Shift working, particularly night shift working, has for many years been recognised as being harmful to health. For the lead author, the impact of night shift working is an area of interest as they have predominantly worked night shifts for over a decade. In 2020, the World Health Organization's International Agency for Research on Cancer (IARC) classified night shift working as being a ‘probable’ class 2A carcinogen (IARC, 2020). In 2007, IARC classified all shift work involving circadian rhythm disruption as probably carcinogenic to humans, but following a meeting in 2019 this changed to specifically target night shift working (Erren et al, 2019). IARC concluded that night shift work is probably carcinogenic to humans based on limited evidence from human epidemiological studies and sufficient evidence of cancer and strong mechanistic evidence in animals (IARC, 2020).
Importance of the circadian rhythm
Shift work is necessary for the provision of health services, especially in hospitals as it is the only way to provide continuous service and care to patients (Turchi et al, 2019). Working at night is inconsistent with the body's biological clock.
Night shift working hugely disrupts the circadian and sleep wake cycles, and disruption of these systems has been proven to be detrimental to health in many ways (Foster, 2022). The cells of the biological clock are present in all organs of the human body, they regulate endogenous biological rhythms, and the body adapts to its circadian rhythm through light stimuli (Szkiela et al, 2021). Melatonin, in particular, functions as a coordinator of biological rhythms by providing the body with vital clues regarding the time of day and the anticipated time of year, while also playing a role in immune-enhancing and anti-tumour activities (Cipolla-Neto and Amaral, 2018).
Circadian rhythm disturbance impacts the endocrine system by changing the rhythm of secretion of certain hormones including prolactin, glucocorticoids, adrenocorticotropic hormone, corticoliberin, serotonin and melatonin (Wright et al, 2013). Melatonin is synthesised primarily in the pineal gland at night and is regulated by the environmental light and dark cycle via the suprachiasmatic nuclei (Kubatka et al, 2018). Exposure to light at night suppresses the release of melatonin and this is associated with disruption of circadian rhythms, a reduction in nonspecific oncostatic (anticancer) effects of the pineal gland and a rise in some hormone levels (Parent et al, 2012).
Melatonin also has an apparent anti-apoptotic signalling function and it is an effective antioxidant that scavenges free radicals and up-regulates several antioxidant enzymes (Kubatka et al, 2018). The impact of living out of synchronisation with our circadian rhythms does not just lead to sleep disruption and tiredness. The further individuals stray from this synchronisation, the more vulnerable they are to chronic health conditions including obesity, type 2 diabetes, cardiovascular disease, cancer, suppressed immunity, dementia and mental illness (Foster, 2022). When lifestyles are aligned with the natural solar day, circadian rhythms allow homeostatic regulation of pathways involved in cell protection (Pariollaud and Lamia, 2020). Most regulatory hormones show strong diurnal rhythms, and some hormones are largely regulated by sleep, some are mainly under circadian influence, others are influenced by light, and some are affected by food intake and fasting (Axelsson et al, 2005). Experimental studies have demonstrated that altered sleep timing, food timing and light exposure typical for shift workers leads to an acute circadian disruption and misalignment of regulatory hormones (Kecklund and Axelsson, 2016). Compared with individuals who work during the day, shift workers are at a higher risk of a range of metabolic disorders and diseases (Lowden et al, 2010).
Sleep–wake cycle disruption
Many studies over the years have demonstrated that night shift workers have shortened sleep duration than day shift workers, and this was more evident amongst workers undertaking permanent night shifts (Sallinen and Kecklund, 2010). Sleep duration, timing and structure varies between individuals (McKenna and Wilkes, 2018). Shift work, and more specifically night shift work, hugely disrupts the normal sleep–wake cycle, leading to shortened sleep duration (defined as 4–7 hours of sleep) and excessive fatigue (Kecklund and Axelsson, 2016). Sleep consolidates memory and disruption of the sleep–wake cycle is linked to an increased risk of developing dementia. Research has shown that during sleep, the glymphatic (glial-lymphatic) system acts as a clearing system within the brain, disposing of beta amyloid, and just one night of missed sleep significantly increases these levels both within the brain and the cerebral spinal fluid, with the middle years, around 40 to 50, being the most affected period of life (Foster, 2022).
