Effects of bright light shock on sleepiness and adaptation among night workers of a hospital in Iran


Background: Night work has many harmful effects on health, efficiency, and industrial safety of workers. Night workers often complained of sleepiness, decrease of performance, and sleep disorder due to the lack of circadian influence that fully encourages night orientation. Objective: This research was conducted in an industrial environment and it accessed the effects of bright-light (BL) exposure on sleepiness during night work. Materials and Methods: This is an interventional study with a cross-over design. A total of 140 night workers with an experience of >1 year at a hospital participated voluntarily in this study. The night workers were divided into two groups, and both groups were exposed to either BL (3000-3500 lux) or normal light (NL) (400 lux) during break times at night work for two consecutive nights. Results: The 15-minute breaks were initiated at 22:00 (before starting work), 24:00, 2:00, and 4:00 h. The range of sleepiness was assessed by the Karolinska Sleepiness Scale (KSS) at 23:00, 1:00, 3:00, and 5:00 h. We used SPSS16 software for data analysis. The results obtained using the paired t-test analysis (P < 0.000) demonstrated that there were significant differences in the rate of sleepiness between the two groups (case and control). Conclusion: The findings of the present study have also demonstrated the feasibility and benefits of photic stimulation in industrial settings which increased the adaptation to night work.

Keywords: Circadian rhythms, shift work, sleepiness

How to cite this article:
Khammar A, Moghimian M, Ebrahimi MH, Abbasi M, Baneshi MM, Yari AR, Hami M, Poursadeghiyan M. Effects of bright light shock on sleepiness and adaptation among night workers of a hospital in Iran. Ann Trop Med Public Health 2017;10:595-9


How to cite this URL:
Khammar A, Moghimian M, Ebrahimi MH, Abbasi M, Baneshi MM, Yari AR, Hami M, Poursadeghiyan M. Effects of bright light shock on sleepiness and adaptation among night workers of a hospital in Iran. Ann Trop Med Public Health [serial online] 2017 [cited 2021 Mar 4];10:595-9. Available from: https://www.atmph.org/text.asp?2017/10/3/595/213117



Shift workers play central roles in many critical aspects of modern life, including medical care, power generation, the military and law enforcement agents, and important commercial sectors, such as manufacturing and public transportation.[1],[2],[3],[4],[5],[6] An estimated 15- 30% of the workforce in industrialized countries operates outside standard daytime hours.[7],[8],[9]

Shift work is associated with a number of negative health consequences, the most prominent of which is disturbed sleep. Night work typically leads to sleepiness and reduced performance; it also increases the number of accidents and health problems.[10],[11],[12],[13],[14] The notable the effects of shift work are the result of the circadian interference such as sleep during the daylight hours and circadian suppression of the metabolism at night.[15],[16] Circadian rhythm has effect on sleep-wake to breath rate and vital capacity and other factor in performance of workers.[17],[18]

Safety is a primary concern of both employees and their employers in most shift-oriented workplace, especially in nuclear power plants or chemical industries.[19],[20] Several factors can be attributed to the lack of adjustment to shift work. The light exposure in many night workers receive on their way home after work has been proposed as major factor in preventing adaptation of the circadian rhythm.[21] among most night workers. Circadian rhythms are generated by a central pacemaker, the suprachiasmatic nucleus (SCN) in the anterior hypothalamus, and are synchronized to the external environment.[22],[23],[24] This pacemaker is regulated by light and does not depend on the timing of sleep/wake cycle.[25],[26] The effect of light depends on the timing of exposure relative to the nadir of the endogenous body temperature rhythm which is usually located 1-2 h before the habitual time of waking up.[27] Pre-exposure of light to nadir triggers a phase delay, whereas the exposure after the nadir induces an advance phase.[28]

