Blood supply to the brain is normally divided into anterior and posterior segments, relating to the different arteries that supply the brain. The two main pairs of arteries are the Internal carotid arteries (supply the anterior brain) and vertebral arteries (supplying the brainstem and posterior brain). Cerebral blood flow increases during sleep. The 4% to 25% increase in brain blood flow in slow wave sleep over that in wakefulness is explicable on the basis of the mild hypercapnia that results from hypoventilation. Brain blood flow increases markedly during REM sleep; this increase was as large as 80% in one study. In goats, Santiago found a 26% increase in brain blood flow during REM sleep, and an increase that was greater than could be explained by the increase in CO2. They also found that brain metabolism in REM sleep was similar to that in wakefulness. This increase in the ratio of brain blood flow to brain metabolism would depress central chemoreceptor activity during REM sleep and might be a factor in reducing ventilatory responses during REM sleep. However, in humans, Madsen and colleagues reported that changes in cerebral blood flow paralleled changes in cerebral oxygen metabolism during both slow wave and REM sleep. Brain blood flow increases during sleep. The 4% to 25% increase in brain blood flow in slow-wave sleep compared with wakefulness is explicable on the basis of the mild hypercapnia that results from hypoventilation. Brain blood flow increases markedly during REM sleep, the rise being as large as 80% in one study. In goats, Santiago and colleagues77 found a 26% increase in brain blood flow during REM sleep and found this to be greater than could be explained by the increase in CO2. They also found that brain metabolism in REM sleep was similar to that in wakefulness. This increase in the brain blood flow brain–metabolism ratio would depress central chemoreceptor activity during REM sleep and might be a factor in reducing ventilatory responses during REM sleep. However, in human beings, Madsen and colleagues79 reported that changes in cerebral blood flow paralleled changes in cerebral oxygen metabolism during both slow-wave and REM sleep.