PURPOSE: Sleep disordered breathing (SDB) has adverse effects on cardiovascular health in adults, partly due to changes in autonomic activity. However, there have been limited studies in children. We analysed the impact of SDB and sleep stage on autonomic control of heart rate in 7-12-year-old children, utilizing spectral heart rate variability (HRV) as a measure of autonomic activity. METHODS: Eighty children underwent overnight polysomnography. Subjects were grouped according to their obstructive apnoea-hypopnoea index (OAHI): controls, OAHI 5. HRV was analysed during Wake, nonrapid eye movement (NREM) 1 2, slow wave sleep (SWS) and REM. RESULTS: Compared with controls, total power, low (LF) and high frequency (HF) power were reduced in all SDB severities during REM. LF/HF ratio was less in MS SDB (median = 0.34; range, 0.20-0.49; p <0.05) versus controls (0.38; 0.26-0.55; p <0.05) and PS (0.39; 0.23-0.57; p <0.05) during SWS. In all groups, total power, LF and HF power were highest during NREM 1 2 while LF/HF ratio was lowest during SWS. Blood pressure was elevated in SDB in all sleep states. CONCLUSIONS: HRV was altered in 7-12-year-old children with SDB, which may signify an overall depression of autonomic tone, perhaps a consequence of their elevated blood pressure during sleep coupled with repeated exposure to SDB event-related cardiovascular disturbance. Further research is warranted to elucidate the long-term effects on the cardiovascular system of subjects exhibiting impaired HRV and elevated BP in childhood.