The proposal of bright light as a countermeasure to physiological maladaptation to shifted sleep/wake schedules was based on some previous laboratory studies.[29],[30],[31],[32] Doses of bright light administered at the beginning of the shift period ranged from 6 h of very bright light (5000-12,000 lux) to more moderate bright light (1230-2000 lux) in short or intermittent time.[33],[34],[35],[36] Field research on the population of night-shift workers supports the effectiveness of bright light treatment during the night-shift despite some reported negative results.[32],[37] A recent study regarding the operators in a truck production plant in Sweden showed the useful effect of bright light on reducing sleepiness and melatonin levels during night shifts and no detrimental effects during off-days.[38] Similar observations were reported for oil workers on rotating shifts.[39] Indeed, permanent night nurses who presented adaptive phase delays also tended to expose themselves to light more during the night (and evening) and less during the day than nonadapted nurses.[40] Some field studies do not support the use of bright lights for shift workers.[41] In a study using a short (120 min) bright light (BL) for night workers on an oil platform, it was concluded that adaptation to night work was not significantly enhanced by the exposure. Instead, the normal indoor light appeared sufficient for rapid adaptation as the workers were isolated from the outdoor light. There are no known moderate and minimum light intensities and duration of exposure that are necessary for the production of phase shifts.[42] Low intensities may also cause a phase shift in the rhythm, but higher intensities appear to push the shift rhythm faster.[43]

The present study evaluated if short (15 min) BL exposure during regular breaks would decrease sleepiness during the night shift.

Materials and Methods

Study design

The participants of our study were medical staff of a hospital in Iran. A total of 140 participants completed the study out of 167 men who work in the night shift. Participants were asked to sign the consent form to confirm that they understood the goals, risks, and potential benefits of the study and their right to withdraw from the study at any time. The study was approved by the Ethical Committee of the Tehran University. Seven night workers had to be excluded in the final analysis because of personal reasons.

The participants had to work for 2 days on a 12-h day shift (18:00 h to 6:00 h), 2 days off work, and 2 days on a 12-h morning shift (6:00 h to 18:00 h), and then the schedule was repeated. The night workers were allowed to enjoy four ~15-min short breaks at night. The participants were randomized to one of two sequences: BL exposure during the first two night shifts followed by normal light (NL) exposure during the next two night shifts, and vice versa. After the 4 days washout, baseline sleepiness was assessed using the Karolinska Sleepiness Scale

Out of two similar rooms, one was modified and BLs were installed in it for use during breaks. Up-light armature was used to install fluorescent tubes in the ceiling and this gave an indirect white light with a mean luminance level of 350 candela m2, generated by full-spectrum light tubes with temperatures of 5000 k. In order to increase luminance, an off-white floor was installed and white textiles were used to cover the walls of the room. This generated a mean exposure of 3000-3500 lux at the eye level (with an 80° gaze angle) for a person sitting down. During their break times, night workers were directed to move to the light room. In the NL condition, night workers went to the room with normal illumination (400 lux). The Karolinska Sleepiness Scale (KSS) was used to obtain ratings of sleepiness. The KSS is an eight-point verbally anchored scale.[44] The participants rated sleepiness with KSS every 2 h during the night shift (at 23:00, 1:00, 3:00, and 5:00 h). The BL or NL exposure initiated at 22:00 (before starting to work), 24:00, 2:00, and 4:00 h.

Ethical issues

The approval of University of Social Welfare and Rehabilitation Sciences was obtained for conducting the study. All participants were presented about the objectives of the study, and their informed consent was obtained.

Data analysis

The statistical analysis was done using SPSS software version 16. Having confirmed the normality from the Kolmogorov-Smirnov test, we conducted parametric tests. Using the paired t-test, we compared sleepiness among the two conditions (sleepiness with BL condition, and sleepiness with NL condition). Statistical significance was considered at P < 0.05.


Out of 167 voluntary participants, 140 successfully completed the study; participants were divided into two groups of 70 people each. In both conditions, the night workers took at least four breaks at work. The average breaks had a means length of 15+/-1 minute. There were no significant differences between conditions regarding the timing of breaks or their frequencies.

