Sleep and stress

Sleep has fascinated all human societies, and questions as to why we sleep, and suggested remedies for sleeplessness, can be found in the earliest written records across global cultures. The examination of sleep from a physiological and experimental perspective, however, is of relatively recent origin. One early milestone was the publication in 1913 of Le probléme physiologique du sommeil by the French scientist Henri Pieron. That Pieron's book remained a standard reference until recently tells us much about how little progress was made during the twentieth century in our understanding of ‘how’ or ‘why’ we sleep. True, Nathaniel Kleitman, known as the ‘father of American sleep research’, made seminal discoveries regarding the effects of sleep deprivation in the 1920s, and in 1953, with Eugene Aserinsky, he discovered the state of rapid eye movement (REM) sleep. The study of the two states of sleep, REM and non-rapid eye movement (NREM) sleep, dominated sleep research for much of the twentieth century. Yet, frustratingly, the real biological significance of these 70–90 min NREM/REM cycles that punctuate our nightly sleep eludes us still. Sleep research has attracted an increasing number of practitioners who specialize in many different sleep pathologies, carving abnormal sleep into at least 83 different types of disorder. But in truth, the questions of how we sleep [1] or indeed why we sleep [2] are only now beginning to emerge. After more than 100 years of largely descriptive science, sleep is being understood at a mechanistic level, and with this new empirical understanding, its role in overall health is now being more fully appreciated. The development of sleep science has led, ironically, to sleep being restored to a status it once occupied in our pre-industrial past, as a key element of human mental and bodily health.

In this theme issue, we consider sleep from two perspectives: (i) our current understanding of the importance of sleep to health and wellbeing and (ii) how sleep has been regarded historically, and how historical studies can inform contemporary science. Although a real understanding of NREM/REM sleep remains a relative mystery, sleep research, largely from the twenty-first century, has shown that sleep can no longer be dismissed as a luxury or as an indulgence that can be curtailed. Rather, it emerges as an essential part of our biology, defining our ability to function optimally during the conscious state. The consolidation of memory and the generation of new ideas are hugely enhanced by sleep; the clearance of toxins from the brain and the rest of the body occur while we sleep; and sleep allows us to prepare physiologically and behaviourally for the demands imposed by activity. The importance of sleep is illustrated by recent findings, showing that the lack of sleep can lead to psychosocial and physiological stress, anxiety, impulsivity, loss of empathy, loss of memory, poor decision-making, loss of attention and a negative focus. If sustained, sleep loss or disruption can predispose individuals to a broad range of illnesses including heart disease, lowered immunity, cancer, metabolic abnormalities and mental illness. Critically, we are now beginning to understand mechanistically how these problems arise and what can be done either to eliminate or mitigate the varied impacts of sleep loss and sleep disruption. Such knowledge is beginning to inform the debate regarding the consequences of sleep disruption in the workplace and whether, as a society, we can justify imposing such stressors upon the workforce.

The term 24/7 has become a slogan, or mantra, which seems to sum up our modern condition, but as Jonathan Crary has argued, it takes its model from a ‘non-social model of machinic performance … that does not disclose the human cost required to sustain its effectiveness' [3, p. 9]. The concept of a 24/7 world is both seductive and dismaying; it captures the realities of global commodity capitalism which pays no heed to the human body clock, while simultaneously emphasizing its sheer unattainability for an individual human life. Recent work in the humanities and social science has challenged the growing dominance of models of social and economic behaviour, which assume highly disrupted or curtailed patterns of sleep [4,5], while historical studies have explored previous social and cultural practices with regard to sleep, including Roger Ekirch's influential study of segmented or biphasic sleep, Sasha Handley's detailed explorations of early modern sleep and Kenton Kroker's history of the development of sleep research, which tracks theories of sleep and dreaming from classical times onwards, and locates the beginnings of experimentation firmly in the nineteenth century [6–8]. All offer valuable perspectives on what the popular press has started to term an ‘epidemic of sleeplessness', a label which is unhelpfully alarmist, and with its medical implications of contagion tends to obscure the social and economic factors which feed into patterns of sleep deprivation.

