedative effects of CDP in an open-field test, a result consistent with that obtained with deprived rats.

The amygdala modulates memory for changes in reward magnitude: involvement of the amygdaloid GABAergic system

Rats with bilateral amygdala cannulae were trained to run a straight alley for a large or small food reward. Muscimol (MUS), a GABA A agonist, or bicuculline methiodide (BMI), a GABA A antagonist, were infused into the amygdala immediately after a reward shift. In Experiment 1, rats in the large-reward group were shifted to the small reward and immediately after received an infusion of vehicle or MUS bilaterally into the amygdala. The runway latencies of the shifted vehicle animals were increased in comparison with those of the unshifted controls. In contrast, the shifted MUS (0.001 nM, but not 0.01 nM) animals displayed latencies that were comparable to those of unshifted animals by the second postshift day. These findings suggest that muscimol dose-dependently attenuated the memory of the reward reduction. In Experiment 2 rats were trained as before except they received bilateral infusions of BMI into the amygdala immediately after reward reduction. The shifted BMI (0.1 nM, but not 0.3 nM) animals displayed increased runway latencies, in comparison with those of shifted vehicle animals, by the second postshift day. These findings suggest that BMI dose-dependently enhanced memory of the reward reduction. In Experiment 3, animals were trained as before except they first experienced a reward increase before receiving post-training injections of vehicle, MUS (0.001 nM) or BMI (0.1 nM). On the next day the reward was reduced. Despite reward reduction, shifted BMI animals persisted displaying low latencies for more trials than did shifted MUS animals. These findings suggest that BMI enhanced memory for the reward increase. More generally, the findings suggest that the amygdala and its GABAergic system is involved in memory consolidation for both positive and negative affective experiences.

Lack of tolerance to contrast-reducing actions of chlordiazepoxide with repeated reward reductions

In each of eight cycles of repeated reward reduction, the performance of rats given brief access to 32% sucrose for 3 days, and then 4% sucrose for 2 days, was compared to rats that received 4% on all 5 days. Shifted rats consumed less than unshifted rats following each shift, with little evidence of diminution of negative contrast across the eight shifts. Acute administration of chlordiazepoxide (CDP, 8 mg/kg) on the second postshift day reduced contrast on each shift with no evidence of tolerance development to these anticontrast actions (Experiment 1a). Acute administration of CDP on the first postshift day had no effect on contrast through the first four shifts, but reliably reduced contrast on the following four shifts (Experiment 2a). There was tolerance to the sedative effects of CDP, as measured in an open field (Experiments 1b and 2b). The data thus show: a) that contrast is not lost with repeated shifts; b) no tolerance develops to the anticontrast actions of CDP and, instead, CDP gains anticontrast actions, in regard to initial contrast occurrence; c) but, concurrently, tolerance does develop to the sedative effects of CDP in an open field.

Resynchronization of the circadian corticosterone rhythm after a light/dark shift in juvenile and adult mice.

Most of the extensive literature concerning the resynchronization of circadian rhythms after a Zeitgeber shift is devoted to the dependence of resynchronization on the mode of the shift and the strength of the Zeitgeber, as well as on the circadian function investigated. Ontogenetic influences have rarely been investigated. Therefore, we studied the resynchronization of several circadian rhythms in juvenile and adult female laboratory mice. We present here the results concerning the corticosterone rhythm. The daily rhythms were determined as transverse profiles (2-h intervals) before as well as 3, 7, and 14 days after an 8-h phase delay of the light/dark cycle produced by a single prolongation of dark time. The corticosterone concentration in serum was determined radioimmunologically. In the control animals the daily patterns were bimodal, with main maxima at the end of the light time and secondary ones just after lights on. Ontogenetic differences were small. In adult mice the amplitude was slightly increased due to an increase in the maximum values, and the time of highest hormone concentrations was slightly phase advanced. In juvenile mice, a distinct daily pattern with a phase position in relation to the light/dark cycle corresponding to that of control animals was present on the 3rd day after the Zeitgeber shift. The daily mean as well as the minimum and maximum values increased initially and reached the values of control animals during the second week. In adult animals, a pronounced daily rhythm with the normal phase position was present only at the 7th postshift day. The amplitude, daily mean, and maximum values were decreased, and the minimum values were increased. The initial values were not reached even after 2 weeks. The results show that resynchronization was faster in juvenile mice compared with adult mice. As a possible cause for the observed age-related differences, a not yet stabilized phase-coupling between various circadian rhythms is supposed.

