Light at night and health: the perils of rotating shift work

THE PERILS OF ROTATING SHIFT WORK

Obesity has been on the rise in the USA and across the globe¹ and is a major public health concern. In an article in this issue of the journal, Kubo et al (see page 327) show that rotating shift work, and in particular longer term (10+ years) rotating shift work, significantly increases the risk of weight gain and obesity.² This is not the first study to report such an association, but is perhaps one of the most thorough and powerful examinations to date.

Recent developments on the international stage have shone a spotlight on the relationship between shift work and cancer as well as other chronic diseases. In response to mounting evidence in animal research and limited evidence in epidemiological studies, in 2007, the WHO’s International Agency for Research on Cancer (IARC) classified shift work involving circadian disruption as probably carcinogenic in humans (group 2A).³ Denmark, after ruling in favour of breast cancer related disability insurance claims among rotating shift workers, has subsequently set a precedent for being the first nation to recognise shift work as an occupational health risk.⁴ Still, while cancer is one of the bleakest disease endpoints, a broader range of outcomes is seen: increases in cardiovascular risk, peptic ulcer disease, chronic fatigue and various sleep problems, a higher abortion and miscarriage rate as well as lower pregnancy rates, higher rates of substance abuse and depression, a greater number of vehicle accidents and higher body weight due to abnormal eating habits and/or metabolism, have all been reported in shift workers.

Because many of the chronic illnesses that have previously been linked to rotating shift work are at least in part mediated through higher body weight and obesity, data like those reported here by Kubo et al² are important. They further emphasise the need to recognise light at night, as encountered by numerous shift workers, as an emerging occupational risk factor.

MAGNITUDE OF THE PROBLEM

Thomas Edison’s invention of the incandescent light bulb in 1879 ushered in the second industrial revolution, allowing people to extend typical daytime behaviour well into the night. Urbanisation has led to a sharp increase in technology and the use of electricity in all facets of daily life. In order to keep up with the fast pace of the modern city, businesses must stay open longer to remain profitable. Moreover, in certain domains, for example, healthcare or security, 24 h service will likely always be needed. In order to cover a full 24 h day, many businesses have turned to a flexible shift system in an effort to foster employee retention and efficiency. Some shifts may be predominantly worked in the evening or overnight, while others may be rotating. In a rotating shift schedule, the times at which the shift starts and stops will vary, depending on the day, and employees may be required to work several overnight shifts per month in addition to their normal day shift.⁵ The efficiency of these types of schedules has made them very popular in the business world, and the prevalence of rotating shift work has increased dramatically, especially in the last 20 years. In the European Union, the prevalence of working at least one night shift a month is 20%.⁶ In the United States, 15 million people regularly work alternative shift schedules, including evening (4.7 million), night (3.2 million) and rotating shifts (2.5 million).⁷ Thus, millions of people are now up at night, working night shifts or simply staying up late. Yet, these revolutionary changes in lifestyle facilitated by the use of electric light may pose new and unexpected challenges to human health.

Mechanisms linking shift work to health: circadian disruption

The ‘biological clock’ in the suprachiasmatic nucleus (SCN) in the hypothalamus controls numerous functions that follow regular, daily patterns (or ‘circadian’ rhythms), including the sleep/wake cycle, body temperature, blood pressure, renal function, hormone secretion and immune function. In addition, the SCN appears to be crucial for behavioural and physiological adaptation to 24 h food availability. Environmental lighting powerfully influences this circadian system in humans. Light entrains the biological clock, as the SCN receives environmental dark/light information directly from the retina. This mechanism appears to have evolved to detect changes in day length/season (for functions such as migration and hibernation). Melatonin is a hormone and marker of circadian rhythmicity intimately linked to the circadian system that demonstrates both cancer- and cardio-protective properties. Mechanisms for the cancer-protective activity of melatonin relate to its influence on other hormones, as well as melatonin’s antioxidant and immune-modulatory effects which appear to also slow down the ageing process and, more generally, delay mortality. The cardio-protective properties of melatonin affect suppression of sympathetic outflow, vascular relaxation and insulin secretion. This complex circadian system has also a genetic component: mutations in the fruit fly Drosophila melanogaster that affect the circadian clock,⁸ and later, human twin studies of abnormal circadian phenotypes showing that much of morning and evening preference is heritable, led to the identification and characterisation of clock genes responsible for circadian behaviour.⁹

SOLUTIONS AND FUTURE DIRECTIONS

Without altering one’s lifestyle, there are surprisingly few practical remedies to minimise circadian disruption in shift workers, apart from optimising shift schedules. Future strategies that could potentially reduce the health risk associated with shift work-induced circadian disruption include: (1) genetic screening tests, which may identify vulnerable populations, as variations in certain clock (or other) genes may make a person more susceptible to the effects of light at night; (2) the use of exogenous melatonin in night workers, which is being discussed, but whether melatonin is safe in the primary prevention of circadian rhythm disorders induced by shift work is currently unclear; and (3) changing light sources or filtering short wavelength (blue) light (the most disruptive to our circadian system) by having night workers wear goggles, which could offer another prevention strategy, albeit perhaps the least practical. Lastly, determining which factors relate to shift schedules and what aspects of shift schedules are most detrimental to health is the next frontier in shift work and disease prevention. To date, no study has examined how specific aspects of shift schedules including length, frequency of rotation and hours worked per week, interact and relate to health and safety. Moreover, few studies have examined how long working hours influence health and safety outcomes in older workers, women, those with preexisting health problems and workers with hazardous occupational exposures. Future research would benefit from a clear and complete description of work schedules and their effects on human health. In the meantime, data like those presented in this issue of the journal² are needed to solidify our understanding of the underlying mechanisms of the health risks associated with rotating shift work: they may provide practical guidance in health counselling and in the development of more effective prevention strategies for shift workers.