I got to point out something here. The vertical axis unit for the time domain signal is milliseconds,and not volts. I am a bit lost here, would the term DC apply to our case here ? Let's we call it the DC offset, can't we remove it by taking the mean of the raw data and then subtracting the values from it.
In the heart rate variability analysis case, they go by 5 minute segments of data, so utmost there will be only 300 seconds data for analysis, we can't go with 1000 seconds data. Also, previously you stated that besides the DC offset, there is an component in the spectrum that has a higher value and you said it doesn't have complete period. How do I remove this component or reduce this irregularity ? Your answer to my questions are really helpful and appreciate your effort. Thanks.