Health Risks Of Night Shifts

One of the news stories in 2016 was about health risks of night shifts. The Bureau of Labor Statistics reported in 2000 that 15 million workers (16.8 % of the working population) were doing alternative shifts (night shift work mixed with daytime shifts). In 2016 they reported 14.8% were working alternate shifts. Among blacks, Asians and Latino Americans the percentage of working alternative shifts was higher, namely 20.8%, 15.7% and 16%, respectively.

Shift work is more common in certain industries, such as protective services like the police force, food services, health services and transportation.

Evidence of health risks of night shifts

There are several publications that showed evidence of health risks of night shift workers. Here is a random selection to illustrate the health risks of night shifts.

  1. A study from 2015 examined the sleep patterns of 315 shift nurses and health care workers in Iranian teaching hospitals. They found that 83.2% suffered from poor sleep and half of them had moderate to excessive sleepiness when they were awake.
  2. This South Korean study examined 244 male workers, aged 20 to 39 in a manufacturing plant. Blood tests from daytime workers were compared to night shift workers. Inflammatory markers like the C-reactive protein and leukocyte counts were obtained. Night shift workers had significantly higher values. The investigators concluded that shift workers have increased inflammatory markers. This is a sign of a higher risk of developing cardiovascular disease in the future.
  3. A Swedish study found that white-collar shift workers had a 2.6-fold higher mortality over a control group of daytime white-collar workers.
  4. Another study compared night workers in the age group of 45 to 54 with daytime workers and found a 1.47-fold higher mortality rate in the night shift workers.
  5. In a study from China 25,377 participants were included in a study that investigated cancer risk in males with more than 20 years of night shift work. They had a 2.03-fold increased risk to develop cancer compared to males working day shifts. Women with night shift work were unaffected with regard to cancer.
  6. A Polish study examined hormones and the body mass index (BMI) among 263 women who worked night shifts and 269 women who worked day shifts. When night shift workers had worked more than 15 years at nights, their estrogen levels, particularly in postmenopausal women were elevated compared to the daytime workers who served as controls. The BMI was also increased in the nighttime workers.
  7. Chronic lymphocytic leukemia (CLL): a study in Spain showed that working for more than 20 years in rotating night shifts was associated with a 1.77-fold higher risk of developing CLL. The authors noted that melatonin levels in that group were much lower than in controls that worked only day shifts. Working in straight night shifts did not show higher risks of CLL compared to daytime workers.
  8. In a Korean study from Seoul 100 female medical technologist who worked nighttime had their melatonin levels tested, which were compared to daytime workers.  They measured 1.84 pg/mL of melatonin for the nighttime workers compared to 4.04 pg/mL of melatonin in the daytime workers. The authors felt that this is proof that the diurnal hormone system has been disrupted. When the melatonin level is altered, the circadian hormone rhythm is also changed.
  9. A group of 168 female hospital employees doing rotating nightshift work in Southern Ontario hospitals were compared to 160 day workers. Cortisol production was assessed. Cortisol production in day workers and in shift workers on their day shift was similar. However, shift workers on their night shift had flatter cortisol curves and produced less cortisol. The authors felt that this disruption of cortisol production would explain why rotating night shift workers have a higher risk of cardiovascular diseases.
  10. A Danish study with female nurses followed 28,731 nurses between 1993 and 2015. Daytime nurses were compared to rotating nighttime nurses and the incidence of diabetes was measured. Night shift workers had a risk between 1.58-fold to 1.99-fold when compared to daytime workers to develop diabetes. The risk for evening shift workers was less (between 1.29-fold and 1.59-fold).

Diurnal hormone rhythm behind health risks of night shifts

Your body has its own rules. It rewards you, if you sleep 7 to 8 hours during the night, but it will penalize you severely, if you turn it upside down. The reason is our built-in diurnal hormone rhythm. A peak of melatonin regulates sleep during the night. Melatonin is released by the pineal gland (on the base of the skull) when it gets dark outside. Daytime wakefulness is regulated by the stress hormone cortisol from the adrenal glands. These two hormones inhibit each other, cortisol inhibits melatonin and melatonin inhibits cortisol. All the other hormones are also regulated according to the diurnal rhythm: testosterone is highest in the morning, human growth hormone is highest between midnight and 3 AM etc.

