Features: Night light or night shift work disrupts the body's biological clock and increases cancer risk

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Night light or night shift work disrupts the body's biological clock and increases cancer risk

Posted: Jan, 2008 (Return to the original web page from this print version)

24-hour gas station: Night light or night shift work increases cancer risk

The World Health Organisation has listed night shift work as a probable cancer-causing agent. Night light significantly reduces the hormone melatonin, which suppresses cancer cells.

The invention of the first commercially viable incandescent light bulb by Thomas Edison 129 years ago (in 1879) was widely revered as a major technological breakthrough. It has drastically reshaped human lifestyles by extending people's activities late into the night. In addition, nearly 20% of the working population in developed countries works night shifts.

However, as indicated by many research studies, exposure to artificial light at night disrupts the body's biological clock, and significantly reduces the production of a hormone called melatonin. This hormone plays important roles in regulating the sleep-wake cycle, emotions and reproduction, as well as suppressing cancerous growth.

People doing overnight work over many years have higher risk of developing cancer, such as breast, prostate or colorectal cancer. In Dec 2007, the World Health Organisation officially listed night shift work as a probable cancer-causing agent. (Related News: Nov 29, 2007)

Even worse than working an overnight shift is flipping between daytime and overnight work, because it is very difficult for the body to reset the frequently disrupted biological clock.

Melatonin is a hormone produced by the pineal gland of our body at night. It is nicknamed the hormone of darkness.

Melatonin is a hormone produced by the pineal gland at night. It is nicknamed "the hormone of darkness".

The pineal gland receives signals from the suprachiasmatic nucleus (SCN), the body's master clock located in the hypothalamous of the brain.

Natural melatonin production rises sharply in the evening, and peaks between 1 am and 3 am.

What is Melatonin

Melatonin is a hormone produced by the pineal gland of our body at night in response to darkness. The pineal gland is a cone-shaped, pea-sized gland located just beneath the centre of the brain. It is also called "seat of the soul" because of the influence of melatonin on emotions.

Melatonin plays an important role in the followings:

	Regulate the circadian rhythms (daily body cycles)

	Regulate the sleep patterns. This includes the speed of falling asleep, duration and the quality of sleep etc.

	Influence hormones in the body that regulate reproduction, the timing of ovulation, and aging etc.

	Anti-aging: Melatonin is a powerful antioxidant, a compound that blocks the action of free radicals (activated oxygen molecules) that can damage cells. Therefore some scientists suggest that it has anti-aging functions.

	Anti-cancer: Some studies showed that melatonin may suppress the growth of certain types of cancer cells, and may stimulate the natural killer cells (a type of white blood cells) to attack tumours. (Source; More info on cancer suppression by melatonin)

For people who sleep "normal hours", natural melatonin production rises sharply in the evening, and peaks between 1 am and 3 am. The peaks become smaller with advancing age after early childhood. (Source) The brain may secrete up to 20 times more melatonin at night than in the day time, hence melatonin is nicknamed "the hormone of darkness".

Major regulators of circadian rhythmn

	Suprachiasmatic nucleus (SCN), the body's master clock

	Pineal gland: secretes melatonin

	"Clock" genes in each body cell

The SCN (body's master clock) receives signals from the eye's light-sensing retina. Under night light, SCN instructs the pineal gland to secrete less melatonin. Photo: retina

Scientists found that there are nine "clock" genes in the body cells of rats. Photo: The clock genes glow in the cells of a fruit fly.

Why artificial light at night is harmful to the body

	Affects the function of the suprachiasmatic nucleus (SCN), the body's master clock that is highly light-sensitive.

	Significantly reduces the night-time production of melatonin

1.	Night light affects the function of the suprachiasmatic nucleus (SCN), the body's master clock

	The suprachiasmatic nucleus (SCN) located in the hypothalamous of the brain is the master clock in our body. It generates circadian rhythms in many body functions. The SCN receives signals from many regions of the brain, as well as from the light-sensing retina of the eyes. Light is the most important stimulus that synchronizes the circadian pacemaker to the environmental cycle.

	The SCN sends information to other hypothalamic nuclei and the pineal gland to modulate body temperature and production of hormones such as cortisol (a stress hormone) and melatonin.

	Scientists have identified nine "clock" genes in the body cells of rats, with some of the equivalents also identified in human beings (Source; More info). The "clock" genes in each body cell receive instruction signals from the suprachiasmatic nucleus (SCN), the body's master clock. (Source)

	These "clock" genes control the daily fluctuations of enzymes and hormones that affect the timing of the functions, division, growth and repair of body cells. Therefore the body temperature, immune responses, digestion, visual and mental sharpness, and pain tolerance of the body undergo cyclical changes throughout the day.

2.	Night light significantly reduces the night-time production of melatonin

	With exposure to artificial light at night, the suprachiasmatic nucleus (SCN) would instruct the pineal gland to reduce secretion of melatonin significantly.

