The nonlinear heart rate variability (HRV) parameter
quantifies autonomic nervous system (ANS) activity based on the
complexity or irregularity of an HRV dataset. At present, among
various entropy-related parameters during sleep, approximate
entropy (ApEn) and sample entropy (SampEn) are not as well
understood as other entropy parameters such as Shannon entropy
(SE) and conditional entropy (CE). Therefore, in this study, we
investigated the characteristics of ApEn and SampEn to
differentiate a rapid eye movement (REM) and nonrapid eye
movement (NREM) for sleep stages. For nonlinear sleep HRV
analysis, two target 10-minute, long-term HRV segments were
obtained from each REM and NREM for 16 individual subjects.
The target HRV segment was analyzed by moving the 2-minute
window forward by 2 s, resulting in 240 results of each ApEn and
SampEn. The ApEn and SampEn were averaged to obtain the
mean value and standard deviation (SD) of all the results.
SampEn provides excellent discrimination performance between
REM and NREM in terms of the mean and SD (p<0.0001 and
p=0.1989, respectively; 95% CI), but ApEn was inferior to
SampEn (p=0.1980 and p=0.9931). The results indicate that
SampEn, but not ApEn could be used to discriminate REM from
NREM and detect various sleep-related incidents.