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The aim of this paper was to study interactions between R-R intervals and respiratory phases in patients with mild obstructive sleep apnea syndrome (OSAS) during night-time sleep using a technique based on joint symbolic dynamics. We investigated overnight polysomnography data in 123 OSAS patients. The R-R time series were extracted from electrocardiograms (ECG) and respiratory phases were obtained from abdominal displacement sensors using the Hilbert transform. Both series were transformed into ternary symbol vectors based on the changes between two successive R-R intervals and the respective respiratory phases. Subsequently, words of length `3' were formed and the correspondence between words of the two series was determined for each sleep stage to quantify cardio-respiratory interaction. We found a significantly higher percentage of similarity in the symbolic dynamics of R-R intervals and respiratory phases during slow-wave (SW) sleep compared to any other sleep stage (slow-wave vs. stage 1, stage 2 and rapid-eye-movement sleep: 20.9±4.7 vs. 15.5±4.2, 17.0±4.1 and 13.4±2.6, p<;0.0001, respectively). In conclusion, joint symbolic dynamics provides an efficient technique for the analysis of cardio-respiratory interaction during sleep.