Sympathetic nerve activity (SNA) in neurons projecting to skeletal muscle blood vessels increases during rapid-eye-movement (REM) sleep, substantially exceeding SNA of non-REM (NREM) sleep and quiet wakefulness (QW). Similar SNA increases to cerebral blood vessels may regulate the cerebral circulation in REM sleep, but this is unknown. We hypothesized that cerebral SNA increases during phasic REM sleep, constricting cerebral vessels as a protective mechanism against cerebral hyperperfusion during the large arterial pressure surges that characterize this sleep state. We tested this hypothesis using a newly developed model to continuously record SNA in the superior cervical ganglion (SCG) before, during, and after arterial pressure surges occurring during REM in spontaneously sleeping lambs. Arterial pressure (AP), intracranial pressure (ICP), cerebral blood flow (CBF), cerebral vascular resistance [CVR = (AP - ICP)/CBF], and SNA from the SCG were recorded in lambs (n = 5) undergoing spontaneous sleep-wake cycles. In REM sleep, CBF was greatest (REM > QW = NREM, P < 0.05) and CVR was least (REM < QW = NREM, P < 0.05). SNA in the SCG did not change from QW to NREM sleep but increased during tonic REM sleep, with a further increase during phasic REM sleep (phasic REM > tonic REM > QW = NREM, P < 0.05). Coherent averaging revealed that SNA increases preceded AP surges in phasic REM sleep by 12 s (P < 0.05). We report the first recordings of cerebral SNA during natural sleep-wake cycles. SNA increases markedly during tonic REM sleep, and further in phasic REM sleep. As SNA increases precede AP surges, they may serve to protect the brain against potentially damaging intravascular pressure changes or hyperperfusion in REM sleep.