The Health Effects of Shift Work

Shift work is associated with a number of health concerns, mainly attributed to changing the body’s natural circadian rhythms. The body’s internal circadian clock adapts slowly to changes in shift schedules and, when you change constantly between day shifts and night shifts, may not get a chance to adjust at all.

Even when a shift schedule consists of nights only, problems still arise due to the way light affects our internal clock, making it difficult to get a proper sleep during daylight hours, and feel alert at night when it’s dark.

The more immediate effects present themselves in the from of a medical condition known as shift work sleep disorder (SWSD).

Shift work sleep disorder is a circadian rhythm sleep disorder, the symptoms of which include:

• Excessive sleepiness when you need to be awake, alert, and productive.
• Insomnia, or the inability to sleep when you need to. This can mean trouble falling asleep, or waking up before you’ve slept sufficiently.
• Sleep that feels non-restorative or insufficient
• Difficulty concentrating
• Lack of energy
• Irritability
• Difficulty with personal relationships

Other health concerns

Aside from the more immediate symptoms above, circadian disruption, and thus shift work, is also associated with many other negative conditions. These conditions often take longer to develop and the connections with shift work are less obvious. 

• Cardiovascular disease
• Cancer
• Diabetes
• Metabolic syndrome
• Obesity
• Depression and mood disorders
• Gastrointestinal problems
• Issues with fertility and pregnancy
• Increased risk of injuries and accidents
• Weakened immune system

For those who would like to read a little further into these health concerns, I have listed below some of the more alarming findings and references to the supporting studies. 

Cancer

The International Agency for Research on Cancer (IARC) stated in a press release[1] that “shiftwork that involves circadian disruption” is considered a Group 2A carcinogen and is “probably carcinogenic to humans”.

A meta-analysis[2] of 61 articles studying the associations between night shift work and the risks of common cancers among women revealed that long-term night shift work increased the overall risk for cancer by 19%. In relation to specific cancers, long-term night shift workers had a 41% increased risk of skin cancer, 32% increased risk of breast cancer and 18% increased risk of gastrointestinal cancer. The same study also showed that for every 5 years of shift work, the risk for breast cancer increased by 3.3%. Nurses analyzed in the study were shown to have even higher numbers: 58% increased risk of breast cancer, 35% increased risk of gastrointestinal cancer and 28% increased risk of lung cancer.

Cardiovascular disease

In a review[3] of 17 studies dealing with shift work and cardiovascular disease risk, shift workers were found to have a 40% increase in risk.

In a study[4] published in the American Journal of Preventive Medicine, researchers found that nurses working rotating night shifts for more than 15 years had a 38% higher risk of dying from heart disease than nurses who only worked during the day. The study also indicated that nurses who worked night shifts for more than 5 years were up to 11% more likely to have died early compared to those who never worked these shifts.

Obesity

Findings in a meta-analysis[5] of 28 studies evaluating the associations between shift work and obesity, demonstrate that shift workers were 23% more likely to be obese/overweight compared to non-shift workers and permanent night shift workers had a 43% higher risk of becoming obese. The same meta-analysis found that shift workers had a 35% higher risk of developing abdominal obesity compared to other obesity types.

A study[6] performed by Di Lorenzo et. al, revealed that the prevalence of obesity was higher among shift workers as opposed to day workers. Shift workers had higher BMI (Body Mass Index) than day workers, and shift work was associated with BMI, independently of age and work duration.

Diabetes

A study[7] performed by Pan et al. compared the association between rotating vs. fixed shifts and type 2 diabetes in 177,000 nurses aged 25–67 years. The risk of type 2 diabetes in participants exposed to rotating shift work for 1–2 years was 5%. This value increased to 20 % after 3–9 years, 40% after 10–19 years, and after 20 years of rotating shifts, participants had a 58% increased risk of type 2 diabetes.

A study[8] of people with type 1 diabetes found that shift work was associated with poorer control of diabetes.

Cognitive decline

A study[9] conducted at Swansea University and Université de Toulouse assessed 3,232 employed and retired workers on tests of speed and memory. The participants were first measured at ages 32, 42, 52 and 62, and then measured again 5 and 10 years later. The findings of the study indicated that shift work was associated with impaired cognition and estimated that working shift work for 10 years resulted in cognitive loss equivalent to 6.5 years of age-related decline. The study also concluded that leaving shift work could reverse the effects, but recovery took at least 5 years.

Depression

5 studies[10] on the effects of working night shifts yielded evidence that there is an increased risk of depression after working the shift schedule for several years. A supplementary meta-analysis of the 5 studies revealed a 42% increase of the risk of depression among persons working the night shift.

A study[11] authored by Carlos J. Pirola PhD, of the Universeidad de Buenos Aires, Argentina revealed that serotonin levels were significantly lower in individuals working rotating shifts compared to those working strictly day shifts. Serotonin is often referred to as the “feel-good” hormone and low levels are associated with anger, depression and anxiety. Serotonin is also believed to play an important role in the regulation of sleep.

Important: Although these studies should prompt people to be conscious of the connection between shift work and depression, it is important to note that the evidence is not strong enough to form any medical recommendations against working shift work for employees experiencing depressive conditions.  

Gastrointestinal problems

In a review[12] of the published literature on the association between shift work and gastrointestinal (GI) disorders, four of six studies reported a significant association between shift work and GI symptoms. Five of six studies showed an association between shift work and peptic ulcer disease and two of three studies showed an association between shift work and functional GI disease.

A study[13] involving 399 nurses revealed that nurses working a rotating shift schedule had a significantly higher prevalence of both irritable bowel syndrome (17%) and abdominal pain (27%) when compared to day shift workers.

Fertility / pregnancy

Research[14] conducted at Harvard University looked at the number of eggs produced by women when their ovaries were stimulated during in vitro fertilization. The data, taken from 500 women, revealed that nightshift workers produced 28% less eggs per session on average. It should also be noted that the same study indicated that women whose jobs involved heavy lifting had 14% fewer eggs.

Immunity

In a study[15] examining the prevalence of common infections among employees in different work schedules, results showed that shift work was associated with a higher risk compared to day work.

In a literature review[16] looking at the effect of shift work on sleep and immunity, researchers concluded that both acute and chronic sleep deprivation are associated with immune changes. And although there is a need for more quality studies, it is likely that shift workers show an increased risk for viral infections because of a possible compromise of the innate immune response and perhaps also of the immune acquired response.

Conclusions

Although this does not include all available studies casting shift work in a negative light, it is important to remember that these studies only reveal a correlation, and do not prove that shift work causes any of the above conditions. Due to the difficulty in conducting such studies, more research is needed to reveal the true mechanisms involved.

Nevertheless, the data is worrisome enough that shift workers should be aware of the various health concerns, and try to adopt healthy lifestyle choices to mitigate them.