In this theme issue of Interface Focus, we look specifically at the close relationship between sleep and stress. As an explicit medical term, stress is of twentieth century origin, dating back, as Mark Jackson has recently argued, in large part to the work of Hans Selye starting in the 1940s [9]. Discussions of the stresses of life, produced by what were seen as the pressures of modernity, however, were prevalent already in late nineteenth century medical and social discourse, and more general concepts of the stresses of life have an even older history [10]. As our historical case studies will show with regard to disrupted sleep, it is possible to trace parallel aspects of experience in the past, which would today be labelled stress-related. The cultural practices and experiences of sleep are always very much determined by historical period and environment, and there can be no easy translation across the centuries. Nonetheless, understanding of earlier practices and philosophies of sleep offers a helpful framework for considering the challenges that face us now in developing both scientific and social investigations of the intersections of sleep and stress.

Our earliest historical paper, by MacLehose [11], focuses on the medieval period, although as quickly becomes clear in this and other papers, understandings of sleep until well into the eighteenth century were heavily influenced by Hippocratic and Galenic medicine, with their theories of internal ‘humours', and external influences, or ‘non-naturals’, of which sleep and waking was one. Although the language and concepts are now alien, at base the model is one of equilibrium and balance, with an emphasis on self-care and preventative medicine. There is also an understanding of the close intricate relationship between health of body and of mind, and of the impact of external environment (broadly conceived), on overall health. As such, it proposes a holistic model which has much to offer current theories of sleep. MacLehose [11] explores cases of mental anguish (angustia) and its effects on sleep, as well as experiences of nightmare (or incubus), and insomnia (insomneitas), already a recognized condition. He also outlines forms of sleep-inducing practices which have a lot in common with current recommendations: music, baths and massage.

Moving into the early modern period, Handley [12] takes six case histories of poor sleepers (four women and two men), drawing out the gender distinctions which could affect both the occurrence and discourse around sleeplessness. Loss of sleep, she notes, was seen not merely as a symptom, but also as a cause of poor mental and bodily health, and the quality of sleep an individual experienced was understood as a barometer of a general state of health. One primary factor in understanding sleep loss which distinguishes this earlier period from our own was that of religion: we no longer believe that sleep loss is a sign of our having offended God. Although sleep was held to be of primary importance for health, its performance, it was argued, should be temperate: ‘slug a beds’ (or sluggards) were roundly criticized, and good sleep habits for the young were seen as the foundation of self-discipline and moderation. The paper speaks directly to that by Illingworth [13] on current work with teenagers on the Teen Sleep project.

Lawlor & Blackwood [14] take us into the eighteenth century, when the Hippocratic–Galenic model was being eclipsed by more modern medicine, based on the nervous system (as first outlined by Thomas Willis). This was also a period of rapid change, when anxieties about the city lifestyles of the upper orders and ‘people of fashion’ gave rise to medical diagnoses of increasing nervous disorders, with sleeplessness viewed as one element of this new challenge to healthy living. The paper looks not only at medical discourse, but also poetry, and the role played by ‘regimen’ poetry in inculcating healthy lifestyles, offering advice on regular and temperate sleep regimes, but also extolling the virtues of industry and exercise: ‘Thout some Fatigue, there's no sound Sleep’.

