Dying for a good night's sleep

Abstract

Modern lifestyles and living environments adversely affect not only diet and physical activity, but also sleep habits, relaxation time, and stress levels. These other features of modern living also affect metabolic health, but have been largely neglected in terms of planning interventions and public health policies to tackle obesity and type 2 diabetes. Over the past few decades, there has been a global shift from rural to urban living—more than half of the world's population now lives in urban areas, whereas 25 years ago less than 40% did so. Urban living alters not only patterns of diet and physical activity, but also sleep patterns. As discussed by Sebastian Schmid and colleagues in a Review in this issue, short duration and disruption of sleep can be detrimental to metabolic health. Disordered sleep–wake cycles—such as in shift workers—might lead to weight gain and metabolic disease, at least partly through neuroendocrine dysregulation that promotes increased appetite, enhanced sensitivity to food stimuli, and, ultimately, overeating. Additionally, noise and light pollution in urban areas can contribute to poor quality sleep, which can also be detrimental to metabolic health even if sleep duration is not decreased. Reduced sleep time might be more directly linked to metabolic dysregulation than merely having effects through changes in diet and exercise; findings from several studies show that glucose disposal and insulin resistance can be impaired in people subject to sleep restriction, and that these impairments are reversible when healthy sleep habits are restored. Many people now spend leisure time watching television or using computers, tablets, or mobile devices. Such so-called screen time has been associated with increased risk of metabolic disease, with reduced physical activity probably at least partly responsible. However, increased screen time probably also adversely affects sleep duration and quality. Another consequence of modern living, particularly in urban areas, is the tendency to work in a high-stress environment. Stress is another under-appreciated factor driving the epidemic of obesity and metabolic disease; in addition to independent effects, it is also likely to contribute through its effect on sleep duration and quality. In such a highly connected world—with laptops and smartphones making communication simpler than ever—many people now work outside of normal working hours. Not only does this compromise in work–life balance reduce sleep and relaxation time, it also heightens stress. As working time bleeds into personal time, many employers expect employees to remain contactable outside working hours, even if it means that limits on working hours are exceeded. In some countries, this problem has been recognised and laws have been introduced in recent years to control when employees can be contacted outside of normal working hours. Long working hours have been associated with an increased incidence of type 2 diabetes, as shown by Mika Kivimäki and colleagues in an Article in this issue. Existing laws in most high-income countries already limit weekly working hours; in the European Union, for example, the working time directive sets limits on the number of hours worked per week at 48 h on average, and also mandates that workers get a minimum of 11 h at rest in any 24 h period. Regulations on night working are more stringent, with shifts limited to 8 h in any 24 h period. However, in countries where laws do exist to limit working hours, most employees can opt out and work longer hours, but few are informed that doing so might negatively affect their metabolic health. Shift work has been associated with increased risk of obesity and type 2 diabetes in observational studies. Part of this association is probably due to associated changes in food intake and physical activity, but stress and disrupted sleep might also be under-appreciated drivers. Kivimäki and colleagues found that the association of diabetes with working hours was only significant in workers with low socioeconomic status jobs: people who are likely to not only work long hours under stressful conditions, but also live in areas where light and noise pollution are great. Unless guidance and regulations are in place and enforced, to protect sleep, such practices will continue to fuel the epidemics of obesity and type 2 diabetes. In view of mounting evidence linking sleep and work stress with metabolic health, clinicians must consider whether these potentially modifiable factors are important in their patients, particularly those otherwise at risk of developing metabolic disease. Additionally, studies on obesity and type 2 diabetes should explore interventions that incorporate increased relaxation time and improved sleep duration and quality. The importance of a good night's sleep for metabolic health should not be underestimated.