	In an experiment conducted in the Bassett Research Institute in New York, blood samples were drawn from female medical students at three different times: during the day, at night after dark conditions, and late at night after the women were awakened and exposed to bright light for 90 minutes.

	It was found that blood drawn in the day time and after the women were exposed to light at night had very low melatonin levels, while blood drawn in the dark was rich in melatonin. (Source)

Night shift work increases breast cancer risk. Photo: breast cancer cell (middle)

The higher the number of years in night shift work, the higher is the breast cancer risk.

Night shift work increases prostate cancer risk. Photo: prostate cancer cells (right side)

Research studies: night shift increases the risks of breast cancer and prostate cancer

Research studies in Denmark and the US on breast cancer and night shift

Exposure to artificial light at night suppresses the production of the melatonin hormone. Low levels of melatonin may either stimulate the growth of cancerous cells in the breast, or increases the production of the female sex hormone called oestrogen. Abnormally high level of oestrogen has been widely linked to breast cancer in various studies.

Remark: For blind women, they are not affected by exposure to light. In a research study conducted in 1998, it was found that blind women had approximately 20 - 50% smaller risk of developing breast cancer than women with normal vision. (Source)

In a research conducted by the Institute of Cancer Epidemiology in Copenhagen of Denmark, researchers studied the medical and employment history of 7,035 women between 30 and 54 years old who had been diagnosed with breast cancer. The key findings are: (Related News: Dec 30, 2000)

	Those women working night shifts were 1.5 times more likely to have breast cancer than the daytime workers.

	The longer a woman regularly worked at night, the higher the risk of developing breast cancer.

In a research conducted by the Fred Hutchison Cancer Research Center in Seattle, USA, 814 breast cancer patients were interviewed on how often they worked night shifts (i.e. starting work after 7 pm and stopping before 9 am) during the 10 years before the diagnosis of the disease.

It was discovered that the more night shifts per week, and the more years the women had spend working at night, the higher the breast cancer risk. (Related News: Oct 17, 2001)

In a Nurses' Health Study conducted by the Harvard University, a total of 78,562 female nurses were interviewed in 1988 on their health status, and were followed up for the next 10 years. During this period, 2,441 nurses (3.1%) were diagnosed with breast cancer. The breast cancer risk was related to the number of years working in night shifts. (Related News: Oct 17, 2001; Web page on Nurses' Health Study)

For those who worked had at least three rotating night shifts per month:

	For 30 years or more: they had 36% higher risk of breast cancer than those who worked day time only.

	For 15 - 29 years: they had 9% higher risk of breast cancer than those who worked day time only.

	For 1 - 14 years: they had 8% higher risk of breast cancer than those who worked day time only.

Research in Japan on prostate cancer and night shift

In a research conducted by the University of Occupational and Environmental Heath in Japan, researchers studied the lifestyle factors of 14,052 male workers between 1988 and 1990. Among them, 11,269 (80.2%) were day workers, 982 (7%) were night workers, 1,801 (12.8%) were rotating-shift workers.

During a follow-up period of 8 years, 31 cases of prostate cancer were reported among these workers. After adjustment for age and other possible risk factors, researchers found that: (Related News: Sept 14, 2006)

	Rotating shift workers had a 3-fold increase in prostate cancer risk than daytime workers.

	Night workers also had a slight increase in prostate cancer risk than daytime workers.

Switch off the lights. Pull the blinds and curtain to sleep in a darkened room. This can increase melatonin production during daylight hours.

Turn off all electronic products in the bedroom before sleeping. Ideally, do not keep any electronic products in the bedroom.

Tips to night shift workers:

	Sleep in a darkened room once they get off from work. Shift-workers who sleep in darkened rooms with their eyes closed can increase melatonin production during daylight hours.

	Try to avoid frequent flipping between day time and night shift work.

	When working on night shift, dim the light as much as possible.

	Try to reduce the frequency on night shifts as much as possible. For example, cancel some non-critical night shift duties. Reschedule some night work to be completed during the day shift. Assign as few people to be on duty at night as possible, if any at all.

	Note: The colour of light that has the least effect on melatonin production at night is red. (Source) However, this is not the usual colour of night light in the working environment.

Tips on a good night's sleep in general:

	Sleep early. Ideally, go to bed before 9 pm for primary school children; before 10 pm for secondary school children; before 10:30 or 11 pm for adults.

	Get a dark night's sleep. Turn off all the lights, and pull the blinds or curtains to screen off any street lights.

	Do not install any night light in the bedroom, even for young kids.

	Keep the pillows, mattress, duvets and bed sheets clean. Frequently wash them to remove the dust mites, moulds and bacteria growing on them. Use dust-mite proven pillow cover and mattress protector.

	Turn off all electronic products in the bedroom, including mobile phone. Ideally, do not keep any electronic products in the bedroom. The electromagnetic waves generated by these electronic products may interfere will those of the human body.