Sleep deprivation
Sleep deprivation has been shown to have an extensive list of consequences, and there is considerable evidence suggesting that insufficient sleep causes many adverse medical and mental dysfunctions (Chattu et al, 2018). In the short term these include reduced cognitive performance, feeling less socially connected and empathetic, more anxious, reduced capacity to remember things, reduced sense of humour and disruption of menstrual cycles. It can lead to higher rates of miscarriage and higher rates of relationship breakdowns (Foster, 2022). Longer-term consequences include immune responses, activating the stress axis resulting in raised blood glucose, raised blood pressure and higher rate of infections and cancers (Foster, 2022). Lack of sleep has been shown to increase the risk of premature mortality, specifically sleeping for less than 6 hours has a tenfold greater risk of premature mortality when compared with those who sleep for 7–9 hours (Chattu et al, 2018). Lim et al (2018) also noted that shift work has been associated with higher risks for disturbances in cardiovascular and neuroendocrine functions, various cancers, sleep problems, fatigue and psychological symptoms. Cheng and Cheng (2017) discussed the link between night shift working, sleep problems and poor mental health. There is an increasing body of evidence demonstrating there is a mechanistic overlap between mental health and sleep circuits. Studies into patients with schizophrenia, bipolar disorder and depression have shown that these patients have a disrupted sleep–wake cycle and by partially stabilising the sleep–wake cycle there is a direct improvement in reduced levels of paranoia and hallucinations (Foster, 2022). This evidence suggests that the sleep deprivation that occurs from night shift working is likely to exacerbate certain mental health conditions, suggesting that consideration should be given to the suitability of night shift working in healthcare staff with a current or previous history of the above.
Chronotypes
Differences in individual chronotypes have been shown to impact the health consequences of shift work. Chronotype is defined as individual variations in sleep/wake times, influenced by environmental light, genetics and human development stages (Skziela et al, 2021). Chronotypes consist of two categories: morning chronotypes where people go to bed early and are early risers, and evening chronotypes, which result in people going to bed late and waking late (Merikanto et al, 2013). Those with an evening chronotype have a higher risk of negative shift work health consequences (Skziela et al, 2021). Foster (2022) suggested that consideration should be given to individual chronotypes when contemplating night shift work employment as this can help to modify potential health risks. The direction of the shift rotation has been shown to affect shift workers. Forward-rotating shifts, whereby the night shift follows the afternoon shift, and the afternoon shift follows the day shift, creates more time in between shifts allowing for improved recovery (Sallinen and Kecklund, 2010; Härmä et al, 2019). The change from night shift working results in ‘sleep debt’ and repayment of this sleep debt is required to enable the body to realign with a normal sleep pattern. Sleeping for 90 to 180 minutes at the end of a stretch of nights is suggested as being an optimal amount of time to aid recovery and allow adjustment back to a normal sleep/wake pattern (Ruggiero and Redeker, 2014).
Obesity and weight gain
Obesity and weight gain is a recognised negative impact of night shift working. Observational studies suggest an association between shift work exposure and increased incidence of cardiometabolic disease (D'Annibale et al, 2021). Epidemiologic evidence has shown that short sleep duration is associated with obesity and Chaput and St-Onge (2014) suggested that this is due to the non-homeostatic drive to eat as subsequently there is more time and opportunity to eat, resulting in increased food intake. There is mounting evidence demonstrating that individuals have improved metabolic health if eating is consolidated (Foster, 2022). Nocturnal eating has been shown to cause disturbances of intestinal motility affecting digestion, absorption and utilisation of nutrients (Lowden et al, 2010). Eating at night, rather than in the daytime, has been shown to result in relative hyperglycaemia contributing to an increased risk of type 2 diabetes for individuals, especially in those who habitually eat at night (Davis et al, 2020). Insufficient sleep has also been shown to enhance an individual's vulnerability to overeat, especially in the current obesogenic environment (Chaput and St-Onge, 2014). Several studies have shown that protein-rich snacks should be consumed when working at night as this is easier for our bodies to digest and aids the regulation of blood glucose levels (Foster, 2022). Reducing the carbohydrate content of a meal and replacing it with protein results in a more stable postprandial metabolic response and also helps in reducing nighttime perturbations in glucose (Davis et al, 2020). Often, protein-rich snacks are not readily available overnight, and high-sugar, high-fat snacks are far easier to source and this also negatively impacts food choices.
