By Justine Renziehausen
What is Circadian Rhythm?
According to a 2016 article, the circadian rhythm refers to the 24-hour cycle of biological systems in humans. The suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for regulation of all bodily systems. Light exposure provides one of the primary stimuli to the SCN. This corresponds to the sleep/wake cycle and suppresses the production of melatonin, a hormone that is produced prior to sleep onset. The SCN is also responsible for synchronizing other biological systems (e.g., body temperature, cortisol, etc.).
This alignment may be individualized, where some people tend to wake earlier and have preferences to complete mentally and physically fatiguing tasks in the morning, while others prefer the late afternoon or evening times, according to a 1976 text. These preferences have been shown to correspond to physiological markers of the circadian rhythm such as body temperature, cortisol, and melatonin production.
A 2015 study suggested that when individuals are forced to work outside of their preferred times, adverse health consequences are likely due to changes in schedules and behaviors. A 2016 study also suggested that circadian rhythm disruption, disturbed sleep, altered eating patterns (i.e., eating at non-optimal times for the circadian phase, eating poor food choices for convenience, irregular eating), and altered lifestyle behaviors (e.g., smoking, alcohol use, and reduced physical activity) may contribute to impaired physiological mechanisms in shift workers.
Understanding Shift Work
Shift work refers to the occupational necessity to work outside of typical sleep-wake hours, therefore disrupting sleeping patterns via altered light exposure. Many occupations, such as healthcare professions, manufacturing, and transportation services require employees to work shifts outside of typical day shifts.
Working these shifts, especially for an extended period, has been shown to have both acute and chronic health consequences. Therefore, the purpose of this article is to discuss risks associated with shift work and potential mechanisms for chronic illnesses.
Shift Work and Injury Risk
Shift work and extended shifts in general (more than 8 hours) are typically required in physically demanding occupations according to a 2017 study. The likelihood of injury increases substantially towards the end of a shift but may be further increased for those working outside of their optimal times.
Shift workers are reported to consistently sleep less and have poorer sleep quality than those who work the day shifts. Recent studies have suggested that this fatigue may be responsible for slips, falls, and other accidents resulting in back pain and other injuries.
Sleep contributes to the healing process via growth hormone production and protein synthesis. As shift workers typically have poorer sleep quality and reduced sleep duration, it’s possible that the recovery process may be impaired, and chronic pain may result. Chronic pain and unhealed injuries may also lead to increased sedentary behavior, increasing the risk of chronic disease.
Further, sleep-deprivation has been repeatedly associated with cognitive impairments (reaction time, emotional regulation, and attention), which may contribute to the risk of injury during a shift.
Shift Work and Metabolic Disorders
There are also several chronic illnesses that have been associated with shift work. A 2016 study found that metabolic disorders such as obesity, diabetes, and cardiovascular disease have been commonly linked to shift work.
A 2010 study found that obesity is more prevalent among shift workers, and that time spent working shifts corresponds to higher body mass index. One potential mechanism that may be partially responsible is disruption of the feeding cycle. Activity during normal sleeping hours forces individuals to eat at irregular intervals outside of their optimal hours. This desynchronizes the circadian rhythm, alters hormone levels that are associated with metabolism (insulin), and ultimately affects the typical rhythm of glucose metabolism.
Interestingly, studies from 1993, 2003, and 2010 examined the differences in eating habits between shift workers and day workers and determined that total calorie intake, as well as macronutrient intake, was often not significantly different between the groups. The differences that were commonly noted involved food selection and eating habits, such as irregular meal frequency and higher consumption of alcohol and sweets in shift workers.
Diabetes Risk in Shift Workers
These metabolic disturbances may ultimately lead to other chronic disorders, such as diabetes. Studies have shown that after age 50, shift workers are significantly more likely to experience insulin resistance, which leads to the development of type II diabetes.
One 2014 review discussed the risk of type II diabetes in shift workers. Researchers concluded that all five studies included in the final analysis indicated a greater risk of developing type II diabetes for those who work the day shift (although confounding factors such as age, body mass index, alcohol consumption, and family history should be reported and accounted for).