The nineteenth century, which laid the foundations for experimental work on sleep, is represented by three papers. Dickson [15] picks up on literature's significant role with reference to sleep, not so much in dispensing advice, but rather in exploring the workings of the mind while in a state of sleep. Her analysis of memory, trauma and dream in Dickens' work shows how, well before Freud, literary works were investigating the intersections of the conscious and unconscious mind within sleep, and the insistent return of traumatic memories which refused to obey the chronology of social time. Taking as her opening case study the example of a sleepless prime minister, Shuttleworth [16] examines the emerging anxieties in the late nineteenth century around sleepless, overworked professionals and sets them in the context of experimental studies, on both human and animal subjects, of the mind in a state of sleep. The paper also considers how worries about poor sleep fed into a rise of ‘chloralism’ (or addiction to the hypnotic drug chloral), thus establishing a vicious cycle which has direct parallels (although not in extent) with the current ‘opioid crisis’ in the USA, and the public health emergency which was declared by the US Department of Health and Home Services in 2017. In a closely linked paper, Beaumont [17] explores the emergence at the fin-de-siècle of the insomniac as both a pathological type and a social archetype. Although, as we have seen, insomnia was a long-recognized complaint, it was only at this period that the term and category ‘insomniac’ came into use. His case study, in keeping with late-century concerns about the stresses of professional life, is that of an overworked and insomniac American dentist who finally takes his own life. Insomnia and suicide, as he argues, were frequently linked at this period, with doctors and dentists as prime sufferers due to their ready access to the various forms of opium, chloral and cocaine available to relieve their stress.

Recent research in sleep science has focused extensively on understanding circadian rhythms and the role of light and darkness in establishing healthy sleep. In our final humanities paper, Ludtke [18] moves into the realm of dystopian fiction to explore the nightmare of ‘total illumination’ in novels by H. G. Wells, Aldous Huxley and George Orwell. These works are set in the context of the history of research, developing from the late nineteenth century, into stress and sleep deprivation (thus foreshadowing current work reviewed by Nollet et al. [19]), as well as the history of the arrival of electric light on our streets. Drawing on this research, and her readings of the dystopias explored in the fiction, she calls for a more nuanced understanding of the division between natural and artificial which often forms an unspoken assumption behind scientific explorations of sleep patterns, or even the most radical critiques of 24/7 culture.

Taken together, the humanities papers form a rich and provocative context for the scientific papers which follow. They introduce forgotten aspects of earlier scientific research, with experiments which anticipate current work, and take us back to previous periods, remote in time, but with relatively sophisticated levels of understanding of processes of interaction between mental and bodily health, and the key role of external factors when considering the role of sleep in both individual and social life. The science has moved on, but current calls for preventative medicine, and the development of regimes of sleep hygiene, have resonant echoes in the past. The wider frames of reference adopted in earlier periods when thinking about issues of sleep provide a challenging stimulus for current work on sleep as we move into more interdisciplinary modes of understanding and analysis. The historical frame also provides a helpful context for considering current anxieties regarding ‘epidemics of sleeplessness', opioid addiction or the impact of technology on sleep and teenage health. We are not, of course, merely repeating the past, but understanding how similar anxieties were framed and responded to in earlier periods will help to place current concerns in perspective.

In the opening essay, Foster [20] provides an overview of recent scientific work on circadian rhythms and the all-important systems of temporal ordering which govern the life of the body and the mind. While earlier work identified the suprachiasmatic nuclei as the ‘master clock’ of the body, current research has shown that there are cellular clocks in every organ and tissue of the body. Foster [20] draws out the implications of this research for sleep, looking at the different forms of sleep disorders, and their impact on emotional responses, cognition, physiology and health, from weight gain to the relationship between sleep disruption and mental disorders. His final section is a call to action, outlining measures which could be adopted by individuals to improve qualities of sleep and by employers to address health problems associated with night shift working. Drawing on historical research on biphasic sleep, he also suggests that our model of human monophasic sleep perhaps needs to be redrawn, showing how insights from the past can help shift agendas and taken for granted assumptions in current scientific practice.

Nollet et al. [19] also offer an overview of sleep research, looking back to the early experiments in the 1890s (also discussed by Ludtke [18]), but focusing on recent experimental research on sleep deprivation. Highlighting the difficulties of separating out the experiences of stress, and sleep deprivation itself, in an experimental setting, they outline a new research framework to assess the impact of sleep deprivation on biology and health, and call for a new sleep-deprivation protocol that avoids stress as a confounding factor in sleep-deprivation studies.

