Heart rate variability (HRV) is reduced in patients with Type 1 diabetes (T1D) 1. Obstructive Sleep Apnoea (OSA) is also associated with alterations in HRV 2. However, little is known about the impact of OSA on HRV in patients with T1D and therefore we aimed to investigate this further. Adult T1D patients without past medical history of OSA were recruited from a single secondary hospital in the UK. Apnoea-hypopnoea index (AHI) was measured using polygraphy. OSA was defined as an AHI ≥ 15 or an AHI 5-14.9 with excessive daytime sleepiness (based on Epworth Sleepiness Scale ≥ 11). Cardiac autonomic function (CAF) was assessed using the ANSAR ANX 3.0 system in response to deep breathing, Valsalva, and standing manoeuvres by spectral analysis of HRV (Low Frequency Area (LFa), Respiratory Frequency area (RFa) and the LFa/RFa ratios). 36 patients were included of which 14 (37%) were OSA positive (6 males), and 22 (61%) were OSA negative (3 males). OSA positive mean (SD) age, diabetes duration, body mass index (BMI), and Hb1Ac were 46.5 (13.6) years, 34.8 (12.0), years, 29.1 (5.8) kg/m2, and 8.7% (1.5%). OSA negative mean (SD) age, diabetes duration, body mass index (BMI), and Hb1Ac were 49.5 (11.8) years, 31.5 (13.4) years, 25.1 (2.9) kg/m2, and 7.8% (0.9%) respectively. Baseline LFa and RFa were higher in T1D only versus T1D with OSA (Median (IQR)): 0.91 (1.48); 0.82 (2.28); p=0.01 vs 0.41 (0.7); 0.28 (0.31); p=0.01) respectively. However, there was no significant difference in LFa/RFa ratio (sympathovagal balance). LFa was significantly lower in T1D with OSA during baseline, Valsalva and standing manoeuvres (p<0.05). RFa was also significantly lower in T1D with OSA during baseline, deep breathing and Valsalva manoeuvres (p<0.05). LFa, RFa, and LFa/RFa response to standing were better in T1D only than T1D with OSA. As compared to T1D only, T1D with OSA had lower deep breathing ratio (median (IQR)):1.18 (0.17) vs 1.22 (0.17); p=0.04), standing ratio (1.09 (0.28) vs 1.23 (0.27); p=0.02), and a tendency for a lower Valsalva ratio (1.13 (0.45) vs 1.41 (0.38); p=0.08). Our findings show that CAF is depressed more in T1D with OSA versus T1D without OSA, which indicates sympathetic and parasympathetic withdrawal in the patients with OSA. It has previously been observed that HRV improves with CPAP in patients with OSA3 and therefore irreversible autonomic nerve damage is unlikely to be responsible for the changes observed in this study. Given that there is an increase in oxidative stress and inflammation in both T1D and OSA this is a potential common mechanism that may impact upon neurotransmission or second messenger systems resulting in the worsening of HRV and CAF in patients with T1D and OSA.