Light and Shiftwork

Status:

completed 02/2024

Aims:

Light is the most important indicator for the human internal clock. Changing light day profiles for shift workers can lead to a desynchronization of circadian rhythms and may influence workers health. The aim of this intervention study was to investigate the short- and long-term effects of dynamic lighting at the workplace and individual lighting recommendations for leisure time in shift workers.

Activities/Methods:

Employees of a company were studied in four field phases between October 2021 and February 2023. Two workplace lighting interventions were carried out

i) during the day in a group of shift workers without night shift in assembly hall 1 and

ii) at night in a group of shift workers with night shift in assembly hall 2.

Employees in other areas of the company and in different shift systems served as a comparison group. After the baseline survey T0 (10/2021-11/2021), the lighting was modified in December 2021. Compared to the baseline situation, in assembly hall 1 the melanopic equivalent daylight illuminance (MEDI) was dynamically increased in the morning by varying light colour and luminous flux and lowered towards the evening. In assembly hall 2, the lighting was dynamically increased at night in the first half of the shift and reduced from the middle of the night shift. For the analysis of short-term effects, baseline T0 measurements were compared with the first follow-up T1 (01/2022-02/2022). Longer-term effects were examined with further field phases one year after T0 and T1 (T2: 11/2022, T3: 01/2023). After T1, further participants were recruited for the comparison group. A randomized subgroup was drawn from all participants of the comparison group to study the effect of individualized lighting recommendations during leisure time with T2 as baseline and T3 as follow-up. The recommendations were communicated individually after T2 and in T3 via study cell phones and other methods. Health endpoints were assessed using both objective measures (actigraphy, hormone levels in saliva and hair, psychomotor vigilance in the middle of the shift) and subjective measures (sleepiness, individual perception of lighting). In addition, photometric measurements were carried out at the workplace as well as individual light measurements. The statistical analyses included the use of algorithms for deriving the endpoints, multiple imputation methods and descriptive analyses. Depending on the endpoint, the orientational modeling included mixed models considering the repeated measurements per individual and adjustment for important confounders.

Results:

Overall, n = 89 people took part in the four field phases, n = 27 were participants in the interventions in the two halls, n = 26 people were selected for the lighting-habits re-commendations during leisure time. Initial analyses show short-term improvements in psychomotor vigilance and sleepiness for the intervention during daytime. For the intervention during the night, longer sleep durations after night shifts were observed in the short term. In the longer term there are indications of higher levels of melatonin and lower levels of cortisol in saliva at the end of the night shift with the night light intervention. Longer sleep durations were observed for early shifts with the recommendations for leisure time. Over the course of the study, the general stress level measured via cortisol in the hair decreased in all groups. Brightness and luminous colour were more frequently perceived as pleasant after the introduction of dynamic lighting.

Conclusions

The initial results indicate that there are no negative influences on health. There are indications of slight positive effects in some of the endpoints.

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