Night work and its association with metabolic syndrome

Night work is associated with a host of health risks because of disruptions to the circadian rhythm. Nyran Bird looks at what occupational health professionals need to know.

More than three million people in UK work at night, an increase of 5% since 2013. According the TUC, the number of night workers aged 50+ has risen to almost one million, with 222,000 aged 60+ (TUC, 2018). The night period is generally recognised as 11pm through to 6am, and those who work a minimum of three hours during these hours are classified as night workers.

Male night workers continue to outnumber females (around 600,000 more) but, with an estimated 80% of all jobs in health and social care undertaken by women (Skills for Care, 2010), significantly more women are now working night shifts.

Other occupations that require night-shift work include: nursing and midwifery, protective service occupations, road transport drivers and elementary occupations such as production-line work, cleaning, and stock replenishment at supermarkets. In fact, the number of retail workers employed through the night as part of their main shift pattern has increased by 50% since 2007. It is believed almost 70,000 individuals work primarily at night in the UK retail sector alone (Shiri et al, 2021).

Night work and sickness absence

Working predominantly nights is associated with higher rates of sickness absence among full-time employees (Van Drongelen et al, 2017). It is believed that the cumulative exposure to night shifts increases the risk of adverse health effects, due to the chronic disruption to the circadian rhythm – a 24-hour cycle that is part of the human body clock, running in the background to carry out essential functions (Suni, 2022). Working nights increases the likelihood of circadian misalignment, which can lead to shortened and disrupted sleep, which in turn yields degraded health (James et al, 2017).

There is evidence of a direct link between short sleep and an increased risk of premature death (Cappuccio et al, 2010). A study, which included around 1.3 million participants across Europe, USA and Asia, recorded over 100,000 untimely deaths over 25 years. Working schedules can often mean sleep does not always align with biological signals, which in turn can deteriorate health as well increasing risk of cardiovascular disease, cancer, accelerated ageing and even early death (Cable et al, 2021). Epidemiologic studies consistently demonstrate that habitual short sleep is also a risk factor for obesity, with sleep deprivation leading to a greater probability of unhealthy food consumption, an increased daily calorie intake, and altered meal timing (Spaeth, 2019).

Metabolic syndrome

Metabolic syndrome (MetS) is comprised of a spectrum of disorders and has become a major public health challenge worldwide (Alberti, 2005). Genetic, behavioural, and environmental factors all contribute to MetS (Wolk, 2007), however physical inactivity, sedentary behaviour and obesity lie beneath the growing epidemic. Furthermore, increasing evidence has led to the suggestion that the introduction of artificial lighting and working at night may contribute to the development of MetS (Maury et al, 2010). This prominence of artificial lighting, especially during night-time hours, can have a detrimental effect on melatonin production (a hormone secreted by the pineal gland in response to darkness) and may increase the risk of developing circadian rhythm sleep-wake disorders (Blume et al, 2019). This has led to MetS being dubbed ‘the circadian syndrome’, and it is believed a name-change could play a much greater role in the future of public health (Zimmet et al, 2019).

MetS, which is predominantly higher in men than women (Regitz-Zagrosek et al, 2006) and increases significantly with age (Hildrum et al, 2007), is often diagnosed if an individual has three or more of the following:

High abdominal obesity. Defined as dysfunctional adipose tissue, it is the most prevalent indicator of MetS and a fundamental measurement of cardiovascular disease (Després et al, 2006). Obesity is mostly due to the consumption of energy-dense refined foods combined with the adoption of a sedentary lifestyle, however there may be other causes such as genetics or medical reasons. Statistics on obesity, physical activity and diet found most adults across England in 2018 were overweight or obese (63%) (NHS Digital, 2020). Night-shift workers have almost three-times higher association with abdominal obesity, independent of age and gender, than daytime workers (Burm et al, 2020).

Hyperglycaemia. A Greek term made of three components – hyper (high), glykys (sweet/sugar) and haima (blood) – it is more commonly referred to as high blood sugar (Mouri et al, 2021). There is mounting evidence of a ‘correlation between melatonin synthesis and glucose metabolism and energy balance’ (Amaral et al, 2014). A circadian misalignment, due to the cumulative exposure to night shifts can lead to an increased risk of elevated blood sugar, and this is confirmed through research that, when investigating dietary interventions for night-shift workers, found that night work is linked with an increased risk of diabetes (Phoi et al, 2019).

Dyslipidaemia. Defined as an elevation in triglycerides and LDL cholesterol, that is accompanied by low levels of HDL cholesterol (NICE, 2015). A longitudinal retrospective study in the UK found that its prevalence doubled between 2009 and 2019 (Bilitou et al, 2022) and permanent night work is associated with high total cholesterol (Dutheil et al, 2020). Lifestyle behaviours that increase the risk of dyslipidaemia include physical inactivity, obesity, cigarette smoking, and the consumption of food high in saturated and trans fats. Excessive alcohol may also be a contributing factor, as alcohol can dysregulate cholesterol, in addition to elevating triglycerides (Capurso et al, 2016). Night-shift work is strongly associated with a binge drinking disorder, with reasons for overconsumption being: self-medication, as a sleep aid, and as a method for coping with stress (Richter et al, 2021).

Hypertension. Known as high blood pressure, this is when the force of blood against the walls of the arteries is too high (WHO, 2021). The number of individuals between the age of 30 and 79 years with hypertension doubled from 648 million in 1990 to 1.278 billion in 2019 (NCD-RisC, 2021). A recent paper looking at the relationship between night shift working and hypertension stated that blood pressure generally rises in the morning and falls in the evening, leading to some researchers loosely referring to this phenomenon as the ‘circadian pattern of blood pressure’ (Douma, 2018). High blood pressure is prevalent among night-workers (De Leew, 2022), and those who have poor daytime sleep are at greater risk (Reigel et al, 2019).

The role of occupational heath

Night-shift work is significantly associated with the risk of MetS (Wang et al, 2014), with reports suggesting the prevalence is between 5% and 20%. One specific finding from an eight-year study concerning hospital night-shift staff confirmed that ‘working nights is associated with metabolic risk factors’ (Cheng et al, 2021). Whilst night-shift working provides numerous business advantages, including increased operational flexibility, working through the night poses considerable health risks to its employees, and employers must do more to ensure their workforce remains well.

The HSE publication Managing shift work states that employers are required to, in line with the Management of Health and Safety at Work Regulations 1999, undertake an assessment of the risks to employees from work activities, which include the number of hours worked and how these hours are scheduled. This may only be a questionnaire, but must be written by a qualified health professional and, whilst employers must also offer a free health assessment to all night workers, this does not have to be accepted by the employee.

Occupational health professionals are key players in assessing the risk of night-shift working, not only to physical health but also the impact it has on mental and emotional health, as well as its effect on energy, sleep quality, appetite, reliance on stimulants/sedatives to function, and the impact it has on social and domestic life.

OH screening provides employers with essential information on whether night shifts are causing harm to health, helping to decide how best to make reasonable adjustments for the individual.

References

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