There is no consistent design of bright light therapy for evening/night shift workers, including schedule, intensity, and duration. Various light intensities, from 1,200 to 10,000 lux, with durations of exposure ranging from 3 to 6 hours, have been used successfully to realign circadian rhythms and improve performance and sleep during the night shift.20,21 Previous studies30,31 have recommended that either intermittent or continuous light exposure begins early in the shift and terminates approximately 2 hours before the end of shift, with the wearing of sunglasses outdoor in the morning was efficacious for phase delay. Importantly, a delayed circadian phase was found to be positively correlated with improved sleep, even in the control group, in subjects during the night shift.31,32 The design of our treatment procedure is easy to put into practice in the real world. Higher intensity and shorter bright light exposure once before work or in a break during the first half of the evening/night shift and a daytime darkness procedure were implemented in our study.

The study of Drake et al.7 found that insomnia or daytime sleepiness is a risk factor for major depression, but it is a much greater risk factor for rotating or night shift workers. A systematic review by Even et al.33 examined the efficacy of light therapy in nonseasonal depression. They found that bright light monotherapy is efficacious in treating seasonal depression, but its efficacy in treating nonseasonal depression is inconsistent. This is the first study to report that in female nurses working rotating shifts, anxiety and depression scores were significantly improved after bright light therapy, even when the change in insomnia severity was controlled. It must be noted that the mean HADS-D score as a group was not above the cutoff point (≥ 8) according to the criteria for depression either before or after intervention in our study, but the scores were high. Besides, the nurses were not diagnosed with depression, nor was the clinical severity of depression evaluated in this study. Some factors may contribute to these findings, such as (1) according to the general criteria of the International Classification of Sleep Disorders, 2nd ed. (ICSD-2)8 for insomnia, depressive symptoms related to nighttime sleep difficulty are commonly reported by insomniacs; (2) treating insomnia in patients with major depressive episode improves mood problems other than insomnia.34–36 It must be mentioned that light therapy is not the only choice of treatment for insomnia or depression. Subjects undertaking shift work can improve insomnia and mood problems through pharmacotherapy, behavioral therapy, or other therapy.

There were some primary limitations in our study. First, this study was not a double-blind study, and “placebo effects” should be considered. The subjects in both groups might work in the same unit, and the use of a sham light box (a light box of a much lower intensity or red light) in the control group would be able to be detected by the controls, who would discern the difference. Therefore, we considered that the “placebo effect” could not have been solved by the use of a sham light box. Second, as this study was performed in a real workplace, it was diffi-cult to measure the differences in endogenous circadian rhythm change and the real intensity and duration of light exposure in groups. Aoki et al.37 reported melatonin could be suppressed by light intensities of around 300 lux. However, compared to ordinary room light (< 250 lux), Martin and Eastman38 found that medium- and high-intensity light, approximately 1,230 lux and 5,700 lux, respectively, for 3 hours, significantly increased the percentage of subjects who adapted to the night shift, as measured by temperature rhythm phase shifts. Larger phase shifts were correlated with more sleep and less fatigue. Third, we only used questionnaires to measure the severity of insomnia, anxiety, and depression in the nurses working shifts. We were not able to confirm the diagnosis of insomnia, because subjects might suffer from shift work disorder (SWD), primary insomnia, insomnia due to mental disorders, or other insomnia-related disorders. According to data from the Detroit tri-county population, 32.1% of night workers and 26.1% of rotating shift workers suffer from insomnia or excessive sleepiness. Thus, the “true prevalence” of SWD in the night- and rotating-worker samples was 14.1% and 8.1%, respectively.7 In a study performed in Norway,39 it was found that 44.3% of nurses working on a three-shift rotation met the criteria of SWD by asking shift work-related symptom questions. Fourth, although the study was a randomized control study, we did not assess the differences in rotation patterns, lifestyle during days off, and off-work periods in the groups. Circadian misalignment might be aggravated by these factors. Rotation patterns such as the number of shifts separated by less than 11 hours and the number of nights worked were found to be positively associated with SWD in the study of Norwegian nurses.39 Lin et al.18 also reported that rotation shift nurses who had at least two days off after their most recent night shift showed significantly improved sleep quality and mental health. Finally, 67.4% of the subjects in the present study were working the evening shift, and 32.6% were working the night shift. In this study, different timings of bright light exposure were employed in the evening and night shift groups. No significant differences were found in the change of insomnia/mood scales across treatment in treatment and control groups. However, since statistical power is reduced by dividing the subjects into smaller subgroups by shift schedule, further studies would be indicated to assess effectiveness under the two different situations.