Shift workers, such as doctors, flight attendants, bartenders and police officers, have been found to be at a greater risk of certain chronic diseases.
Shift work has its own demands that set it apart from jobs with traditional working hours. Shift work has its benefits; it can be more convenient from a child care perspective, is sometimes better paid and can allow workers time for other activities, such as study.
However, the medical and scientific communities are continually reporting that shift work can increase the risk of certain disorders and have a negative impact on the overall well-being of employees.
In this article, we take a look at what has been reported recently about the effects of shift work, what reasons could possibly be behind these findings and what people working shifts can potentially do to lower their risks of various health problems.
Shift work in the US
Shift work tends to be classified as any work schedule that involves hours that are irregular or unusual in comparison with the traditional daytime work schedule that usually occurs between 6 am and 6 pm.
The term shift work can, for this reason, refer to working evenings, overnight, rotating shifts or irregular employer-arranged shift patterns.
According to an article published in 2000 by the Bureau of Labor Statistics (BLS), over 15 million (16.8 %) full-time wage and salary workers are employed working alternative shifts. Of these, the most common alternative shifts are evening shifts, with working hours usually between 2 pm and midnight, and irregular shifts with a constantly changing schedule.
In contrast, the University of California-Los Angeles (UCLA) Sleep Disorders Center reports more than 22 million Americans work evening, rotating or on-call shifts.
Recently, the BLS reported that the proportion of full-time wage and salary workers employed working alternative shifts now sits at 14.8%. This figure is supported by a poll conducted by the National Sleep Foundation (NSF) in 2005, which found 14% of Americans work shifts.
While there has been a slight drop in the number of white Americans working these hours - from 16.2% in 1997 to 13.7% in 2004 - the proportion of black, Asian and Latino Americans working alternative shifts has remained largely the same. In May 2004, the percentages for these groups were 20.8%, 15.7% and 16%, respectively.
Shift work is most commonly found within industries that provide services around the clock, such as food services, transportation, health services and protective services like the police force.
At first glance, it appears as though the main factor connecting shift workers is that they work different hours to the typical "9-to-5" routine. However, multiple studies report that there is something else that connects bar staff, long-distance truck drivers, nurses and police officers - an increased risk for certain diseases.
Previous studies reported by MNT
Medical News Today have reported on various studies associating shift work with an increased risk of certain health problems. These associations have ranged from the somewhat predictable to the surprising.
In July 2014, a meta-analysis published in Occupational and Environmental Medicine suggested that shift workers face an increased risk of type 2 diabetes. In particular, people working rotating shifts face an increased risk of 42%.
A 2014 study suggested shift work may raise the risk of type 2 diabetes.
The authors theorized that rotating shifts made it more difficult for workers to maintain a regular sleep-wake cycle, negatively affecting sleep quality and potentially weakening insulin resistance.
Then, in November 2014, another study published in the same journal suggested that shift work could impair the functioning of the brain.
Study participants who were currently working or had previously worked shifts scored lower in tests assessing memory, processing speed and overall brain power than participants working traditional daytime hours.
"The cognitive impairment observed in the present study may have important safety consequences not only for the individuals concerned, but also for society as a whole," wrote the study authors, "given the increasing number of jobs in high hazard situations that are performed at night."
More recently, a study published in the American Journal of Preventive Medicine reported that female nurses working rotating night shifts for 5 or more years could be at an increased risk of all-cause and cardiovascular disease mortality.
In addition, working rotating night shifts for 15 years or more was found to potentially raise the risk of lung cancer mortality.
A quick perusal of these studies indicates another factor that shift workers are likely to have in common - disruptions to the sleep-wake cycle. Shift work can lead to workers sleeping at strange or varying times of day, potentially resulting in reduced amounts of sleep.
But how much of an impact can sleep disruption have on an individual's health?
What are circadian rhythms?
"Some of the most serious and persistent problems shift workers face are frequent sleep disturbance and associated excessive sleepiness," report the NSF.
In 2007, the World Health Organization (WHO) decided to classify night shift work as a potential carcinogen due to its disruption of the body's circadian rhythm. This fact places great importance on circadian rhythms, but what precisely are they?
Jet lag is caused by the disruption of circadian rhythms from crossing multiple time zones.
"Most of the processes that occur in the mind and body follow natural rhythms," state the UCLA Sleep Disorders Center. "Those with a cycle length of about 1 day are called circadian rhythms."
Bodily states affected by circadian rhythms include body temperature, wakefulness and certain hormone levels. These are controlled by the suprachiasmatic nuclei (SCN) situated in the middle of the brain.
The SCN regulate these bodily states according to various times in the circadian cycle, sometimes referred to as the circadian clock, or "body clock." For example, body temperature rises toward the end of sleep and falls at around the time that sleeping normally occurs.
