Cardiovascular Activity One of the possible functions of sleep is to give the heart a chance to rest from the constant demands of waking life. As compared to wakefulness, during NREM sleep there is an overall reduction in heart rate and blood pressure. During REM sleep, however, there is a more pronounced variation in cardiovascular activity, with overall increases in blood pressure and heart rate. Additionally, changes in blood flow that cause erections to occur in males or swelling of the clitoris in females is characteristic of REM sleep. The underlying reason for these considerable neural and physiological variations in REM sleep is currently unknown, and may be a by-product of REM-related changes in nervous system activity or related to dream content.