Introduction

We spend a significant part (about a third) of our lives sleeping, which is essential to our physical and psychological well-being. Sleep, however, is a fragile state that can easily be impaired by psychological stress or physical illness. For up to 10% of the general population, difficulty falling and/or maintaining sleep occurs several times a week (i.e., chronic insomnia). Some of these problems may be due to existences of obstructive sleep apnea syndrome, a condition that affects over 10% of the population, or due to restless leg syndrome (RLS)/periodic leg movement syndrome (PLMS), sleep-related involuntary leg movements often associated with an abnormal sensation in legs. Excessive daytime sleepiness (EDS), parasomnia, and sleep problems associated with medical/psychiatric conditions are also common. Narcolepsy is a primary EDS disorder affecting about 0.05% of the population. EDS is also often secondary to a severe insomnia associated with obstructive sleep apnea.

Many different pathophysiological/etiological mechanisms for these sleep disorders are considered, and the International Classification of Sleep Disorders (ICSD) lists over 84 different types of disorders (Table 29.1). These sleep-related problems are often chronic and negatively affect the subject's quality of life. In a 24-hr society that encourages sleep deprivation, daytime sleepiness is also an emerging issue even in healthy subjects. Accidents due to sleepiness are now well recognized as a major public hazard. The emergence of clinical sleep medicine has proceeded rapidly during the last 30 years with the awareness of these sleep problems.