Arousal from sleep produces an acute increase in heart rate and blood pressure (O’Driscoll et al 2004). We tested the hypothesis that this increase is mediated by the sympathetic nervous system, by measuring the acute heart rate response to arousal from sleep in autonomically challenged patients; specifically Multiple System Atrophy (MSA) patients who have both central sympathetic and parasympathetic neuropathy, and Pure Autonomic Failure (PAF) patients who have a peripheral sympathetic neuropathy. After ethical approval and written informed consent, we studied 10 autonomic failure (AF) patients (5 MSA patients; median age, (range): 59 (48 – 68 years), 5 PAF patients; 52 (37 – 72) years) and 10 Controls (58 (42 – 71) years). Each subject underwent a night of polysomnography (SOMNOScreen, S-Med, UK). Sleep stages were scored according to standard rules by a researcher blinded to the subject group. Spontaneous arousals (5 to 15 seconds of wake EEG) were identified from stable sleep with no apnoeas / hypopnoeas in the preceding 3 minutes. The RR interval was quantified pre and post arousal. To test the effect of arousal on the RR interval in AF patients compared to Controls, a mixed-model regression analysis was used; data were expressed as median (range). Please refer to Table 1. The number of arousals analysed per patient was: MSA, 4 (1 – 7); PAF, 4 (2 – 5); Controls, 3 (1 – 6). The RR interval at baseline (60 s pre arousal) was: MSA, 1041 (762 – 1097) s; PAF, 938 (570 – 999) s, Controls, 995 (758 – 1179) s. There were no significant differences in the response to arousal across the 3 time periods (AF patients vs. Controls: coefficient of variation (CV) 14.8, 95% confidence interval (CI) -8.9 – 38.6; p=0.22, MSA patients vs. Controls: CV 10.6, 95% CI -18.9 – 39.9; p=0.48, and PAF patients vs. Controls: CV 17.3, 95% CI -14.4 – 49.1; p=0.29). In conclusion, although our sample size was small, our data indicates that the RR response to arousal from sleep was greater in the controls compared to the autonomically challenged patients, with the smallest response occurring in the peripherally de-innervated PAF patients. 3 time periods.