High prevalence of depression among Obstructive Sleep Apnea (OSA) patients has been observed in both community and clinical populations. Due to the overlapping symptoms between both disorders, depression is usually misdiagnosed when it is correlated with OSA. Identifying shared biomarkers between OSA and depression within signals obtained from sleep recordings will allow for faster and efficient diagnosis of the comorbidity. Phase coupling between respiratory sinus arrhythmia (RSA) and respiratory has been proposed to be an alternative measure for assessing autonomic nervous system (ANS) activity and therefore it can be used as a measure of depression. Overnight electrocardiograms (ECG) and breathing using plethysmography were recorded from control subjects (38 CONT) , OSA subjects with major depression (40 OSAMDD+) and OSA subjects without major depression (40 OSAMDD-) . The interbeat intervals (RRI) and respiratory movement were extracted from 5 minute segments of ECG signals with a single apneic event during REM and NREM sleep. Hilbert transform was used to extract instantaneous phases of the RSA and respiration. Then time dependent phase coherence (λ) between RSA and Respiration was obtained. Heart rate variables (HRV) were also calculated. During REM sleep, higher λ was found in the OSAMDD+ group compared to the OSAMDD- group (p<0.05) and lower λ was found in the OSAMDD+ group compared to the CONT group (p<0.05). During NREM sleep, higher λ was found in the OSAMDD+group compared to both OSAMDD- group (p<0.01) and CONT group (p<0.05). Moreover, higher normalized high frequency (nHF) and lower normalized low frequency (nLF) were found in the OSAMDD+ group compared to the OSAMDD- group (p<0.05) during both REM and NREM sleep. Our findings suggest that depression has lowered the sympathetic activity when it is accompanied with OSA allowing for stronger synchronization between RSA and respiration.