When you work daytime shifts, your diurnal hormone rhythm is unchanged. But if you work night time shifts, your hormones have to adapt. This is very similar to traveling east or west where you cross several time zones. Your internal diurnal hormone system has to adjust to these changes. Typically it takes 1 day to adjust to a 1-hour time zone difference.

In people who work permanent night shifts, the hormone changes stay adjusted and there is no further switching. But most employers want to be “fair” to everybody, so they introduced the rotating night shifts, which as all the publications cited above show is the worst thing you can do. It messes with your diurnal hormone rhythm, and some people never switch completely to the new hours worked. They don’t get enough daytime sleep because the kids are loud during the day etc. The rotating shift workers are running the highest risk of getting cancer, diabetes, cardiovascular diseases, obesity, cancer, leukemia, and they have low levels of melatonin.

Health Risks Of Night Shifts

Health Risks Of Night Shifts


When shift workers work constant night shifts, this is less stressful to our system than the more common rotating shift work. This is where you work night shifts for a period of time, then the schedule switches to day shift, and you keep on rotating. The least health risks are associated with regular daytime work. People exposed to rotating night shifts suffer from poor sleep. They have a higher risk of gaining weight, getting obese and acquiring diabetes in time. They are at a higher risk for heart attacks, strokes and cancer. All-cause mortality is about twofold higher than for workers who work day shifts.

The underlying problem seems to be a disturbance of the diurnal hormone rhythm. Normally this regulates our waking/sleeping rhythm and keeps us healthy. But with nighttime work melatonin production weakens, cortisol production is reduced and hormone rejuvenation during rest periods suffers greatly. This weakens the immune system, allows cancer to develop and leads to chronic inflammation causing cardiovascular disease and diabetes. The remedy to prevent this from happening is to catch little naps whenever you can during the day and, if at all possible, work daytime shifts permanently.


LED Streetlights Can Be Harmful

The American Medical Association (AMA) has just released a new policy recommendation and statement that LED streetlights can be harmful. After meticulous reviews of the research on the effect of white LED streetlights and their effect on health the AMA decided that it needed to publish its recommendations so that city fathers have some guidelines for illuminating their cities.

Cost and energy savings from white streetlights

Many cities in the US have already switched from the yellowish traditional streetlights to the high-intensity LED lighting. This was done unilaterally without consulting of health professionals or the public. The cities wanted to save money by using white LED lights. But as we will discover below there are serious health consequences to this.

Historic lighting solutions

We did not always have streetlights. The intensity of light is measured by the Kelvin units (K). The color composition is called the “color temperature” (CT). Before electricity was invented we burnt candles and wood at night. This produced a light of 1800 K, which was yellow and red with hardly any blue. Next came incandescent light bulbs. They have a color temperature of 2400 K, which contains less blue and more yellow and red. The new light source is the LED light, which contains a lot more blue color and appears to us as white light. The color temperature is around 4000 K to 5000 K.

LED streetlights can be harmful to your health

Who would have thought that switching streetlights could have anything to do with your health? The newest LED street lights are emitting large amounts of blue light. Our eyes perceive this as white light, but it also creates night-time glare, much more so than conventional lighting. Eye discomfort from blue-rich LED lighting can interfere with visual acuity and cause road hazards. However, this is not all. The blue-rich LED streetlights happen to operate at the wavelength that suppresses melatonin during the night. It has been estimated that the white LED lamps cause interference with the important circadian sleep rhythm. This effect is much stronger with the white LED lights compared to the conventional streetlights. The interference with the sleep rhythm means that there is less sleep time. Sleep quality is perceived to be unsatisfactory. There is daytime sleepiness and impaired daytime functioning. White light LED lights can even cause obesity.

