  Human heart rate fluctuates in a complex and non-stationary manner.
Elaborating efficient and adequate tools for the analysis of such signals has
been a great challenge for the researchers during last decades. Here, an
overview of the main research results in this field is given. The following
question are addressed: (a) what are the intrinsic features of the heart rate
variability signal; (b) what are the most promising non-linear measures,
bearing in mind clinical diagnostic and prognostic applications.