The development of type II diabetes in shift workers is thought to be a result of changes in meal timing and frequency and altered insulin and glucose circadian rhythm.
Shift Work and Hypertension
Shift work is also associated with the development of hypertension and cardiovascular disorders. Several studies from 2014-2017 have reported increased risk of developing hypertension for shift workers. Additionally, the chance of developing hypertension significantly increases as time spent working non-optimal shifts increases.
For example, a 2017 study determined that the rate of hypertension among individuals with 20 or more years of shift work was 16.4%, compared to 4.6% for those with 10 or less years. This was thought to also be impacted by length of shifts, overall stress, and changes in hormone production.
Melatonin is a primary regulator of the circadian rhythm via light/dark cycles and plays a role in regulating hypothalamic-pituitary-adrenal axis (HPA axis) activity. Not only does melatonin itself impact blood pressure, but so does a hormone produced via the HPA axis (cortisol).
It has been hypothesized that light exposure is ultimately responsible for changes in blood pressure and the development of hypertension, however, it is more likely due to a cascade of events.
Further, a 2010 study suggested metabolic diseases/abnormalities, such as diabetes, may lead to the development of cardiovascular disease/coronary heart disease as well.
Shift Work and Cardiovascular Disease
It has been hypothesized that shift work is also linked to the development of some cardiovascular diseases (CVD), although evidence is limited, as direct causal relationships are difficult to determine. While there is evidence to support a link between shift work and coronary heart disease (CHD), no studies to date have established the same relationship with CVD overall.
One 2010 review discussed potential mechanisms for the development of CVD, to describe pathways that are connected to shift work and disease. Along with increased stress, less sleep, and less time to recover from strenuous occupational demands and fatigue, the authors discussed behavioral changes that may play a role in disease development such as smoking and altered nutrition.
The review showed that the prevalence of smokers amongst shift workers is between 10-40%, although that number has decreased in recent years. It was postulated that smoking is a strategy to feel more alert while on the job. Additional studies have shown that shift workers tend to choose less nutrient dense foods, eat out of convenience, and eat at irregular intervals. It was also found that they also have higher cholesterol and triglyceride levels.
As previously mentioned, there are also relationships between shift work and the development of hypertension and type II diabetes. As smoking, hypertension, prolonged stress levels, high cholesterol, high triglyceride levels, and metabolic syndrome are known to be risk factors for the development of CVD, it may be assumed that shift work and CVD have some relationship, although there may be other, confounding factors.
Individual Differences in Shift Work Impact
Individuals may also be affected by shift work differently based on several factors such as age, genetics, chronotype (preferences for certain time of the day), and physical activity levels. The length of shifts, years of shift work, and shift rotations may also impact the severity of consequences.
For example, a 2015 study found those who worked only night shift as opposed to those who worked rotating shifts had higher odds of risk ratios. Future studies examining both effects of shift work and potential intervention strategies should consider all potential confounding factors.
Additionally, many of the cardio-metabolic disorders are related to each other, and the development of chronic diseases may be due to a series of events and prolonged stressors. Future research should account for years spent working shifts.
Intervention Strategies for Shift Workers
Although shift work affects many people, only a limited number of intervention strategies have been studied to mitigate adverse health effects. Melatonin supplementation has been considered to reduce some of the negative health risks and improve sleep.
A 2014 study suggested that there is evidence to support improved sleep with melatonin compared to a placebo in nurses. The study also suggested that caffeine supplementation may improve alertness while working, but there is no evidence that this will impact acute or chronic injuries/disease.
Additionally, exercise interventions have been shown to significantly improve BMI and blood pressure, which may decrease the risk of the development of metabolic and CVD. Interestingly, it was also shown that increased physical activity was associated with increased consumption of fruits, indicating potential relationship between activity level and healthier food choices.
Final Thoughts
The circadian disruption caused by shift work is likely to result in several adverse health effects. Extended time in circadian disruption may worsen symptoms and lead to the development of chronic diseases.
Although currently limited, intervention strategies should be evaluated to help mitigate the effects of shift work. As exercise interventions may be a promising strategy, various forms of exercise and exercise timing should be further evaluated.
Future studies should also be sure to include samples across all age groups and sexes, as these groups may have different responses.
