Physiological effects

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The electroencephalogram shows lower delta power and increased beta activity than normal during sleep in primary insomnia, suggesting that there may be a reduction in the homeostatic drive to sleep as well as hyperarousal. Intrusion of alpha waves is common and may be due to a lower threshold for arousal to stimuli such as pain or noise. It is associated with awareness of thoughts during sleep, and with the perception that sleep is unrefreshing.

The peak melatonin secretion is reduced in primary insomnia in proportion to its duration rather than its severity. PET and SPECT scans have shown increased cerebral blood flow during sleep, with less reduction in the metabolic rate in the anterior cingulate and medial prefrontal cortex during wakefulness compared with NREM sleep.

The oxygen uptake is increased in stages 3 and 4 NREM sleep, even when arousals are allowed for, in those with primary insomnia compared to normal subjects. This suggests that the hyperarousal disorder underlying this condition affects both sleep and wake-fulness. The heart rate is faster in all stages of sleep as well as in wakefulness in primary insomnia, and the pattern of heart rate variability indicates an increase in sympathetic and a reduction in parasympathetic activity. Hypertension is also commoner in those with insomnia than in normal subjects.

The serum level of noradrenaline is raised throughout the night in those with insomnia, particularly when their sleep efficiency is low, and it is likely that sympathetic activity is increased throughout the day as well. The ACTH level is raised throughout the day and night and there is a significant increase in plasma cortisol IL-6, and TNF alpha levels are increased, particularly in the daytime, but also during sleep.

These abnormalities may be detectable even before insomnia has become established. In subjects who undergo polysomnography and who subsequently develop insomnia, there is raised sympathetic activity with, for instance, an increased heart rate. This suggests that there is a trait to respond in this way to stressful situations.

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