Night shift work and cancer
Night shift working as a direct cause of cancer is yet to be proven in humans. Accumulating evidence points to an adverse effect on circadian disruption on cancer incidence and progression (Pariollaud and Lamia, 2020). Most epidemiological studies have found that higher night work intensity is associated with a higher risk of breast cancer in women (Skziela et al, 2021). Breast cancer is the most common cancer diagnosed in women worldwide and its incidence continues to rise internationally (Johns et al, 2018). Independent cohort studies of both night workers and shift workers have observed increased incidence of prostate, endometrial epithelial malignancies, and non-Hodgkins's lymphoma (Pariollaud and Lamia, 2020). A study undertaken by Skziela et al (2021) demonstrated that working more than three consecutive night shifts had increased detrimental health effects and working night shifts for more than 10 years, especially in individuals over the age of 40 years, was consistent with an increased incidence of breast cancer and that every additional year of night shifts increased the risk of breast cancer.
It has been suggested that the main factors that contribute to higher cancer incidence in shift workers include the disruption in metabolic and immune homeostasis at the cellular level, both autonomous and nonautonomous (Pariollaud and Lamia, 2020). Muscogiuri et al (2022) added that suppression of melatonin secretion also plays a pivotal role, leading to alterations in circadian rhythm patterns that are detrimental in terms of appetite regulation and dysfunction in metabolic signaling and cell growth in cancer. There is also mounting evidence from experimental observations suggesting that exposure to light at night induces physiological changes that influence tumour development (Parent et al, 2012). Bright light at night, however, can be beneficial as it acts as a stimulant helping to reduce sleepiness and fatigue (Briggs, 2023). Several studies have shown a direct improvement in vigilance and reduction in fatigue following exposure to bright light at night (Alfonsi et al, 2021). Overall, there is accumulating evidence that points to an adverse effect on circadian rhythm disruption on cancer incidence and progression (Pariollaud and Lamia, 2020).
Obesity is another well-documented risk factor for cancer and obesity is also intricately linked to metabolic syndrome. Obesity increases the likelihood of metabolic syndrome, which causes an overall state of inflammation, and this is an additional risk factor for developing cancer (Pariollaud and Lamia, 2020). Although there are several studies linking breast cancer and night shift working particularly among female nurses, little evidence has been accrued regarding cancer at other sites among males; however, there are a few studies that have documented associations with prostate and colorectal cancer (Parent et al, 2012). Other negative health consequences involving night shift working surround the body's immune system. During the night, the immune system is turned down making individuals working at night more susceptible to infections (Foster, 2022). Interestingly, skin permeability alters and at night skin becomes more porous and prone to pruritis resulting in an increased risk of skin infections and impaired healing if the skin is damaged (Foster, 2022).
Increased risk of accidents
Reduced alertness and an increased incidence of accidents, especially for night shift workers, is another well researched area. Many individuals undertaking shift work experience excessive sleepiness whilst they are awake, resulting in an inability to stay alert and focused and has a detrimental effect on performance and safety resulting in workplace accidents and errors (Alfonsi et al, 2021). One well-researched area involves driving ability following a night shift. Many night shift workers drive after a night shift and they are at an elevated risk of drowsiness-related motor vehicle crashes (drowsy driving) due to circadian disruption and sleep restriction (Lee et al, 2016). Long wake episodes coinciding with an adverse circadian phase on a background of chronic sleep loss has been shown to negatively impact driving ability (Mulhall et al, 2019). Night shift work has been shown to directly increase driver drowsiness, degrade driving performance and increase the risk of near-crash driving events (Lee et al, 2016). Sleepiness has been shown to increase reaction time, reduce vigilance, alertness and concentration (Road Safety Observatory, 2011). Approximately 20-30% of all motor vehicle crashes are sleep related, making sleepiness one of the leading causes of preventable deaths on the road (Mulhall et al, 2019). In a national survey published in 2017 involving junior doctors, a startling 57% had had a road traffic collision or near miss on their commute post night shift (McClelland et al, 2017).
