Despite the exponential growth in heart rate variability (HRV) research, the reproducibility and reliability of HRV metrics continues to be debated. We estimated the reliability of 11 metrics calculated from 5 min records. We also compared the accuracy of the HRV metrics calculated from ECG records spanning 10 s to 10 min as compared with the metrics calculated from 5 min records. The mean heart rate was more reproducible and could be more accurately estimated from very short segments (<1 min) than any of the other HRV metrics. HRV metrics that effectively highpass filter the R–R interval series were more reliable than the other metrics and could be more accurately estimated from very short segments. This indicates that most of the HRV is caused by drift and nonstationary effects. Metrics that are sensitive to low frequency components of HRV have poor repeatability and cannot be estimated accurately from short segments (<10 min).