A review was made of the literature relating sleep apnoea to sympathetic nervous system (SNS) activity, as inferred from catecholamine levels or muscle sympathetic nerve activity (MSNA). Twenty-four studies were located. Most studies reported an elevation of norepinephrine and MSNA, both during sleep and wakefulness among individuals with sleep apnoea. However, studies rarely controlled for known confounders of sympathetic activity, making the validity of the findings questionable.