Do Time Domain and Frequency Domain Metrics Track Together?
Changes in Time Domain and changes in Frequency Domain don't always correlate with each other. Normally, HF and RMSSD/SDNN tend to track together, but not always. Certain responses in the nervous system have more impact on some values than others.
Additionally, Time Domain and Frequency Domain respond differently to artifacts and signal noise. It is much easier to correct artifacts for the Time Domain than it is for the Frequency Domain - just something to consider.
Another similar example is comparing basic heart rate to HRV values. Usually, if heart rate slows down, HRV goes up, and vice versa. However, there are some cases in which the opposite happens. Often, an increase in heart rate corresponds to stress on the body, which typically decreases HRV, but sometimes other factors influence the nervous system and HRV independent of heart rate such as breathing patterns or immune system function.