Modifiable behaviours
One strategy for mitigating the detrimental effects of sleepiness includes napping during break times. Napping has been adopted as an effective countermeasure to sleepiness and fatigue in many other safety sensitive industries but has not had widespread acceptance in health care (Geiger-Brown et al, 2016). Having a 20-minute nap during the night shift improves a number of functions including psychomotor performance, brain function, energy and mood, improving patient safety (Neville et al, 2017). Naps should be kept to a maximum duration of 30 minutes as this helps to avoid slow-wave sleep followed by a period of grogginess on waking, known as ‘sleep inertia’ (McKenna and Wilkes, 2018). Napping is an evidence-based practice that has been shown to make the workplace safer and reduce drowsy driving on the way home (Geiger-Brown et al, 2016). Having a nap before the first night shift starts also helps reduce sleepiness and fatigue (Briggs, 2023).
Fatigue is a symptom closely related to sleepiness, consisting of feeling tired without energy and is another common complaint amongst night shift workers that can negatively impact patient and staff safety (Alfonsi et al, 2021). The consumption of caffeine has been shown to be beneficial in improving sleepiness and fatigue as it improves several aspects of cognitive performance including orientation, attention and reasoning (McKenna and Wilkes, 2018). A ‘little and often’ approach, consuming 20 mg of caffeine every hour, is advised although consumption of higher amounts has been shown to have adverse effects (Briggs, 2023).
Recommendations for practice
There is an overwhelming body of evidence demonstrating the harmful effects of shift working, specifically night shift working, negatively impacting individuals at an exponential rate. Within the healthcare setting, night-time working is essential in order to provide care. In other industries, working at night is often not essential, it is a practice adopted to improve efficiency for financial gain and given the potential health risks involved, raises questions of whether this practice should be allowed to continue. With an increased section of the population employed in jobs requiring shift work, consistent health and dietary guidance is urgently required to support the health and wellbeing in employes working outside standard daytime hours (D'Annibale et al, 2021). Foster (2022) suggested there is a need to implement frequent health checks for night shift workers and that employers should also be screening their employees to ensure they are suitable for and capable of undertaking night shift work and regularly review individuals for deleterious signs.
Conclusion
For the authors, the discussed findings posed quite a revelation. The vast amount of proven negative health effects was extremely concerning. It is evident that individuals are knowingly being put at risk with no robust systems in place to help improve the health and wellbeing of this at-risk group of workers. Urgent increased awareness among healthcare workers is needed. A review of current systems within the NHS allowing individuals to continuously work permanent night shifts needs to happen as even though 24-hour care is a necessity, rotating posts significantly reduce the overall night shift burden thus proving less harmful. In addition, other changes that would make a significant improvement include ensuring designated naps are encouraged during night shifts and suitable napping spaces are provided. Also, health promotion specific to night-time working wellbeing, offering dietary advice and good sleep hygiene is also an area of focus.
The authors intend to present these findings to their senior management team within their NHS local Trust as well and add to the curriculum of the local higher education institution, in a bid to raise awareness and improve current working conditions.
KEY POINTS
- Within healthcare settings, night-time working is essential in order to provide care, but it is important to be aware of the potential negative effects
- These include short-term effects on alertness and mood but also longer-term impacts on mental and physical health
- Night shift working can lead to weight gain and problems with the digestive system, cardiovascular problems and higher risk of infections
- In 2020 the World Health Organization's International Agency for Research on Cancer (IARC) classified night shift working as being a ‘probable’ class 2A carcinogen
CPD reflective questions
- After reading this article what practices would you consider changing on your next night shift?
- If bright light reduces sleepiness, should we ask hospital managers to equip staff rooms with day light bulbs for night shift workers?
- After reading this article, what do you feel is your individual chronotype?
- Why is melatonin a vital part of the sleep–wake cycle?