All the participants were male with a mean age of 30.36 years (range 21-45 years).

The KSS was completed at 23:00, 1:00, 3:00, and 5:00 h. Then, the amount of sleepiness peaks rose at 5:00 h followed by 3:00 h. This increase in sleepiness was consistent over the 4 days of study. [Figure 1], [Figure 2], [Figure 3], [Figure 4] show the comparison of sleepiness score among BL and NL treatment.

Figure 1: Night 1: intervention 1

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Figure 2: Night 2: intervention 1

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Figure 3: Night 1: intervention 2

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Figure 4: Night 2: intervention 2

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[Figure 1], [Figure 2], [Figure 3], [Figure 4] display and compare the sleepiness score for the BL and NL conditions

In [Table 1], significant differences were detected between BL and NL conditions. This difference is found by comparing both groups at these times: time 1-time 3; time 1-time 4; time 2-time 3; and time 2-time 4. Furthermore for the sleepiness variable, significant differences were detected between NL and BL in the two groups (time 1-time 2) and (time 3-time 4) (P < 0.05, both comparisons), whereas the expected difference between time 1 and time 2 in group 2 (to be in the control condition). Furthermore, the sleepiness score at second night BL was significantly less than that of sleepiness score at first night BL (P < 0.05). In [Table 2], showed the treatment effect, carry over effect and wash out period.

Table 1: Comparison differences in rate of sleepiness between two groups in four nights of study

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Table 2: Treatment effect &##9;carry over effect, period effect in this study

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This research has shown how effective the exposure to BL was when levels of sleepiness in shift workers were decreased during their break periods. The results of the present study were consistent with those of a similar study conducted in Sweden where exposure to two 20-min periods of bright light during the night had significantly reduced subjective sleepiness (measured using the KSS questionnaire [38],[39],[40],[41],[42] or measured using the SSS questionnaire.).[45]

The findings of the present study also revealed reduced levels of sleepiness during the second night of intervention as compared to both the first night and the baseline which can be attributed to the adaptation to shift work as a result of the exposure to BLs. There was consistency in this decrease in sleepiness during consecutive nights as compared to results from other similar studies that have shown an increase in adaptation to the night shift due to exposure to BLs during five consecutive nights of the night shift work but disappeared during the weekend.[38]

It is noteworthy to say that in spite of the short period of exposure to BL with four 15-min breaks, significant differences were found. In our study, there was no real placebo and we classify this as one of our limitations. This is because the participants in the study had ample knowledge of the study procedure. This might have affected the results.

The present study showed the positive effects of utilizing bright lights during work on decreasing levels of sleepiness and this supports the use of light stimulants on adaptation to night shifts.

It is economical to give industrial work places rooms furnished with BLs instead of supplying the entire workplaces with BLs. More so, previous studies showed no disruptive effects of post-exposure to bright lights. These are capable of having important applications regarding the safety and productivity of the workplace and the workers as well. However, the moderate dose, intensity, and timing of BL exposure have not yet been determined and there is need to verify them before utilization of BL stimulants at industrial work places.


Short-term bright light could be an effective treatment for decreasing sleepiness and adaptation to the shift work. We suggested that the intervention must be continued and examining the prevalence of various sleep disorders in future study must be done before and after the intervention.


The authors gratefully acknowledge the cooperation of the study participants and their managers. They thank the volunteers and people involved in data collection. The authors express their deep appreciation towards help contributed in any manner to this study, and a special thanks is extended to Professor Hossein Kakoei, wishing him eternal peace.

Financial support and sponsorship

This study was supported by grant No 9352 in University of Social Welfare & Rehabilitation Sciences.

Conflicts of interest

There are no conflicts of interest.



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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ATMPH.ATMPH_108_17


[Figure 1], [Figure 2], [Figure 3], [Figure 4]


[Table 1], [Table 2]

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