What is the difference between RR-intervals and NN-intervals in HRV-data?
The difference is rather simple. NN-intervals refer to the intervals between normal R-peaks. During a measurement, artifacts may arise due to arrhythmic events or faulty sensors, for example (Citi, Brown &amp; Barbieri, 2012). This may lead to abnormal R-peaks, which may in turn distort the statistical measures. To ensure reliable and valid data, only normal R-peaks are selected. Alternatively, the abnormal R-peaks can be corrected. 

In practice, however, RR-intervals and NN-intervals are synonymous (Tarvainen, 2014; Wiki). The use of "NN-intervals" is merely used to emphasize that normal R-peaks were used. 



References

Citi, L., Brown, E. N., &amp; Barbieri, R. (2012). A real-time automated point-process method for the detection and correction of erroneous and ectopic heartbeats. IEEE transactions on biomedical engineering, 59(10), 2828-2837.

Tarvainen, M.P. (2014). Kubios HRV version 2.2: User's Guide. 

https://en.wikipedia.org/wiki/Heart_rate_variability