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Working the night shift: Biorhythm of 'morning people' disrupted more than that of 'night owls'

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(Vienna, 29 May 2019) Artificial light negatively impacts the biorhythm of people working night shifts and affects their melatonin levels. This increases their risk of developing chronic disease. This is the main finding of the "Nurses' Health-Study II", a longitudinal study conducted by a research group led by Eva Schernhammer from the Division of Epidemiology at the Medical University of Vienna's Center for Public Health. However, what was new was the finding that their health risk was dependent upon their personal chronotype. It was clearly shown that working night shifts causes much less disruption in "night owls" than it does in "morning people". The study has been published in the journal "Cancer, Epidemiology, Biomarkers and Prevention".

It has long been known that shift work is bad for your health. The natural rhythm of countless biological functions of people who work night shifts on a rotational basis is disrupted, increasing their probability of developing diabetes or cancer relative to people doing normal daytime work. This is probably also due to a disruption in the production of the sleep hormone melatonin, since people cannot produce enough of it in artificial light.

In a study conducted with her research group, Eva Schernhammer, Head of the Division of Epidemiology of the Medical University of Vienna's Centre for Public Health, has now shown that artificial light causes a type of "jetlag" in people working night shifts, due to the disruption to their circadian rhythm. The new and somewhat surprising finding is that a person's individual chronotype determines the extent of the disruption, therefore possibly providing an indication of increased health risk.

For the study, Schernhammer first of all collected specific data from 130 participants in the American longitudinal study entitled "Nurses’ Health Study II", which has been running since 1989. This involved fitting a photometer, which can directly measure the incidence of light onto the retina, onto the heads of the test subjects for a period of seven days. They were only allowed to take the photometer off for sleeping. In addition to this, regular urine samples were taken from the women to measure their melatonin levels. The quality of their sleep and their chronotypes were also determined by means of questionnaires.

When the results were analysed, it was found that night-shift workers actually receive a much higher dose of light than during the daytime and that their melatonin levels are erratic. Their individual chronotype was recognised as being an important factor in determining the extent of the burden. Under normal day/night conditions, so-called "night owls" have a slightly less pronounced melatonin rhythm than "morning people", whose rhythm – and consequently health risk – is much more severely affected than that of "night owls" under the conditions associated with working night shifts. This means that the biological clock of a "night owl" working night shifts is not disrupted as much as that of a "morning person".

This study forms a good basis for follow-up investigations looking at the correlation between individual chronotypes and working hours. Schernhammer had already shown in a previous study that the risk of developing diabetes increases in "morning people" with the number of years they work night shifts. Conversely, the same is not true for "night owls" (https://www.ncbi.nlm.nih.gov/pubmed/26109502).

Says Schernhammer: "These new data make an important contribution towards 'Precision Prevention'. As with the chronotype, we now want to identify other individual factors to achieve a targeted reduction in the health impacts of a disrupted body clock, which is an increasingly common phenomenon in our 24/7 society."

Service: Cancer, Epidemiology, Biomarkers and Prevention
"Shift Work, Chronotype, and Melatonin Rhythm in Nurses". P. Razavi, Elizabeth E. Devore, Archna Bajaj, Steven W. Lockley, Mariana G. Figueiro, Vincent Ricchiuti, W. James Gauderman, Susan E. Hankinson, Walter C. Willett, Eva S. Schernhammer.