The nature of arousal in sleep

Abstract

SUMMAR Y The role of arousals in sleep is gaining interest among both basic researchers and clinicians. In the last 20 years increasing evidence shows that arousals are deeply involved in the pathophysiology of sleep disorders. The nature of arousals in sleep is still a matter of debate. According to the conceptual framework of the American Sleep Disorders Association criteria, arousals are a marker of sleep disruption representing a detrimental and harmful feature for sleep. In contrast, our view indicates arousals as elements weaved into the texture of sleep taking part in the regulation of the sleep process. In addition, the concept of micro-arousal (MA) has been extended, incorporating, besides the classical low-voltage fast-rhythm electroencephalographic (EEG) arousals, high-amplitude EEG bursts, be they like delta-like or K-complexes, which reflects a special kind of arousal process, mobilizing parallely antiarousal swings. In physiologic conditions, the slow and fast MA are not randomly scattered but appear structurally distributed within sleep representing state-specific arousal responses. MA preceded by slow waves occurs more frequently across the descending part of sleep cycles and in the first cycles, while the traditional fast type of arousals across the ascending slope of cycles prevails during the last third of sleep. The uniform arousal characteristics of these two types of MAs is supported by the finding that different MAs are associated with an increasing magnitude of vegetative activation ranging hierarchically from the weaker slow EEG types (coupled with mild autonomic activation) to the stronger rapid EEG types (coupled with a vigorous autonomic activation). Finally, it has been ascertained that MA are not isolated events but are basically endowed with a periodic nature expressed in non-rapid eye movement (NREM) sleep by the cyclic alternating pattern (CAP). Understanding the role of arousals and CAP and the relationship between physiologic and pathologic MA can shed light on the adaptive properties of the sleeping brain and provide insight into the pathomechanisms of sleep disturbances. Functional significance of arousal in sleep, and particularly in NREM sleep, is to ensure the reversibility of sleep, without which it would be identical to coma. Arousals may connect the sleeper with the surrounding world maintaining the selection of relevant incoming information and adapting the organism to the dangers and demands of the outer world. In this dynamic perspective, ongoing phasic events carry on the one hand arousal influences and on the other elements of information processing. The other function of arousals is tailoring the more or less stereotyped endogenously determined sleep process driven by chemical influences according to internal and external demands. In this perspective, arousals shape the individual course of night sleep as a variation of the sleep program.k e y w o r d s micro-arousal, NREM sleep, K-complex, cyclic alternating pattern (CAP)

On the impact of arousals on the performance of sleep and wake classification using actigraphy

“…Alternatively, if we cannot reliably detect the presence of arousals during sleep but instead consider positive classifications on arousal epochs as correct, the precision of the classifier increases by a maximum of 31.3%. These findings are in line with earlier observations that body movements may follow cortical arousals during sleep [9]. However, the physiological consequences of these events are not limited to body movements.…”

“…However, the physiological consequences of these events are not limited to body movements. In fact, the sympathetic surge corresponding to the occurrence of an arousal is also manifested at cardiac level [9]. Interestingly, recent work in sleep/wake classification showed that the addition of cardiac features to actigraphy increases sensitivity to the wake class [7], [17].…”

“…The second problem is related to the occurrence of arousals. An arousal is defined as a rapid shift in the EEG frequency and can be accompanied by an increase of electromyographic (EMG) activity, cardiac frequency or body movements [9], [10]. This means that many sleep epochs will share some physiological characteristics of wake, in particular in regard to the presence of body movements.…”

Interaction between neocortical and hippocampal networks via slow oscillations

“…K-COMPLEXES AND SLEEP SPINDLES-Irregular, relatively sharp potentials occur sporadically in the scalp EEG during non-REM sleep (Loomis et al, 1938;Roth et al, 1956) and under anesthesia (Amzica and Steriade, 1997;Steriade and Amzica, 1998b). This pattern, termed K-complex (Loomis et al, 1938), can arise either spontaneously or be evoked by stimuli of all modalities, including slight positional changes in bed (Halasz et al, 1985;Halasz et al, 2004). Again, in light of the mechanisms of slow oscillations the origin of the K-complex can be redefined.…”

“…Because sensory stimuli can trigger UP states (Petersen et al, 2003), this might explain why at least some K complexes are associated with sensory inputs in sleeping subjects (Halasz et al, 2004).…”