Night Shift: Shuffling On All The Way To Cancer

5. September 2017
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Those who work at night not only turn their biorhythm upside, but also increase their risk of cancer. Responsibility for this could lie in a lack of the sleep hormone melatonin, which is involved in DNA repair.

It has already long been discussed as to whether frequently working at night is carcinogenic. Ten years ago the International Agency for Research on Cancer classified night shifts, which hinder having a normal day-night rhythm, as being “probably” carcinogenic. The World Health Organisation (WHO) shares this view as well. The precedence was among other things a study published in 2001, which had shown that nurses who worked over 30 years at night had a 1.5-fold higher risk of breast cancer than their colleagues who slept at night and worked during the day.

At night, DNA repair works better

Epidemiologists have now examined molecular mechanisms which are thought to be behind the increased breast cancer rate among women working at night. One current study suggests that the human body can more effectively repair damaged DNA at night than it can during the day. Parveen Bhatti from the Fred Hutchinson Cancer Research Center in Seattle and his colleagues discovered in a previous study of nurses and other employees that people who worked frequently at night excreted less 8-hydroxydesoxyguanosine (short: 8-OH-dG).

8-OH-dG is formed when the DNA component oxidises guanosine. “This occurs in the body routinely, since reactive oxygen is released during normal metabolic processes. This reactive oxygen reacts with the DNA and can provoke mutations there”, the scientists explain, describing the process involved. DNA repair enzymes remove the harmful 8-OH-dG and replace it with unoxidised guanosine molecules.

Light disturbs the biorhythm at night

If a person excretes 8-OH-dG in sufficient quantity through the urine, scientists assume that their DNA repair is intact. Bhatti and his colleagues suggest that the decreased 8-OH-dG excretion in women who are often working at night could be one of the factors involved in their increased breast cancer susceptibility. DNA repair also often entails involvement of the hormone referred to as a “sleep hormone”: melatonin.

“Between 19:00 and 20:00 melatonin concentration in the body starts to rise. It reaches its peak between 1:00 and 2:00 at night and then keeps dropping right up until the early morning hours”, according to the epidemiologists. Light leads to the pineal gland in the brain reducing melatonin production. Light from laptops and smartphones affect the synthesis of the sleep hormone as well. “Melatonin synchronises the physiological processes and the rhythm of the human body”, says Bhatti. The hormone is comparable to the moon, which rhythmically coordinates the tides.

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Head of Study ParveenBhatti. © Robert Hood / Fred Hutch

Lower melatonin levels correlate to reduced 8-OH-dG secretion

The researchers hypothesised that human sleep among other things serves a role in the body’s defence against cancer. In previous studies, both melatonin and the DNA 8-OH-dG DNA repair system were seen to be associated with nucleotide excision repair (NER). With this system, the body corrects defective sites that produce a kind of “hump” in the DNA molecule, thereby disturbing the helix structure.

Bhatti and his colleagues have now investigated whether the melatonin level correlates with the excretion of 8-OH-dG in the urine. For this purpose, the team used data from 50 women who had participated in his study last year and had shown particularly low melatonin values. The investigations confirmed Bhatti’s suspicion: night-working women with low melatonin levels were also excreting about 80 per cent less 8-OH-dG than their night-sleeping colleagues.

Do not take melatonin as part of your own self-treatment

At the moment, however, this phenomenon is only an observed correlation. Bhatti and his colleagues want to investigate a causal link in a further study. In so doing they wish to clarify the question whether the administration of melatonin can increase nightly 8-OH-dG excretion and thus presumably improve DNA repair. If this is the case, long-term studies should clarify whether the use of melatonin in shift workers has a lasting positive effect on breast cancer risk. Bhatti warns specifically against creating one’s own self-help treatment initiative involving intake of melatonin based on available data. To this end, little is known about the benefits, about potentially beneficial dosage and about side effects that such an intake could bring with it.

Until reliable data is available Bhatti advises all shift workers, even more so than other people, to refer to the usual advice given for stable health: enough sleep, good nutrition and regular exercise.

Sources:

Shift work and chronic disease: the epidemiological evidence
X-S. Wang et al.: Occupational Medicine, doi: 10.1093/occmed/kqr001; 2011

Oxidative DNA damage during night shift work
Parveen Bhatti et al.: Occup Environ Med, doi: 10.1136/oemed-2017-104414. 2017

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Image copyright: Dave Herholz, flickr / Licence: CC BY-SA

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Melatonin doesn’t have much of a downside and is useful for helping shift worker’s sleep without the aid of harmful pharmaceutical alternatives. If it improves DNA repair, more’s the better.

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