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Physiologic regulation during sleep varies with the state of the brain and is influenced by different stages of sleep. On the other hand, control of circulation during sleep requires coordination of the respiratory and the cardiovascular system. All these physiologic systems need to continuously interact with each other and at the same time each one has its own regulatory mechanisms, which adds more complexity to the sleep physiology. In normal circumstances, the heart rate (HR) is constantly being adjusted within each respiratory cycle and depends on the breathing frequency, known as respiratory sinus arrhythmia. 1 To date, little is known about the neural mechanisms of sleep-specific central commands. It is known that different sleep stages modulate regional blood flow at all levels of the nervous system. The strong relation between blood flow and metabolism in the central nervous system indicates that sleep …