Animals can also be affected by white LED light. The bright outdoor LED lights disturb many species that need a dark environment. Poorly designed LED lighting disorients some birds. Insects, turtles and certain fish species have also been found to be disturbed in their habitat by white LED lights. US national parks have adopted lighting designs that reduce the effects of light pollution on wildlife. So far nobody has shown any concern, what light pollution does to humans!

AMA guidelines to reduce impact of white LED lights

The AMA has come out with new guidelines to help reduce the impact of white LED light. Instead of high-intensity white streetlights the AMA recommends to communities to use the lowest emission of blue light possible to reduce glare. The AMA has recommended a color temperature of no more than 3000 K. This type of light has more of a mix of blue, green, yellow and red in it and very little white. In contrast the CT of bright LED streetlights have 4000 K to 5000 K. This contains a lot of blue light, which appears as white to the eye. Seattle and New York belong to the cities that recently had their streetlights retrofitted to the white LED streetlights. According to the AMA guidelines these lights should now be refitted again. The AMA also recommends that the lights should be shielded to minimize glare and the detrimental effect on humans and the environment. There should also be a possibility of dimming the lights in off-peak periods.

Evidence from medical literature that LED streetlights can be harmful

Here I am reviewing what has been found in the medical literature about white LED lighting that is now preferably used in street lighting.

  1. A Chinese lab tested epithelial cell lines derived from human lenses under a variety of light sources. The lab had previously established that oxidative stress on epithelial cells from human lenses was the most important factor for developing cataracts in humans. Human epithelial cells were cultured in the presence of different LED light frequencies. A high color temperature of 7378 K caused damage to the epithelium while controls of lower color temperatures of 2954 K and 5624 K LED lights did not. The authors concluded that white LED light with a high color temperature could cause significant photobiological damage to human lens epithelial cells. This in turn can cause cataracts and legal blindness.
  1. Another research group studied human epithelial pigment retinal cells in tissue culture. They exposed the tissue cultures to various light frequencies, namely white, blue, green or red LED light. Reactive oxygen and DNA damage were assessed. The white light showed the highest amount of damage to the human retinal cell in culture.
  1. In a trial on senior long-term care home residents researchers found that intermittent exposure to bright light (versus red light) showed positive cognitive effects on the seniors. It is not clear why with respect to cognitive function there is a positive finding using white LED lights, while the other tests mentioned above showed negative effects on the lens and the retina of the eyes.
  1. In another investigation blue light filters were employed in night shift workers and the investigators detected beneficial health effects. In animal experiments the same results were found.

Normally in shift workers cancer, heart disease, metabolic disturbances, depression, anxiety disorders and reproductive problems are found.

LED Streetlights Can Be Harmful

LED Streetlights Can Be Harmful


Despite the evidence in the medical literature showing that yellow lights are better than the newer white LED lights, cities have been converting the yellow streetlights to white LED streetlights. It makes economic sense, but harms people. The American Medical Association is attempting to convince authorities to use common sense. The AMA wrote guidelines and hopes that city fathers will reconsider their decisions. If you are concerned, voice your opinion by talking to your political representative.

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Light Can Interrupt Your Circadian Rhythm

A light bulb company from Florida has decided to put warning labels on the light bulbs they manufacture to tell you that artificial light can have health consequences: light can interrupt your circadian rhythms.We do not easily see that it should matter whether you use artificial light at night or not.


What we do know is that in the evening when we close our eyes and shut out the light the melatonin production gets elevated, we get sleepy, and we fall asleep. During our sleep the immune system receives a boost from the higher melatonin blood concentration, while cortisol takes a rest and levels are lower overnight. Melatonin is also a powerful anti-cancer agent and this would fit in with the study that found that a loss of the clock gene in shift workers was correlating to a worse prognosis regarding their breast cancer.

