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Shift work, light-at-night, and melatonin: Characterizing a new cancer-related occupational exposure
June 2005
| Principal Investigator: |
Marilyn Borugian (B.C. Cancer Agency) |
| Co-investigators: | Kristan Aronson (Queen's University); Richard Gallagher (B.C. Cancer Agency) |
For more information about this development grant project, please contact Dr. Marilyn Borugian.
Issue
Exposure to light-at-night (LAN) may represent a novel and widespread potential risk factor for cancer, in particular for shift workers, through the suppression of nocturnal melatonin (MLT) secretion.
In order for researchers to investigate LAN as a possible risk factor, a way to estimate workers' lifetime LAN exposure is needed. Years of shift work may be appropriate for this purpose, but direct measures of workplace light exposure are needed to develop this approach. This study addressed the feasibility of 24-hour light exposure measurement for an extended period in natural residential and occupational settings, and looked at whether years of shift work can be used to estimate workers' LAN exposure.
Key findings
- It is feasible to measure light intensity exposure in nurses over extended periods of time (7 days) in natural occupational and residential settings.
- Self-reported data on years of shift work were correlated with light and melatonin levels.
- Melatonin levels in those working rotating shifts were abnormal, with low levels during the rest/sleep times and high levels during work hours.
Objectives
- To explore the feasibility of 24-hour exposure measurement for an extended time period in natural residential and occupational settings.
- To compare light exposure patterns and melatonin levels by shift worked.
- To assess whether years of shift work can be used as a surrogate for exposure to light-at-night.
Method
Five day-shift office workers and 17 nurses working either steady day shift or rotating night and day shifts were recruited.
Each participant wore a device to measure and record light intensity exposure for 7 days, completed a questionnaire on bedroom lighting and occupational history and provided 3 saliva samples in one 24-hour period for melatonin levels. Five nurses agreed to repeat the protocol 3 months later.
Results
Acceptability of wearing the light measurement device and compliance with the 24-hour, 7-day protocol and salivary melatonin sampling procedure were satisfactory in this occupational group.
Nurses working rotating shifts had irregular light exposure patterns and abnormal MLT levels, compared with those working steady day shifts. The day shift group demonstrated a very regular daily pattern, while the rotating shift group showed pattern disruptions, including reduced minimum and maximum light exposure during night shifts. Nurses working rotating shifts also had higher than normal MLT levels on arising and during work.
The numbers of years of shift work were correlated with MLT and light levels.
Conclusions
This study demonstrates that nurses and healthcare office workers are willing to wear a monitoring device continuously for a week, in both occupational and non-occupational settings. This suggests that future measurement studies of this nature would be feasible.
The study also suggests that self-reported “years of shift work” is associated with irregularities in both light exposure and melatonin levels, and would be a useful measure for estimating LAN exposure in workers. Due to the small size of this pilot study, these observations must be interpreted with caution, as confounding variables or chance cannot be ruled out as possible explanations for the differences reported.
The irregular light exposure patterns and abnormal MLT levels associated with rotating shifts are consistent with the alertness and insomnia problems that are commonly reported by shift workers.
Future directions
The data from this project supports the feasibility of future larger scale studies of LAN exposure in the workplace. Larger studies are needed to confirm the finding that shift work is related to possible differences in LAN exposure and may be a valid surrogate measure. Future research may also explore whether high waking melatonin levels in rotating shift workers help to explain problems of fatigue and alertness, and may be addressed by measures to improve shift work adaptation.
Publications and presentations
Borugian, M, Gallagher, R, Friesen, M, Switzer, T, Aronson, K. (2005) Twenty-four-hour light exposure and melatonin levels among shift workers. Journal of Occupational and Environmental Medicine 47: 1268-1275.
Three presentations were conducted on the methods and results:
1. On October 6, 2003, Dr. Borugian presented at research-in-progress for the Cancer Control Research Program of the BC Cancer Agency, with a focus on methodology.
2. On March 26th, 2004, Dr. Borugian presented to the School of Occupational Hygiene at UBC. The audience expressed considerable interest in the topic and contributed comments and feedback on procedures and measurement issues.
3. On May 19, 2004, Dr. Borugian was an invited speaker at the 11th Annual Occupational Hazards to Health Care Workers conference (Vancouver Canada), held jointly by the School of Occupational and Environmental Hygiene at UBC and the Department of Environmental and Occupational Health Sciences at the University of Washington. The information was very well-received, and numerous attendees commented on the forward-looking nature of the research.