Cardiovascular variables were monitored in 15 healthy young subjects during night sleep and in waking state during the day. Forearm and calf blood flow, blood pressure, and interbeat interval were measured at regular intervals with simultaneous monitoring of electroencephalogram, EOG, and chin, calf, and forearm EMG during sleep. In eight subjects the same variables were monitored during the day in recumbent positions. Large variations in all recorded cardiovascular variables were observed during sleep. Calf and forearm blood flow and conductance varied independently and reached increases of up to 700% of the resting value. Variations during the day were less frequent and smaller; heart rate also showed large variations. None of the changes showed any association with the sleep stages. The origin of the variations is uncertain; during sleep the neuronal networks integrating and patterning cardiovascular response may not be exerting their control. As a result the cardiovascular variables behave in an independent fashion. A parallel is drawn with dreams, which consist of disjointed and distorted information encoded during waking hours.