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Comparisons of indices for HRV between short and long nap groups. As parameters for HRV, SDNN [standard deviation of NN (RR)), LF/HF (low-frequency domain/ high-frequency domain), and HF (high-frequency domain) were determined. These parameters were natural logarithm transformed to achieve a normal distribution. Each column shows the mean ± standard error. P values were determined using Student’s t-test.