There are other diseases that can develop when the circadian rhythm is not maintained. Here are a few examples: neurodegenerative disease, cancer, depression, and sleep disorders.

This link shows how the internal central clock in the suprachiasmatic nucleus of the hypothalamus is responsible for keeping time inside of us. The suprachiasmatic nucleus is situated just above the optic chiasm, hence the name.

The clock gene influences the peripheral clock via the clock genes in each organ to be synchronized. If you disregard your internal clock, expose yourself to prolonged artificial lighting and delay going to sleep in time you will create a disorganized central/peripheral rhythm, which weakens the immune system, disrupts your normal hormone rhythms, and ultimately this can lead to disease.

Breast cancer from interrupted circadian rhythm

Women in California were followed with regard to developing breast cancer and nightly exposure to artificial light.

In this study the authors found a 1.34-fold higher breast cancer risk in premenstrual women exposed to high levels of ambient light at night compared to women who were not.

Similarly, a 2014 study showed a difference with regard to breast cancer rates in women who were working night shifts and women who worked normal hours.

The study showed a loss of clock genes in shift workers with breast cancer that was associated with a worse prognosis of their breast cancer compared to those who were not shift workers and had normal clock genes.

Is it a good idea to tell people that light bulbs can be harmful?

Fred Maxik, the Florida based Lighting Science Group chief officer thinks it is a good idea. Dr. Paolo Sassone-Corsi, the director of the Center of Epigenetics and Metabolism at the school of medicine at University of California Irvine who has authored many studies on the negative effect of artificial lighting on the circadian rhythm also thinks that labeling light bulbs is a good idea. He cautions that there are even more powerful light sources like TV’s and computers that can disrupt the circadian rhythm the later it gets in the evening. “And think about how many people look at Facebook at 2 a.m. That is way more disruptive, but this is certainly a good start; we need to keep increasing awareness in a larger population that light at the wrong time of day can harm you” Dr. Sassone-Corsi added.

It is somewhat nebulous what effects a disruption of the circadian rhythm has in our system. But we do know that sleep deprivation can cause overeating and obesity, memory loss, short attention span, diabetes, depression, car accidents, sudden cardiac arrest and sudden cardiac death due to deadly electrical heart rhythms (ventricular fibrillation).

This 2015 study suggests that melatonin should be used to treat people who are involved in shift work. This will help to reset the circadian rhythm to normal.

Origin of circadian rhythm disorders

Sleep disorders can start in childhood, usually when school starts with structured days and normal bedtime hours, followed by school holidays and weekends where late bedtime hours or irregular sleep habits are the rule. Circadian rhythm disorders are not disorders of sleep quality, but rather disorders in timing of sleep (Ref.1). Circadian rhythm disorders start usually when the child enters school, but can develop as late as in adolescence. 10 to 18% of children and adolescents have circadian rhythm disorders. It is not known why some children find it easy to switch between the irregular sleeping habits of summer to the regular sleeping habits during school days. But others are not able to switch and have problems in school with inattention, daytime sleepiness, irritability, hyperactivity and combativeness. Circadian rhythm disorder tends to persist and can turn into adult circadian rhythm disorder. There are also morning types and evening types (in medical lingo morning chronotypes and evening chronotypes).

Delayed sleep phase disorder

One of the most frequent subtypes of circadian rhythm disorder is the delayed sleep phase disorder. This is what is often found in adolescents who push for a later and later bedtime. The fact that they sleep in until 11 AM or 1 PM reinforces it. They like to shut their windows with a black curtain to keep the sunlight out. At night they like to spend time in front of a computer or the TV. They like to go to sleep only at 1, 2 or 3 AM. They may not be aware what is happening to them: the lack of morning sunlight exposure in the early morning hours of the day leads to a delayed setting of the dim light melatonin onset (DLMO) at the end of the day, which is regulated through the suprachiasmatic nucleus (a part of the hypothalamus). By taking frequent blood or saliva melatonin levels researchers have been able to measure corresponding melatonin levels at the time of the DLMO.

