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Effect of post-term age on HRV parameters in quiet sleep in the supine (A–D) and prone (E–H) positions in infants born very preterm, preterm and term. Values are mean ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001. HF, high frequency; HRV, heart rate variability; LF, low frequency.
