diff --git a/README.md b/README.md
index 5e12e3b..529fd3d 100644
--- a/README.md
+++ b/README.md
@@ -38,3 +38,5 @@ https://www.researchgate.net/figure/HRV-time-domain-measures_tbl1_320078994
 https://link.springer.com/article/10.1007%2Fs00521-019-04278-7
 
 https://terpconnect.umd.edu/~toh/spectrum/Smoothing.html
+
+https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5624990/