Long haul flight to different time zones upsets the body's biological clock, incurring a heavy toll on both the passengers and airline crews.

Night time traveling causes a lot of stress for both the drivers and the passengers. A lot of car accidents occur at late night.

Fast food should be avoided, especially in late night. Our digestive system should take a rest at this time, as well as the workers in fast food stores.

It is a time to review whether it is worth to work overnight to get the newspaper printed in the morning. Most of us rely on electronic media to get the time-critical news anyway.

With wide-spread usage of refrigeration, it may not be necessary for the fresh meat and produce suppliers to work in midnight to get the products available in the market at 7 am.

Reflections on the prevalence of night shift works - Do we need them?

In developed countries, nearly 20% of the working people are working night shifts. This is equivalent to millions of workers worldwide. The major industries that engage a lot of night shift staff include:

Industries

Workers on night shift

1. Medical and health care

Doctors, nurses, other hospital and clinic staff, paramedics, workers in nursing home and rehabilitation centre

2. Emergency services and disciplined groups

Police, firemen, customs and immigration officers, workers in prisons

3. Public utilities

Emergency repair units (telecom, gas, electric and water companies etc.)

4. Transportation

Pilots, airline crews, taxi / bus / lorry / public vehicle drivers, sailors, 24-hour operated container terminal workers, airport / railroad workers

5. Publishing and media

Workers in newspaper, TV or radio station, and publishing house

6. Finance

Stock or forex trader

7. Meat and produce suppliers

Slaughter house workers, wholesalers of fresh produce and fish (start working in the very early morning in Asian countries)

8. Retail

24-hour pharmacies, convenience stores, fast food stores, serviced gas stations etc.

9. Entertainment

Night clubs, pub, karaoke, cinema, casino

10. Security and property management

Security guards (residential and commercial buildings), emergency repair (plumbers, electricians etc.)

11. Tourism

Workers in hotels, inns and lodges

12. Others

	Certain factories with 24-hr operation, e.g. spinning mills, wood pulp mills, oil refineries etc.
	Translation company (24-hour service to global customers)
	Information technology workers (24-hr support to computer systems)
	24-hr service hotlines (e.g. suicide prevention, report on credit card loss etc.)
	Outreaching social workers

Man is a diurnal animal

Human beings is born to be a diurnal animal (i.e. function during the day time). However, in order to maintain the so called civilised structures of our modern society, we sacrifice the basic human right of millions of people worldwide to sleep at night.

In return, most of the night services that we get are either not time-critical or what we should not have at all. For example:

	No one should be travelling late at night, except for emergencies, as our brain should be taking a rest instead of enduring the stress of driving a car or travelling as a passenger.

	We should be sleeping instead of "enjoying" entertainment in a night club / pub in mid night. Forcing our bodies to stay awake at odd hours is more like a torture than an entertainment.

	From 11 pm to 6 am, we should not eat any food, particularly fast food with high fat and calorie. Our digestive system needs to take a rest during this period.

	It is not absolutely necessary for us to get a piece of newspaper at 8 am in the morning. Making the newspaper available a few hours later make no big difference, as most of us rely on electronic media (e.g. internet, radio, TV etc) for the time-sensitive news anyway.

	We should earn a living during the day, when our brain functions the best, instead of burying ourselves overnight in a dealing room to trade stocks or foreign currencies in another time zone.

	Why can't we make the fresh meat and produce available at around 11 am in the market instead of 7 am. For any consumption in the morning before that, we can simply buy the products the day before and put them into the refrigerator for just a few hours.

Is the invention of light bulb a blessing or a curse? Our precious resting time at night has now been stripped to a rock bottom level.

To protect public health, night shift works should be regulated to prevent non-critical assignments.

Early to bed, and early to rise, makes a man healthy, wealthy and wise.

Night shift work is against human nature, hence should be regulated to protect public health

Is the invention of light bulb a blessing or a curse? In the modern society, not only does man has to work painstakingly during the daytime, but the precious resting time at night has also been stripped to a rock bottom level.

If there is a choice, no one with sanity would choose a night shift work, as it is against our basic instinct and physiological need to sleep at night. Taking into account the negative effect on health, the real salary of many night shift workers may actually be extremely low, or even negative.

Nowadays, the prevalence of night shift work in even non-essential and non-time-critical industries, such as 24-hour convenience store and fast food chain, is a vivid example of company bosses and big corporations trying to maximise profits at the expense of the precious health of their employees.

Health is priceless, something that money cannot buy. It is time to revisit our priorities and treat ourselves in a more humane way - simply go to bed at night according to our natural biological clock.

To protect public health, government bodies should review the labour policies and prevent the engagement of employees in night shifts where not absolutely necessary. In addition, if all of us turn off the lights before 10 or 11 pm and go to bed, it would save a lot of unnecessary electricity consumption, hence much better off for the environment.

"Early to bed, and early to rise, makes a man healthy, wealthy and wise." - an American proverb

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