Influence of Melatonin Treatment on Human Circadian Rhythmicity before and after a Simulated 9-Hr Time Shift

The hormone melatonin is currently proposed by some investigators to be an efficient means for decreasing the impairing effects of jet lag. Eight healthy male subjects, aged 20 to 32, underwent a 9-hr advance shift in the isolation facility of our institute during two periods each of 15 days' duration. In a double-blind, crossover design, subjects took either melatonin or placebo at 1800 hr local time for 3 days before the time shift and at 1400 hr for 4 days afterwards. The time shift was simulated on days 7 and 8 by shortening the sleep period by 6 hr and the following wake period by 3 hr. Body temperature was recorded every 90 min, and urine was collected at 3-hr intervals all day and night. Melatonin treatment enhanced the resynchronization speed of some, but not all, hormone and electrolyte excretion rates for several days after the time shift. The adaptation speed of the temperature rhythm significantly increased during one postshift day. In addition, the circadian temperature rhythm had a significantly higher amplitude under melatonin treatment than under placebo after the time displacement. For the placebo group, the rhythm of 6-hydroxymelatoninsulfate excretion exhibited an advance shift in five subjects, whereas the other three showed a delay shift, and adjustment did not achieve more than one-half of the expected value within 8 days. A significantly different adjustment could be observed in the melatonin-treated group: Seven subjects underwent an advance shift of the expected 9 hr within an average of 8 days. The results suggest that melatonin treatment can accelerate resynchronization of the melatonin excretion rhythm after eastward time zone transitions. The improvement is not, however, sufficiently great that we can recommend melatonin for the alleviation of jet lag.

Effect of Varied Taste Experience on Negative Contrast in Consummatory Behavior

Rats shifted from 32% to 4% sucrose consume less 4% sucrose than animals that experience only the 4% solution. Previous experiments have suggested that a stress/emotional factor may be causally related to this negative contrast effect on the second postshift day, but not on the first postshift day. The present experiment was concerned with the possibility that contrast on the first postshift day is related to a neophobic response to the postshift solution. Results showed that giving animals experience with variously flavored (seven different flavors) 32% sucrose during the preshift period reduced degree of contrast when the animals were shifted to 4% sucrose. These data are considered in terms of solution novelty, specific loss of "sweetness", and caloric loss as contributors to negative contrast.

The effects of morphine in the consummatory contrast paradigm.

Rats shifted from 32% to 4% sucrose show a negative contrast effect, licking significantly less than animals that receive only 4% sucrose. The effects of morphine sulfate (0.5, 1.0, 2.0, 4.0, 8.0, and 16.0 mg/kg) on negative contrast were investigated in four experiments. Contrast was reduced on both the 1st and 2nd postshift day by the 4.0 and 8.0 mg/kg doses, but the effects were less robust than those seen with the benzodiazepines. The effects of morphine on contrast were dissociable from simple increases in sucrose consumption. Naloxone (0.25, 0.5, and 1.0 mg/kg) had no effect on contrast or sucrose intake. However, the contrast-reducing effect of morphine (4.0 mg/kg) was blocked by pretreatment with naloxone (0.50 mg/kg). The results are discussed in terms of other anxiolytic screening paradigms that have obtained "partial anxiolytic effects" using morphine.

Chlordiazepoxide and the determinants of negative contrast

Previous experiments have shown that the negative contrast effect in consummatory behavior that occurs when rats are shifted from 32% to 4% sucrose is alleviated by the tranquilizer chlordiazepoxide (CDP). However, in these experiments, CDP was effective on the second postshift day but not on the first postshift day. The three experiments described in this paper suggest that this differential effectiveness of CDP is not due to the difference in preshift-postshift retention intervals on Day 1 (24 h) and Day 2 (48 h), but is due instead to the necessity of some degree of experience (about 5 min) with the postshift solution. These results, combined with those of an earlier study which showed elevated corticosterone in shifted animals on the second postshift day but not on the first postshift day, suggest that negative contrast may be a dynamic process, involving sequential processes of detection, evaluation, and conflict over the postshift period. It was further suggested that CDP becomes effective in moderating contrast only when the conflict stage is reached.

Correlation of corticosterone elevation and negative contrast varies as a function of postshift day

In three experiments, rats shifted from 32% to 4% sucrose consumed less of the 4% sucrose than did rats that had received only the 4% solution. Experiment 1 showed that this negative contrast effect was not accompanied by a corticosterone elevation on the first day subsequent to the shift. Experiments 2 and 2a showed that corticosterone levels were substantially elevated on the 2nd postshift day and that there was a tendency for degree of elevation in corticosterone to be related to degree of lick suppression. These results are discussed in terms of other data suggesting that anxiolytic drugs and disinhibitory stimuli are more effective in alleviating contrast on the 2nd postshift day than on the 1st postshift day. It is suggested that, in the present paradigm, reaction to stimulus change may be the primary determinant of contrast on the 1st postshift day, but emotional processes related to reward loss and/or conflict develop by the 2nd postshift day.