Night shift work has been a common feature of industrial economies for decades, and it has long been known that working through the night can negatively impact upon health. In recent years, the evidence base about these health impacts has expanded considerably. Here, Professor David Ray introduces this evidence and highlights how employers and policymakers should be paying increased attention to this issue. 

• There is a growing body of evidence for the adverse mental and physical health impacts of night shift working
• Recent research has revealed a wider number of health conditions that are related to night shift work, including a number of cancers
• The implications of this new evidence for employers, policymakers, and workers, makes a wider conversation about managing and mitigating the impacts of night shift work essential

Many people now work night shifts – in developed countries it is one in five. There have been concerns about the impacts of night shift work in mental and physical health, but until recently a causal link was elusive. There are many reasons why people work nights, including increased pay rates, lack of choice, or family responsibilities. Shiftwork is associated with other health risks, including smoking, and lower socioeconomic status.

Nightshift work increases the risk of mental health issues, including mood disorder, and sleep disorder. In addition, there is an increase in the risk of metabolic diseases including obesity, and diabetes, malignant diseases, including breast and prostate cancers, and inflammatory diseases, including asthma.

New information identifies that night shiftwork carries a clear, and significant risk to mental and physical health, even after other factors such as smoking, have been taken into account.  Importantly, we now understand why shiftwork carries the risk. This is misalignment of the biological clock in the worker with the external light-dark environment. This is a major advance, as it offers a rational target for intervention to protect the health of nightshift workers.

The negative health impacts of night shift work

Nightshift work is unpleasant, and typically attracts higher rates of pay in compensation. Some nightshift workers are unable to continue in role due to immediate difficulties in coping.

Adverse mental, and physical health outcomes have been associated with nightshift workers for many years, but as a group nightshift workers have been marginalised, and there is a lack of evidence-based advice to help nightshift workers with immediate problems associated with fatigue, sleep disorder, and nutrition; and a complete lack of health surveillance, and harm mitigation to protect workers from developing serious, long-term health issues such as cancer.

As 20% of the working population work shifts, and the need for out of hours work is set to increase rather than decrease, it is essential that research efforts are directed to protect the health and well-being of nightshift workers.

A further pressing need is ensure immediate safety for the worker, and the safety implications of their work. It is striking how major industrial accidents tend to occur during the nightshift; Chernobyl, Texas City, Exxon Valdes. Recognition and management of these immediate risks have important implications for safe night shift work, and the potential to avoid massive liabilities, and reputational damage resulting from accidents occurring on the night shift.

The journey home after nightshift is also a dangerous time interval, with very many near-miss incidents reported, including sleeping while driving.

Recent research shows wider health impacts of shift work

Nightshift work has increased in prevalence with the advent of the 24 hour society. With the general reduction in work-related injuries, and associated illnesses, the implications of nightshift as a risk independent of other risks, eg air pollution has been raised.

Advances in population health science, coupled to larger cohorts of nightshift workers, has allowed the contribution of nightshift work to be measured. For example, many studies now report increased risk of breast cancer in night-working women.  Following this association further studies have sought to identify how nightshifts could cause cancer. Here, animal models subject to simulated shiftwork changes in the light-dark were also found to have increased risk of cancer, and faster progression of breast cancer, for example. These studies help to link the observations made in nightshift workers to a mechanism of action, which might offer ways to intervene, and reduce the risks of nightshift work.

It is thought that the core body clock exists to maintain alignment with cycles in body function with the external light-dark cycle as we move through day and night. Nightshift work involves living against the clock, with activity, and light at the time our body clock would be expecting dark, and sleep. This misalignment also occurs, to a lesser extent, in people with extreme chronotypes; chronotype referring to preference for morning, or evening activity (morning larks, or evening owls).

Sure enough people with extreme chronotypes also show an increase in risk which provides independent evidence that it is the circadian misalignment that is the relevant factor in nightshift work responsible for disease risk, for example. This new understanding is very important, as it now offers a clear path to find people with increased risks associated with shiftwork, who may need to consider that work schedule, or undergo additional health screening. It also offers the chance to develop interventions that will reduce the risk of nightshift work.

Implications for policymakers, employers and workers

Very many people work shifts, and the need for work out of hours is not going to diminish. There are implications for employers, in terms of avoiding excess accidents at work, or in transit between work and home. Here we see a wide role for the HSE in protecting the health and well-being of nightshift workers, and reducing the risks to others, eg road traffic collisons during commuting. It is striking that no effective mitigating strategies are in place to protect society from the adverse impacts of necessary nightshift work.

For shiftworkers there is massive unmet need helping them to manage their shift schedule, mainly in terms of coping with sleep, and eating against their internal body clock. The emergence of strong evidence that night shift exposures increase the risk of physical illnesses including cancer, metabolic diseases, and inflammatory conditions is new. We propose that nightshift working should be recorded and managed by employers through updated occupational health functions.

The development of these diseases is irreversible, and so prevention is very important. For example, in the Netherlands breast cancer is a recognised industrial disease for female nightshift workers, with implications for compensation.

Directing effort to mitigating the risks of nightshift work will be cost-effective, and offer immediate improvements in well-being, and reduce longer term health risks.

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