Michael H. Bonnet

Hyperarousal and insomnia: State of the science

In the past few years it has become increasingly clear that insomnia is a chronic disease that interacts with many other medical conditions. As our ability to examine complex physiological activity during sleep has increased, additional evidence continues to suggest that insomnia is associated with inappropriate physiological arousal. It is now known that patients with primary insomnia have increased high-frequency EEG activation, abnormal hormone secretion, increased whole body and brain metabolic activation, and elevated heart rate and sympathetic nervous system activation during sleep. This activation can be measured throughout the day and night and is chronic. Other research suggests that insomnia, probably based upon the associated chronic physiologic arousal, is associated with increased risk for medical disorders such as depression, hypertension, or cardiac disease. An animal model that has used odor stress to produce poor sleep in rats has identified specific activated brain sites similar to those found in human brain metabolic studies to suggest that insomnia is a state in which sleep and arousal systems are both simultaneously active. The animal studies have also shown that the inappropriate arousal can be blocked by lesions in the limbic and arousal systems. It is hoped that these findings can be extended to identify new compounds that improve insomnia by acting at these sites of abnormal brain activation.

Hyperarousal and insomnia

Pvimary OY psychophysiological insomnia has alternatively been viewed as either a predominantly psychological problem OY as a predominantly physiological problem. Several early studies of patients were not able to document physiological differences, but more recent studies have found that many patients with primary insomnia take longer than control subjects to fall asleep on daytime nap tests despite feeling fatigued and they have elevated metabolic rate throughout both night and day. Other recent studies have found that increasing physiological arousal level for a week in novmal sleepers produced the majov secondary symptoms reported by insomniacs. In contrast, producing the disturbed sleep of insomniacs in a group of novmal sleepers did not produce the typical pattern of secondary symptoms. Taken together, evidence is presented which supports the contention that primary insomniacs suffer from a disorder of hyperavousal and that the elevated arousal produces the poor sleep and other symptoms repouted by patients. It is therefore suggested that new treatment strategies directed at veduction of arousal level be considered in these patients.

Impact of naps and caffeine on extended nocturnal performance

It was hypothesized that alertness and performance during an extended work period would be improved by an afternoon nap and the subsequent use of caffeine during the night. Twelve young adults received a 4-h afternoon nap and caffeine during the night during one session and four 1-h naps during the night in a second session. After an afternoon nap, subjects had increased objective and subjective alertness, increased oral temperature, and increased performance on complex tasks like logical reasoning and correct additions when compared to the condition that allowed four nighttime naps. It was concluded that the specific scheduling of a nap period in preparation for an all-night work shift where sleep would not be allowed could result in increased alertness and performance as well as a less conflicted work situation.

Metabolic rate and the restorative function of sleep

It has been shown that performance improves during sleep more rapidly after sleep deprivation than during normal sleep. Recovery after sleep deprivation is also characterized by a different sleep stage distribution and by a decreased metabolic rate compared to a normal night of sleep. The present study examined whether sleep during a period of increased metabolic rate was less restorative than a normal period of sleep. Twelve normal young adults participated for two 4-day periods. In each period, an undisturbed baseline sleep night was followed by baseline testing. On the subsequent night, Ss slept a total of 212 min, starting at 0200 h after consuming placebo or 400 mg of caffeine. After the nap, subjects remained awake for 41 h until beginning an undisturbed night of recover sleep. During the period of sleep loss, subjects remained at the sleep laboratory where they performed computer tests and MSLTs. The caffeine produced a significant increase in metabolic rate during the nap. Although total sleep time was held constant, there was an increase in stage 1 and a decrease in stage 4 sleep after caffeine use. Performance on addition and vigilance tasks was significantly decreased after the caffeine plus nap condition. However, no significant difference were found on the MSLT, and subjective vigor was significantly greater following the caffeine and nap combination. It was concluded that the beneficial effect of sleep in maintaining performance was decreased by increased physiological activation during sleep.

Physiological Basis of Insomnia

Forty years of psychophysiological research have shown that patients with primary insomnia have elevated levels of physiological arousal as indicated by a wide range of variables including brain and whole-body metabolic measures, cardiac measures, and hormone and endocrine measures. Current research has attempted to refine our understanding of the relationship between physiological arousal, poor EEG sleep, psychological status, and subjective report of insomnia. Experimentally induced chronic physiological arousal in normal adults produces mood and personality changes seen in insomnia patients and therefore provides a model of how chronic insomnia could develop in physiologically susceptible individuals. Studies showing that poor sleep, as found in insomnia patients, by itself does not produce the arousal, mood, and personality characteristics found in clinical insomnia and imply that (1) the symptoms produced by chronic physiological arousal are not mediated by the poor sleep itself and (2) the symptom complex associated with psychophysiological insomnia is not really a sleep disorder but rather an arousal disorder. Finally, the importance of physiological arousal as the harbinger of insomnia has been enhanced by the finding that normal adults with no reported sleep disorder who were found to have EEG-defined situational insomnia also had elevated heart rate and cardiac spectral activity – an indication of predisposition to the later development of insomnia. In total, such studies show the primary importance of physiological activation as the harbinger of both the poor sleep and subjective dysphoria that characterize primary insomnia.

Studies of Fatigue and Human Performance in the Laboratory

Many laboratory studies of fatigue and performance have been done in the last 100 years. Most have examined changes in performance at different times of the day and night (circadian time) or after various types of sleep loss. In general, psychomotor performance declines during early morning hours and improves during the normal day. Performance also declines as time awake increases. These general rules are modified by a number of factors. Certain performance tests such as those requiring vigilance, reaction time, and working memory appear most sensitive to fatigue. However, performance is also modulated by several variables including the length of performance requirement, knowledge of results, and task proficiency level. Performance is also dependent upon environmental influences such as activity level, light level, posture, motivation, and availability of naps and drugs. Finally the close relationship between sleep, fatigue, and performance breaks down in sleep disorders or other situations where the continuity of sleep is destroyed.