Circadian clocks are determined by a number of factors. Light appears to play a significant role by providing a time cue, which results in blind people sometimes having difficulty with sleep. Circadian clocks and the SCN can also be influenced by factors such as exercise, hormones and forms of medication.
However, there are numerous things that can disrupt people's circadian rhythms and cause them problems. The most common example is jet lag caused by traveling across a number of different time zones.
If an individual was to fly from the US to Europe, for example, their circadian clock would not match up with the time of the country they ended up in. Programed by cues specific to living in the US, the individual would end up feeling sleepy in the afternoon and awake at nighttime, rather than at the expected time for the country they were in.
People that work shifts can experience similar problems to jet lag. These can be due to working overnight, or due to successive rotating shifts restricting the hours available for sleeping.
Symptoms related to jet lag - apart from sleep problems - include indigestion, irritability and reduced concentration. While most people experience a mild form of jet lag twice a year - around the switch to and from daylight saving time, according to UCLA - shift workers are prone to experiencing circadian disruption much more frequently.
Why might shift work increase disease prevalence?
Carla V. Finkielstein is an associate professor of biological sciences at Virginia Tech who is currently researching circadian control of cell proliferation.
When asked why shift work might increase disease prevalence, Prof. Finkielstein told MNT longer working hours and staying awake for many more hours each day mean that bodies are no longer truly synchronized with their environment anymore.
Circadian disruption caused by shift work has been shown to affect blood pressure and blood lipid levels.
"Ideally, our physiology would reset to this new condition," she said, "but it might not be able to do it and the consequences of this are the onset of diseases associated with abnormal circadian rhythmicity."
She added that television, computers and longer hours of social activity also contribute to what is referred to as "social jet lag," as well as many new diseases and disorders that are more prominent in Western societies.
Prof. Finkielstein previously worked on a study finding a protein that regulates circadian rhythms - human period 2 (hPer2) - may protect against cancer development through its interactions with another tumor-suppressing protein.
A study published in Occupational Medicine in 2011, from X-S Wang and colleagues from the University of Oxford in the UK, aimed to investigate the mechanisms behind shift work and chronic diseases with a secondary review of existing studies.
Associations between chronic disease and circadian rhythm disruption were found within the review. In particular, the authors noted that increased exposure to light suppresses the secretion of melatonin, which may increase the risk of cancer in numerous ways, including altering endogenous sex hormone concentrations.
In addition, shift work was noted by the authors to be linked with behavioral and lifestyle factors that are also known risk factors for certain chronic diseases. Various studies have shown that shift workers are more likely to have a high body mass index (BMI), to smoke and experience stress.
At present, however, findings in this area have been inconsistent. "Notably, there have been few large prospective, population-based investigations of the relationship between shift work and cancer or CVD," wrote the authors.
This implies that a lot of research is required before definitive answers can be found - a sentiment that Prof. Finkielstein agrees with. She told us:
"More occupational work and assessments need to be done to identify those key environmental factors that lead to these outcomes so that better prevention strategies can be developed. Imagine if just changing your work habit would result in a successful prevention strategy? This would be an extremely cost-effective strategy for reducing disease incidence."
What can be done to protect shift workers' health?
The majority of people working shifts do so because it is a necessity of the job rather than out of personal preference, evidenced by a report from the BLS.
The UCLA Sleep Disorders Center offer a number of suggestions for people working rotating shifts to try:
- On the last few days of an evening shift, delay going to bed and waking up by 1 or 2 hours. This should make it easier to adapt to a new shift schedule
- Allow extra time to adjust to a new work schedule, without reducing time for sleeping
- Wearing dark glasses or special goggles can help workers avoid daylight and other light cues after finishing a night shift
- Try to stay on the same sleep schedule every day of the week, even on days off, to help align the body clock with the work schedule.
The general message from the experts is that getting enough good-quality sleep and minimizing disruption to the body's circadian rhythms are the best ways for shift workers to stay healthy.
However, it can be difficult to reprogram the body's natural instincts. Based on data that is currently available, Prof. Finkielstein believes that limiting the years spent working at night is the best option for workers to reduce their risk of developing chronic diseases.
"The more exposure to circadian disruption - for instance, by working longer hours at night for longer periods of time - correlates with the onset of more aggressive diseases, from cardiovascular disease to various types of cancers," she told MNT.
Although this field requires more in-depth and focused studies, a growing body of evidence continues to indicate that measures need to be taken to protect a significant section of the population.
In their 2011 study, X-S Wang and colleagues concluded that present and future findings "should ultimately inform employment guidelines and public health interventions aimed at reducing risks among shift workers."