How Does Night Shift Work Impact Health?
Did you know that our ancestors were nocturnal? They used to stay awake in the night to hunt without worrying about dangerous predators and sleep during the day. Now, our circadian rhythms are lined up with the sun. That's why as soon as the sun sets, our bodies start getting ready for rest, and we end up feeling sleepy.
Circadian rhythms are biological cycles that coordinate essential mental and physical functions, such as sleep and hunger. The circadian clock is regulated by a part of the brain called the Suprachiasmatic Nucleus (SCN).
The circadian clock is also influenced by temperature. When the body temperature drops around the afternoon and late evening, it induces drowsiness and sleep.
Sleep is induced by a hormone called melatonin, which is produced in low-light conditions. Bright light conditions during the day suppress melatonin production and promote wakefulness.
People who work the night shift have disrupted circadian rhythm and have an increased risk for the following conditions:
- Obesity
- Cardiovascular diseases
- Mood changes
- Digestive issues like diarrhea, constipation, stomach discomfort, peptic ulcers
- Cancers, especially breast cancer
- Epilepsy in people who are predisposed to the condition
- Sleep problems
- Chronic fatigue
- Depression
Working the night shift is carcinogenic to humans, according to multiple studies conducted the world over.
Several studies show that disruption in the night’s sleep can reduce melatonin levels and increase the risk of tumor growth.
Night Shift Work—A Risk Factor for Breast Cancer
Some animal studies have shown that exposure to light at night led to the growth of breast cancer.
The risk of breast cancer among nurses and other night-shift workers seems to be higher than their counterparts who worked day shifts.
A study published in the Journal of National Cancer Institute in 2001 reported that women who work in rotating night shifts for at least three nights per month, along with day shifts, have a moderately high risk of breast cancer.
Further, the risk seems to be higher when the night shifts per week increase!
This increased risk is attributed to the messed-up melatonin levels in the body.
In addition to promoting sleep, melatonin also stops tumor growth and protects against the spread of cancer cells.
When melatonin levels decrease in the body, it results in an imbalance of inflammatory cytokines, increased mutations in the cells, and oxidative damage (due to free radicals).
These events can all trigger cancer development.
A reduction in melatonin also affects estrogen levels, which further increases the risk of breast cancer.
How Does Genetics Influence Rotating Night Shift Work and Breast Cancer Risk?
A long duration of shift work throughout the years is associated with estrogen and progesterone-positive tumors.
When the circadian rhythm is altered, it changes the expression of the CLOCK genes. This also influences the production of reproductive hormones.
The NPAS2 Gene
The Neuronal PAS Domain Protein 2 or NPAS2 gene is the largest circadian gene. It plays a vital role in sleep homeostasis and circadian rhythm regulation.
This gene also regulates the cell cycle and works with certain other genes for repairing DNA. The NPAS2 gene shows a strong association with breast cancer.
rs2305160 (Ala394Thr) is an SNP (Single Nucleotide Polymorphism) in the NPAS2 gene.
Among women with little or no exposure to shift work, the A allele (AA or AG) is associated with a significantly lower risk of breast cancer.
However, among women with AA genotype who had worked >2 years of rotating night shifts, the risk of breast cancer was nearly 3 fold compared to women with the same genotype with <2 years of night shift work.
| Genotype | Implication - > 2 years of rotating night shifts |
| AA (Thr/Thr) | ~3 fold increased risk of breast cancer |
| AG (Thr/Ala) | Slightly increased risk of breast cancer |
| GG (Ala/Ala) | Normal risk of breast cancer |
Use Xcode Life’s Free Gene Tool To Find Out If You Have The Risk Genotype!
The RORA Gene
RAR-Related Orphan Receptor A or the RORA gene is located on chromosome 15 and regulates genes involved in the body’s circadian rhythm.
rs1482057 is an SNP in the RORA gene. A study published in 2014 showed that SNP rs1482057 was associated with breast cancer in postmenopausal women.
Women who have at least one A allele and had a history of working night shifts in their lifetime had a higher risk of developing breast cancer.
Conversely, women having the CC genotype and working night shifts showed a decreased risk of breast cancer.
| Genotype | Implication |
| AA | Increased breast cancer risk on night shift work |
| AC | Increased breast cancer risk on night shift work |
| CC | Decreased breast cancer risk on night shift work |
The CRY2 Gene
Cryptochrome circadian regulator 2 or the CRY2 gene gives instructions to produce a protein involved in regulating the body’s circadian rhythm.
rs2292912 is an SNP in the CRY2 gene, located on chromosome 11. Night shift working increased the risk of breast cancer in women who carried the CG genotype of rs2292912 SNP.
| Genotype | Implication |
| CG | Increased breast cancer risk on night shift work |
| GG | Decreased breast cancer risk on night shift work |
| CC | Decreased breast cancer risk on night shift work |
Regulating Your Sleep-Wake Cycle To Reduce Breast Cancer Risk
- Reduce your night shift hours per week
Since working night shift hours increases the risk of breast cancer in women, one of the most effective ways to lessen this risk is to reduce working night shifts.
Switching with a colleague’s shift, alternating your night shifts with day shifts, or switching jobs can be a few ways by which you can reduce your night shift hours.
- Get a minimum of 7-8 hours of sleep every day
Apart from disrupting the sleep-wake cycle, disturbed sleep or poor quality of sleep in people who work night shifts can increase their risk for breast cancer.
So, if you are working a night shift, ensure you get your 7-8 hours of sleep every day. If you have trouble sleeping, consult your doctor about supplements that can help you catch up on your daily sleep.
- Reduce other risk factors for breast cancer
People working the night shift must try and reduce other risk factors of breast cancer.
A healthy diet with lots of fruits, limited alcohol consumption and smoking, adequate physical activity, and reduced exposure to harmful chemicals can help reduce breast cancer risk.
Summary
- The circadian rhythm regulates our body’s sleep-wake cycle.
- A disturbed circadian rhythm increases the risk of developing health conditions like obesity, cardiovascular diseases, gastrointestinal problems, etc.
- According to many studies and published reports, working the night shift is carcinogenic as it decreases melatonin production - melatonin helps stop tumor growth.
- Working the night shift has been particularly associated with an increased risk for breast cancer.
- Women with certain changes in genes like NPAS2, RORA, and CRY2 have an increased risk of breast cancer when on rotating night shifts.
- Regulating your sleep-wake cycle, getting adequate sleep, following a healthy diet, and limiting alcohol consumption and smoking can help lower breast cancer risk.
Reference:
- https://pubmed.ncbi.nlm.nih.gov/28541391/
- https://academic.oup.com/jnci/article/93/20/1563/2519563
- https://pubmed.ncbi.nlm.nih.gov/19095474/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3405398/
- https://cebp.aacrjournals.org/content/27/1/25
- https://blogs.cdc.gov/niosh-science-blog/2021/04/27/nightshift-cancer/
- https://academic.oup.com/jnci/article/93/20/1563/2519563
- https://www.frontiersin.org/articles/10.3389/fonc.2020.01006/full
- https://www.uniprot.org/uniprot/Q99743
