We spend roughly one-third of our lives asleep. Although the primary function of sleep remains a topic of debate among researchers, there is agreement that sleep is important for the rejuvenation of many bodily functions. The endocrine and immune systems are particularly active during sleep, cellular repair mechanisms are activated, and the growth hormone and cortisol rhythm are both coupled to sleep. Sleep also plays an important role in influencing mental function.
With aging, sleep often becomes more sensitive to external and internal disturbances. With many disorders, such as those accompanied by pain, sleep is usually disturbed as well. In addition to these daytime disorders, there exist a large number of disorders that have their origins in sleep. In these primary sleep disorders, sleep itself or a regulation during sleep is impaired or disturbed.
Primary sleep disorders are categorized in a few major groups, including insomnia, which constitutes the largest group and refers to cases where there are problems initiating or maintaining sleep or where early morning awakening occurs on a chronic basis. The second-largest group includes sleep disorders characterized by respiratory problems during sleep. If, for example, snoring is accompanied by respiratory cessations, obstruction of the upper airways, or changes in blood gases (oxygen and carbon dioxide), then this becomes a health risk, leading to cardiovascular consequences. Combined with other factors, such as obesity, recent studies have also demonstrated strong associations with metabolic dysfunction, including type 2 diabetes. The third group constitutes movement disorders during sleep, such as the feeling of muscle activity and periodic movements of the limbs during sleep.
Other known disorders include sleepwalking, teeth grinding, and frightening dreams. Currently, more than 80 different sleep disorders with various definitions and degrees of severity have been identified as clinical entities, most of which are chronic disorders that need lifelong treatment.
Diagnosing this range of sleep disorders requires biomedical equipment and, thus, engineering expertise. Diagnosis may be performed at specialized sleep centers or at home. For example, the treatment of sleep-disordered breathing requires ventilation equipment, oral appliances, and neurostimulators. Because of this breadth, sleep disorders present a challenge to the BME community as well as an enormous opportunity for companies active in this field. Many large companies have grown through producing diagnostic equipment and therapeutic equipment for sleep disorders. This new field of sleep medicine dates only to the 1980s, but has expanded enormously over the last 30 years. With links to neurology; psychiatry; respiratory medicine; cardiology; ear, nose, and throat medicine; pediatrics; and anesthesia, it is likely that, in a few years, sleep medicine will evolve into its own standing discipline.
This special issue of IEEE Pulse is devoted to reviewing recent developments in the application of engineering methodologies and technologies in the diagnosis and treatment of sleep disorders. In addition, the IEEE Pulse website features a roundtable discussion among several internationally renowned leaders in the field that highlights the state of the art and future directions.
Most of the current efforts in the diagnostic area have been directed toward the development of more ambulatory and nonintrusive methods of monitoring sleep that can be applied in domiciliary environments, away from traditional hospital-based polysomnography. In his article, Hirshkowitz provides a brief review of the history and evolution of the instrumentation used in polysomnography. Gari Clifford’s article discusses the latest developments in noncontact methods for monitoring sleep, while Conor Heneghan focuses on new devices that use radiofrequency sensors. Jané presents an overview of the broad spectrum of electromechanical devices currently used to treat obstructive sleep apnea—these range from classical continuous positive airway pressure (CPAP) devices to intelligent adaptive positive airway pressure (such as automatic positive airway pressure) machines.
In addition, Nayak and Fleck provide a fascinating look into state-of-the-art developments in dynamic imaging using magnetic resonance scanning of the upper airway in patients with obstructive sleep apnea during both wakefulness and sleep. Although upper airway collapsibility has been considered the primary factor that mediates the occurrence of sleep-related breathing disorders, recent studies have shown that other mechanisms can also play an important role. From these new findings has emerged the notion that there are multiple phenotypes of sleep apnea, and that, in some of these phenotypes, the relative stability of the chemoreflex control of ventilation could also be an important contributor to various degrees. Nemati, Orr, and Malhotra also present a mathematical modeling approach to better characterize ventilatory control stability.
Finally, while sleep-related breathing disorders in children are becoming progressively more recognized as a public health problem, much of our current knowledge of the physiology and underlying biophysical phenomena in sleep apnea has come from studies of adults. The article by Amin et al. highlights the ongoing efforts by five NIH-funded research centers to obtain a better understanding of the anatomy and function of the pediatric upper airway that could allow for identification of the sites of obstruction and create computational models that can be virtually manipulated to plan and predict the results of surgical and other therapeutic interventions. What is noteworthy about this consortium of researchers is the multidisciplinarity of their backgrounds and the methodologies employed in their projects—in particular, many of the investigators are bioengineers with expertise in computational fluid dynamics, signal processing, control theory, and imaging. The models and technologies developed by these investigators apply not only to the pediatric upper airway in the context of sleep apnea but also to the upper airways of children with developmental (e.g., Down syndrome, subglottic stenosis, and microagnathia) and endocrine (e.g., polycystic ovary syndrome) abnormalities.
The contributions outlined here show that the medical field is increasingly recognizing the importance of sleep disorders in all aspects. There are sleep disorders as primary health problems and sleep disorders as consequences of other medical problems. Because effectiveness during the day is crucial, diagnosis and treatment of such disorders is more important today.
Sleep-disordered breathing is a chronic disorder that can be diagnosed easily and treated effectively, relieving most symptoms. Because sleep-disorderd breathing is a functional disorder and not a biochemical problem, it is an ideal scenario for developing new tools, algorithms, and devices to diagnose this condition. A diagnosis can be made with equipment using extensive measurement technologies to gain new insight into pathophysiology. Diagnosing can also be done with smaller and cleverly designed devices, such as those linked to smartphones, which make use of the information that can gained by monitoring movement and pulse readings. Biomedical technology is not restricted to diagnosis alone as in many other conditions, but, here, CPAP technology as well as new nerve stimulation devices provides a wide field for more technical applications. We are convinced that sleep disorders present an expanding field for the application and development of BME methods now and even more in the future.