Restricted and disrupted sleep: Effects on autonomic function, neuroendocrine stress systems and stress responsivity

Summary

Frequently disrupted and restricted sleep is a common problem for many people in our modern around-the-clock society. In this context, it is an important question how sleep loss affects the stress systems in our bodies since these systems enable us to deal with everyday challenges. Altered activity and reactivity of these systems following insufficient sleep might have serious repercussions for health and well-being. Studies on both humans and rodents have shown that sleep deprivation and sleep restriction are conditions often associated with mild, temporary increases in the activity of the major neuroendocrine stress systems, i.e., the autonomic sympatho-adrenal system and the hypothalamic-pituitary-adrenal axis. Sleep deprivation may not only have a direct activating effect by itself but, in the long run, it may also affect the reactivity of these systems to other stressors and challenges. Although the first signs of alterations in the way people deal with challenges under conditions of restricted sleep appear to be on the level of emotional perception, chronic sleep restriction may ultimately change the fundamental properties of neuroendocrine stress systems as well. Understandably, few controlled studies in humans have been devoted to this topic. Yet, experimental studies in rodents show that chronic sleep restriction may gradually alter neuroendocrine stress responses as well as the central mechanisms involved in the regulation of these responses. Importantly, the available data from studies in laboratory animals suggest that sleep restriction may gradually change certain brain systems and neuroendocrine systems in a manner that is similar to what is seen in stress-related disorders such as depression (e.g., reduced serotonin receptor sensitivity and altered regulation of the hypothalamic-pituitary-adrenal axis). Such data support the view that insufficient sleep, by acting on stress systems, may sensitize individuals to stress-related disorders. Indeed, epidemiological studies suggest that sleep complaints and sleep restriction may be important risk factors for a variety of diseases that are often linked to stress, including cardiovascular diseases and mood disorders.

Introduction

Chronically restricted sleep is a widespread and serious problem in our society.1, 2 An increasing number of people experiences regular sleep loss due to our modern around-the-clock lifestyle, increased work pressure, and psychosocial stress. A steadily increasing flow of information and stimuli, full agendas and busy schedules are a growing challenge for many of us, and often the time for recovery and sleep seems insufficient. Over the last century the average time per day spent on sleep has decreased by $1\frac{1}{2}$ hours and the pressure on sleep time will continue to grow, in a society where working night shifts and staying up late for leisure are generally accepted and common practice.² Importantly, frequent sleep loss is not only common among adults but is a growing problem even among young children.3, 4, 5 In various countries, up to one fifth of children below age 16 report feeling tired almost every morning due to early school hours and going to bed late in the evening.⁵

Repeated sleep loss may have many, yet largely unknown, repercussions for health and well being. Controlled studies have shown that acute sleep deprivation strongly affects cognitive performance and emotionality.⁶ Recent experimental studies in healthy subjects show that successive nights of restricted sleep result in a gradually accumulating decline in cognitive function.7, 8 Whereas subjects may initially recover from these effects after subsequent sleep, frequent or chronic sleep loss may induce neurobiological changes that are not immediately evident but accumulate over time, ultimately with serious health consequences. Sleep complaints and restricted sleep have been identified as risk factors for various disorders including, for example, cardiovascular diseases9, 10, 11 and psychiatric disorders.12, 13, 14

In this paper we will review and discuss the effects of restricted and disrupted sleep on autonomic function, neuroendocrine stress systems and stress reactivity. The question of how disrupted and restricted sleep affects the autonomic and neuroendocrine stress systems in the body is important for two reasons. First, the activation of the body's stress systems and the release of stress hormones allows us to adapt and survive in a continuously changing and challenging environment. These stress hormones not only support metabolic processes and physical activity under acute stress but also affect brain function, cognition and mood. Therefore, effects of sleep loss on these stress systems may have direct functional consequences for the way we perform and deal with everyday challenges. Second, chronic activation of the neuroendocrine stress systems has been associated with the development of various diseases, in many cases the same categories of diseases that have been linked to insomnia and sleep disruption. It might thus be that some of the proposed health consequences of chronically restricted and disrupted sleep are mediated by over-activation and altered regulation of stress systems.

Clearly, the relationship between activity of stress systems and sleep (loss) is complex and bidirectional. In everyday life, stress and insufficient sleep often go hand in hand and make up a vicious circle in which stress keeps a person awake and the inability to sleep may increase the feelings of stress. In such a condition it may not always be possible to separate cause and consequence. However, numerous experimental studies have been performed to separately address the relationship between stress and sleep in both directions. Because the main issue of this review is the consequences of restricted and disrupted sleep, it largely focusses on the relationship in one direction: the effects of sleep loss on stress systems.