Psychophysiology of Sleep Disorders
Understanding the disordered presupposes a fundamental knowledge of the normal reference state or behaviour. It should come as no surprise, therefore, that our understanding of the psychophysiology of sleep disorders is limited, given that our knowledge of the characteristics and processes associated with normal sleep physiology and related variations in cognition can, in many respects, be considered elementary. Until relatively recent times, the development of knowledge about sleep was restricted by conceptual limitations and behavioural barriers. The emphasis on wakefulness as the state during which the most important behavioural and cognitive accomplishments occurred diminished the importance of sleep as a behaviour worthy of or even requiring investigation. This attitude was reflected in the view of the waking state as 'the sole portion of … existence that "counts" in any way, sleep appearing as "time out" from the game of living' (Kleitman, 1963: p. 3) — an attitude reinforced by the behavioural inertness characteristic of the sleeping organism. Still, the influence of sleep on waking behaviour is undeniable and evident both in its timing and duration. The occurrence of sleep can be considered to be subject to limited volitional control since it is possible to remain awake for long periods of time or to select convenient times and environments for sleeping. However, at times the pressure for sleep is irresistible and sleep will occur despite the known likelihood of potentially deadly consequences, as while driving an automobile. The amount of sleep obtained has also been shown to be important for normal waking activities. The adverse effects of reduced sleep on alertness and performance are well documented (Monk, 1991; Gillberg and Akerstedt, 1994), and, in the extreme, the long-term absence of sleep may lead to death (Horne, 1988; Everson, 1995; Bentivoglio and Grassi-Zucconi, 1997). From these observations, two important inferences can be drawn: 1) sleep is not simply a passive state during which physical activities are suspended; and 2) sleep processes interact with those occurring during waking in complementary and synergistic ways to maintain and extend life. Until the mid-20th century, insight into the nature of this codependence was prevented by the inability to obtain information regarding ongoing processes from the sleeping organism. Technologic developments during the past half-century have largely removed obstacles to these efforts. Notable among these developments has been the ability to conduct recordings of central nervous system and related autonomic and peripheral nervous system activities over the extended time periods that sleep typically occurs. More recently, these measures have been supplemented with assessments of state-related variations in brain metabolism using imaging procedures.
The interest in applying these technologic tools to chart variations in sleep physiology across the night was accompanied by an equally intense curiosity regarding what these measures might reveal about the occurrence of mental activity during sleep. The long-standing belief that experiences of mental activity continue during sleep in the form of dreams suggested that sleep was not a mental void. Furthermore, since these mental experiences occur during times of sustained disengagement from the environment, the nature of psychophysiological relationships under these circumstances could be different than those present during wakefulness. The advent of the new methods brought the study of these relationships within the range of effective experimental control. Investigations using these new procedures revolutionized the study of sleep, and the findings from this research have forced a redefinition of concepts regarding physiological and psychological activities and events that can be considered normal.
Although it is apparently the natural order that the states of sleep and wakefulness remain separate, their codependence renders them inseparable and interactive. Some aspects of these state interactions are fixed, and others more variable. Since the existence of higher mammals can be considered a closed system generally dichotomized into states of sleep and wakefulness (although conditions exist which do not clearly fall into either of these categories, such as coma and anaesthetic states), interactions based on state duration are constrained and essentially hydraulic in nature — that is, increases in time spent in one state are at the expense of time spent in the other.
It can be assumed that an important determinant of state duration is the time needed to satisfy functional requirements. Consequently, an important source of sleep-wakefulness interactions derives from the manner in which within-state behaviours (or processes) affect, or are reflective of, the realization of these requirements. Broad examples of such variations would include the nature of waking activity (e.g., resting vs. vigorous physical activity) or sleep quality (e.g., quiescent vs. disturbed). Such variations have both immediate within-state effects as well as longer-term, between-state consequences.
The relative inaccessibility of sleep processes to systematic study delayed the recognition of these interactions and consequences as significant determinants of behaviour. This delayed acknowledgement was evident in the absence of sleep-related variables among factors considered contributory to mental disorders in the initial publications of the Diagnostic and Statistical Manual of Mental Disorders (DSM) in 1917 and 1952 (DSM-I). The publication of DSM-I occurred near the time that rapid-eye-movement (REM) sleep was discovered (Aserinsky and Kleitman, 1953) — a revelation that effectively launched the new era of sleep research. Fifteen years later (1968), DSM-II appeared, and for the first time sleep disorders were included (referenced under 'Special Symptoms'). By 1979, an understanding of sleep had developed to the point that a 'Diagnostic Classification of Sleep and Arousal Disorders' could be published. Beginning with the publication of DSM-III one year later, the involvement of sleep disorders in the determination of mental status has been consistently and more fully represented.