Changes in cardiovascular variables during sleep reflect alterations in autonomic system activity. During NREM sleep there is an increase in parasympathetic tone which, in REM sleep, is accompanied by an inhibition of sympathetic activity. Therefore there is a reduction in resting heart rate during NREM sleep, which decreases further during REM sleep. However, sympathetic tone varies during the latter and intermittent tachyarrhythmias are commonly seen. Similarly, arterial blood pressure decreases during NREM sleep and is variable during REM sleep. Peripheral resistance falls during both phases of sleep, and thus cardiac output decreases. This is particularly marked during early morning REM sleep and may explain the increased incidence of sudden death in the early morning of patients with cardiorespiratory disease. Pulmonary artery pressure increases during sleep and reflects the changes in PaO2. Cerebral blood flow increases during NREM and REM sleep, and this may reduce central chemoreceptor activity.
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