Foster's [20] call for direct action is addressed, in part, by Illingworth's [13] paper on adolescent sleep, which explores recent scientific findings with regard to circadian physiology in adolescence, and highlights the general levels of sleep deprivation amidst this age group, and the bidirectional relationship between mental health and sleep loss in teenagers. The paper outlines some of the practical measures that have been taken to address these issues, from curtailing evening screen use, to experimenting with later school start times. Another option Illingworth [13] outlines is using a sleep education programme to persuade teenagers to go to bed earlier. A level of scepticism rightly underlies her observation, however, that improved sleep practices might be difficult to maintain. As the example shows, science can demonstrate the health and educational benefits of enhanced sleep, but effective intervention is another matter. Science and medicine at this point need enhancement from other disciplines and agencies that can offer understanding of the social, cultural and psychological factors at work, and thus help to frame strategies of intervention which have some chance of success.

The final two papers are opinion pieces from the medical front line. Goodwin [21], a psychiatrist, argues strongly for both genetic predisposition and environmental adversity as factors in the onset of severe depression in the young. Like Illingworth [13], he is concerned with patterns of sleep in adolescence, but here in relation to the emergence of major depressive episodes and their possible prevention by early intervention. Sleep disruption, he suggests, is a neutral symptom, which it is easier for a person to acknowledge than emerging feelings of low mood. Work with an app to encourage better sleep hygiene in the young has had promising results. Like Foster [20], Goodwin calls upon public health policy makers to pay more attention to the huge potential health benefits of improved sleep habits.

Fitzpatrick [22], by contrast, offers very personal reflections on the experience of being on-call at nights throughout the 35 years of his career as an obstetrician. He is, in a sense, a human guinea pig. Given the emphasis on the need for undisrupted sleep in the preceding papers, his conclusions are both intriguing and quite surprising. While acknowledging the real gains that emerged in clinical practices from the implementation of the European Working Time Directive, such as the cutting down of excessively long shifts, he nonetheless believes there are arguments to be made for the 24 h shift in fields such as obstetrics where the continuity of care across an extended period can be required. Contrary to many findings on the health risks of disrupted sleep, and the body's failure to adapt to disrupted or reduced sleep, he suggests that it is possible to train the body and mind to adapt (although he does admit an increased toll on bodily energies with age). The suggestion opens up again the question, originally raised by the historian Roger Ekirch and considered again in these papers by Foster [20], of whether monophasic sleep is essential for human health, or whether we might indeed embrace biphasic or polyphasic models.

With the UK's departure from the European Union, and the question of whether we will continue to abide by the European Working Time Directive still to be resolved, the findings of recent sleep research take on political urgency. As Illingworth [13] points out, the UK has identified a reduction in sleep deprivation as a public health priority, while, in Australia, the total annual cost of inadequate sleep was estimated to be $66.3 billion in 2016–2017 (https://www.sleephealthfoundation.org.au/files/Asleep_on_the_job/Asleep_on_the_Job_SHF_report-WEB_small.pdf, August 2017). Despite this growing appreciation of the impact of sleep loss, the mechanisms by which these problems will be addressed remain unresolved. In these papers, ranging across scientific disciplines and historical periods, from the medieval to current times, we offer a new framework for thinking about the social and cultural importance of sleep, and its benefits for bodily and mental health. We hope that the unusual juxtapositions of material will stimulate thought, and lead to further research and policy initiatives.

Data accessibility

This article has no additional data.

Authors' contributions

Both S.S. and R.F. contributed equally.

Competing interests

We declare we have no competing interests.

Funding

The theme issue is based on the papers delivered at a conference, ‘Sleep and stress, past and present’, at the Royal Society in December 2018, funded by the ERC project ‘Diseases of modern life: nineteenth-century perspectives’, under the European Union's Seventh Framework Programme, ERC grant agreement no. 340121.