We know from research with astronauts in space travel that melanopsin is produced in the blue-light-sensitive photopigment in the ganglion cell layer of the eye. Melanopsin travels from there along the optic nerve into the suprachiasmatic nucleus, where it helps to set the circadian rhythm for the day.

The day is defined by first opening our eyes when we wake up, getting the first melanopsin dose in the circadian rhythm headquarters of the hypothalamus; two yours before we fall asleep we have the dim light melatonin onset where melatonin is just starting to rise, which makes us gradually tired. Maybe the cave men and women sat around the fireplace and told each other stories. We could listen to soft music in a less brightly lit area.

Treatment of delayed sleep phase disorder

It is important to note that people with a delayed sleep phase disorder (DSPD) do not have a sleep disorder: they have normal sleep at an abnormal time (Ref.1)

Here is how the sleep specialist treats delayed sleep phase disorder.

  1. Exposure to sunlight or to blue LED light at the time of awakening for 20 minutes to 1 hour is key to resetting the circadian rhythm to an earlier point than has been the case. This involves that the child, adolescent or adult has to get used to setting an alarm clock to a desired time in the morning. In order the preserve the resetting of the circadian rhythm towards the evening it is important that from 5 to 6 PM in the evening exposure to the bright lights is avoided. This includes light emission from TV’s, i-phones or computers. The eye would otherwise reset the circadian rhythm via the melanopsin mechanism to a later time.
  2. Melatonin treatment is used to advance or delay circadian rhythms. Melatonin also has a sedating effect, but only about 20% respond to that within 30 minutes by falling asleep. The doses in commercial products override the circadian rhythm effect. Sleep experts use much smaller doses of melatonin to reset the internal clock. Thinking of an adolescent who goes to sleep at 2 AM, the DLMO would be at midnight. To phase advance an individual like that a small amount of melatonin (0.5 to 1.0 mg) would be given at 6 to 8 PM (that is 4 to 6 hours before the DMLO point or 6 to 8 hours before the previous bedtime). The morning exposure to bright light works together with the early evening dose of a tiny dose of melatonin, which by itself is not enough to put the person to sleep at that time.
  3. Supportive sleep hygiene methods: It is important that the parents understand the underlying problem. If necessary, they may have to seek the advice of a sleep expert and discuss the details with him/her. 2 hours prior to bedtime the child needs to be exposed to dim light, which is light that does not have blue light in it. The level of dimness is such that reading is difficult. No TV, no cell phone or I pad is allowed. In this dim light atmosphere melatonin is expressed normally and will be produced and released by the pineal gland in higher amounts. Establish a regular bedtime with which all family members can agree. This is best kept on school days, holidays and weekends. If you would sleep in, you would switch your time machine in your head to another time zone further west and it would be an effort to switch it back! There are many children and adolescents who can switch back and forth easily, but the person with DSPD cannot switch easily and would get stuck again in the familiar late sleeping pattern.

Avoid cola and other caffeinated beverages, including green tea, as they stimulate. The bedroom should be dark, quiet and comfortable. Sound machines have not been shown to enhance sleep (Ref.1).

Light Can Interrupt Your Circadian Rhythm

Light Can Interrupt Your Circadian Rhythm


Circadian rhythm disturbances are more common than previously thought of. There is a certain percentage of children who enter the school system that develop delayed sleep phase disorder. This often stays with them into adolescence and can even carry on into adulthood. Two simple tools have been shown to treat this: early morning light exposure for 20 minutes to 1 hour and a small dose of evening melatonin to reset the circadian rhythm. There likely are thousands of untreated people with circadian rhythm disorders. As not all circadian rhythm disorders are the same it is advisable to seek the advice of sleep disorder expert, if sleep patterns are problematic.



1. John H. Herman, Chapter 5, 35-43. “Circadian Rhythm Disorders”

Principles and Practice of Pediatric Sleep Medicine

Second Edition. Stephen H. Sheldon et al., 2014, Elsevier Inc.